TSP #157 - Teardown, Repair & Analysis of a Rohde & Schwarz FSEK 20Hz - 40GHz Spectrum Analyzer

hi welcome to the signal path in this episode I have another product repair for you guys this is a road insurance spectrum analyzer fseek a 30 this is a 20 Hertz to 40 gigahertz spectrum analyzer so it's still considered a millimeter wave instrument it is absolutely enormous is the largest instrument in the lab right now and it weighs probably about 80 pounds so it's reasonably old you can see it still have a has a floppy disk on it it could be equipped with quite a few options including tracking generators unfortunately this one doesn't have it you can see quite a bit of discoloration here on the LCD screen and the screen is super dark so you can't really see anything it does turn on but it's a it I think has had some water damage in some places so we're going to have to take it apart completely unfortunately it looks like I'm gonna have to tackle the LCD screen before we look at anything else inside just because we can't really tell what's happening on here unless we fix that so let's go and see if I can take the front panel off take a look and see if this discoloration here has damaged the LCD screen anyway if not maybe we can clean it and then change the backlight and see what shows up and from there we can go ahead and do some RF debugging and here's the front panel assembly it comes out quite nicely as one unit the inside of the instrument is really interesting I'm definitely going to show you and hopefully we can take a detailed look and see how that works I don't think there is really any information unfortunately available I don't think rajeshwar shares any block diagrams or anything like that so the front panel button it looks like it's a hard button but it's not actually too soft power button and you can see if I look the other side and it is indeed just a little board here in this corner that is responsible for that so nothing really unusual there is there this is the cable that connects from the front panel connector over here directly to the instrument you have to of course disconnected interesting to see there are ferrite beats directly on the outside to capture any transient effects or anything on the outside of the coal axis that's how on the ground essentially it's covering both conductors so we're going to put that aside very carefully because this has a unique connector that is not as SMA basically the LCD screen connection is a little unusual it's almost like handmade cables this could not have been inexpensive to produce and there's a little resistor trapped in this shrink tube over here so yeah this is some kind of a conversion cable that they must have made to make it work with this LCD maybe there was some change in the production so that they had to just retrofit something else a little bit unusual for sure and then we have the little box over here which has the backlight inverter in it so we can just take that and hopefully it should be easy to replace so I'm upgrading this module to have LED instead of CCFL tube it's going to be pretty straightforward because if you look at this little inverter board it's very clear how it is constructed so we don't have to reverse engineer every little bit on here all we care about is that it's clear that there's going to be three VDD pins and three ground pins by the way they are laid out so we can just steal power from this and tip these are typically between 12 to 15 volts and that's more than enough to connect to a DC DC converter to perform the function of running the LEDs instead of the CCFL so I'm just going to use one of these modules I've done this before in some of the other videos and I've just simply taken the tube out and replaced it with a row of LEDs and you can see that the tube used to be in here so it's pretty straightforward stuff and this is the glass that sits in front of the LCD screen isolating the internals of the instrument from the outside world and by the look of it when you you know when you see it first it doesn't really look like much but this is basically the same as a microwave door it has a mesh between two layers of glass that protects e/m from entering and exiting the instrument and of course this mesh is really really fine in a microwave oven the size of the holes can be much larger because you're only protecting yourself around 2.4 gigahertz you just have to be about 10 times smaller than the wavelength that the instrument is operating on now this mesh is really fine because you want to protect a broad spectrum all the way above 40 gigahertz and the reason this discoloration has happened is because there was a crack in one of the layers of the glass and that meant that moisture had gone in overtime and then there's some discoloration of the machine if you look very carefully you can kind of see the mesh between the two pieces of pass and this just broke when I was fiddling around with it so this is basically going to be now useless and we have to replace it with an equivalent if you want to maintain be in my performance of the instrument so I'm gonna have to go on eBay and see if I can find one and change it and this rose that you see around here this metallic edges it connects to that mesh and then the whole thing is grounded creating a Faraday cage and that protects the instrument now obviously this is going to lose now some opacity as a not going to be perfectly transparent as a result of the fact that there's a mesh but that's a small price to pay if you want to protect the instrument from any electromagnetic interference so let's see if I can find a replacement I also wanted to show you what this screen actually looks like under the microscope so you can appreciate how to find this meshes and how well it needs to work in order to protect any signal from entering them instrument and interfering with any of the internal measurements so this is really essentially making a really good Faraday cage so here's the mesh this is a fairly high magnification a factor of 200 so you can see how fine these wires are and then of course woven together like fabric so that they block all diem field and we can do a measurement we can see the opening in one direction is probably about point four millimeter yes and in the other direction a little bit smaller that's maybe about 0.32 or so there this so considering that the wavelength of eeehm wave at 30 gigahertz in free space is about 10 millimeter you can see that it is quite a bit smaller than any of the frequency range this instrument would operate in and this is exactly what it needs to have if I can move around here a little bit and we can go to the part that is damaged and you can see the damaged part there it is you can see the mesh as the present and the green color very indicative of presence of copper when oxidization happens is most likely what's going on you can see the lamination has started the laminate is quite pretty actually and yeah so this is obviously still there but it's starting to degrade and you can see some shifts here this is where the wires used to overlap and there's been some pull on it from where damage has happened so yeah well I actually did find a replacement and I put it back together on the swing itself so I'm putting back the unit let's go and take a look at the unit and turn it on and see if it works and here is the bottom of the unit as you can see it is enormous I mean look at it compared to the size of my hands are very heavy and large instrument with any boards in it now the very back this is a bottom so you have the main motherboard piece here which connects all the different cards together and you can see they're essentially all in parallel by the traces here an individually addressable by the main motherboard which is over here where the operating system and hard drive and everything else is as well as the connections to the outside world like GPIB serial and parallel and so on so each of these boards performs different functions as I said and they're written over here we have a digitizing I have Section IQ candy modulation which unfortunately is not populated that would be a pretty nice addition to this unit if you were there there's a couple of other ones frequency generation synthesizers and so on so we will have to take a look I have no idea if it works so there's no information that I could find about the block diagram of this unfortunately if you have it please share it with me and I will post it so that other people can find it too so over here we have the front end I've already put the screen back and replaced everything was really boring so I didn't really record that for you guys but here's the front end connection going into a mechanical switch now we've seen RF mechanical relays in the front of these units before keysight does the same thing and i believe this is used for alignment internal amplitude and frequency alignment you can see that it has three cables connected to it there is one cable that's coming from here this is a low frequency cable by the look of it and therefore it's not responsible to do anything you know above a couple of hundred megahertz or maybe a gigahertz or so and the front end relay can select the alignment signal or the front input and route it to the rest and then the second and third and so on converters can all be self aligned and do amplitude balancing it can also do a complete amplitude calibration of the front and mechanical attenuator and this finally making the mechanical itinerary is made by hundreds ooh it looks like so it's interesting to see them using a none reach to front end block but that's not unusual vendors often use whatever they can get their hands on Agilent is a bit of an exception because they'd like to make everything even they used off-the-shelf components of course and here's a micro lambda incorporated block here I believe this is a geek tuned filter I haven't looked it up yet here's a diplexer in the front which is a solid-state switch that most likely switches the front and perhaps by passing the first converter this is typically how it is done and so that when you're in low frequency you actually up convert to your second converters frequency and if you are within the second converters frequency then you don't do anything so it's interesting to see how all of these things actually work together and I'm going to flip it so we can see the other side there is another main RF block there looks like perhaps the first converters there but really there's nothing unusual in terms of architecture it's just going to take a really long time to reverse-engineer it if that it would be necessary I can see some oh interesting here we can select whether it's a color on a monochrome display and whether we can do with hardware a selection of the pallet interesting okay perhaps at some point the screen must have changed and they must have moved to a color screen I don't even know if this screen is color I haven't looked at it yet but anyway looks good let's flip it to the other side and here's the other side absolutely massive power supply I think the motherboards on the other side of the power supply and here are all the different cards there's some adjustments and noise here that you can you can touch on of course and change things but we're not gonna fiddle with it and as some of these things are the eke to end unit and the a0 assembly it says they're a match so they obviously calibrated together so you can change those modules just independently floppy drive here fan here that blows across all the modules really nothing unusual you can see the connectors perhaps from your angle maybe it's difficult to see whether it is the connectors at the very bottom of the motherboard in the vertical a piece that I showed you there that's where some of the boards are missing not hopefully that because someone's taking them out but because they just didn't have those options okay here's the moment of truth let's give it a try the light is on that's a good sign okay oh look at that that is nice and bright very nice copyright wouldn't show us in Munich yeah I was just in Munich maybe about two years ago it's a beautiful place are there go is initializing a bunch of things don't see any issues so far so system is booting I do hear something doesn't sound very nice from the back it could be a fan or it could be the mud or the hard drive we'll have to take a look mister bullying but that the screen is nice and bright as you can see I don't know if it's doing some bizarre thing on the camera but let's see how there you go okay now it's completely blue so good synthesizers are successful it's finding all the DSPs that are inside each of the bore so it's detecting all the board's these are all good signs and there is okay looks good doesn't seem to be doing anything unusual so far there's some death pics or something else to do but I think that's normal and other than that it looks fine let's see oh no that's not fine I take it back there is a problem there you go we have an L oh one level that's not good so there is something wrong with it well let me fill the round with the firm a little bit and see if I can find out any more diagnostic information so we can get some information here by going under info and hardware version you can see that more FS ek 20 shows up even though this is FS ek 30 but and it shows a bunch of of the ports that are installed nothing really unusual and if I go into system messages you can see that we have a frequency unlock condition the egg loop is not locked and this thing these things as you've seen I've done quite a few repairs and synthesizers and PLL loops of multi loops and this can be very complicated even when there is a block diagram so without the block diagram oh that's going to be pretty tricky this shows up but it's really annoying because I you cannot get rid of it in a easy straightforward way I have to keep pressing clear so that's it let me see I was going to under info so if I go look look at the options it has the low phase noise option installed but that's really the only option that's there so it doesn't have IQD modulation and all of those things but nonetheless it's delay like I said it a 40 gigahertz instrument if it an if we can get it to work so there's no point putting signal in it if the hello is unlocked because with yellow unlocked you basically don't have anything so now let's see what we can do well an easy way to start is to check you know from the very beginning all of these instruments have some kind of a reference so there's a crystal oscillator oven control crystal oscillator and there is some kind of an internal 10 mega Rohan megahertz loop that starts all of the synthesizers because everything has to be based on that initial 10 mega our signal and then it gets multiplying there's gigs and ratios and so on that create many many different frequencies feeding across the entire instrument and I'm sure some of those can be traced and tracked directly in the unit as I go on and poke around so let's go ahead and do a little measurement here and there and see what we can get so a few interesting development so I put unit on its side in order to get from the back the ten megahertz input-output and connected to my frequency counter right over here and I noticed that when I turned to the side for some moment about maybe about five minutes TLO on level went away and then but I unfortunately can't replicate that anymore of course as an intermediate problem is the worst kind of thing to try and fix but I'll show you what happens up so right now unfortunately again it went away so it is still saying hello unlevel i just restarted that to see if he goes away again so it's booting and the idea here is that by looking at this signal ten megahertz you can get an idea if the internal crystal is working as that's the beginning of the PLL as I explained earlier and see what kind of accuracy and and we can get from that but at the same time I'll tell you it's connected to the Fluke PM six six of eight five R which is a rubidium based instrument so it's extremely accurate and there you go you can see that the unit has powered on and it is very very close to ten megahertz it's actually a little bit further than I would have thought it would be which is a bit surprising but you can see it's kind of sitting there and we're gonna wait until the crystal in here warms up and see if it settles to a nice value so I waited for some time and this guy is now just over ten megahertz but it still moves around a little more than I would have expected an oven controlled crystal a stir to move around so it's a little bit surprising now it could be that this is not directly from the crystal remember it could be that this is actually a VCO divided down sometimes they do that they don't directly take the crystal out but they're taking it from another loop which could be the reason why it's doing this but anyway my intuition says it's a it's a little bit strange well one way to easily find out is I'm going to spray some cold air onto the oven control crystal acid in that frequency should stay fairly stable because that's the whole point so let me give you a bird's eye view of where the oven control crystal is is right here I connected to this board we can actually take that board out if you want to so I'm going to spray something alright and and see what we get we go I'm just gonna spray some coolant on it like so there we go so I can see a bit of frost on it it's okay oh that's interesting so the frequency shifted by quite a lot oh well this you know be able to see that yeah sorry about this here we go take a look yeah it shifted by quite a lot this is not how a crystal should behave so indeed there is a direct connection and he moved a lot and shouldn't be doing that so my intuition again says something is wrong there and there was another interesting thing I found out that I tried before I was spraying it I was just feeling the temperature and this oven is not hot at all I mean you would this would have to be at you know 4045 degrees Celsius the case temperature and it can get pretty hot and no I I'm I had my finger on it and it's just basically essentially sitting in room temperatures a little warmer but it doesn't look like the oven is running which is a little surprising and because I get the signal so obviously it is powered but it could be that it is powered from a different supplier so now we have a few pieces of information at the same time it worked for some time after I kind of turned the unit to its side it doesn't seem to be over nice or at least the oven doesn't seem to be working the frequencies not quite stable so I think what we should do is you should take this board out and take a look at it and see what's inside of it it would be nice to see what's in it anyway and see if we can get any further so here is that module out so this module is obviously responsible for converting the USC XO module into some of the other PLL functions that are there you can see the VCO mixer divider phase detector and interface controls are there as a few inputs and outputs at the bottom and I see on the instrument that one of them is labeled 732 megahertz or so so it may be that it is even responsible for some amplification of some of the other elbow pads but this is a low phase noise option a 120 that's probably what we were seeing when we looked at it now I was pretty happy when I took it out and I was looking at it looks nice but then I flipped it over and then I saw this now that is not a good thing it probably means that someone has been fiddling with this before and there's something might have been wrong with it although this a solder point here that holds this case in does not seem to have been removed maybe somebody was just looking and see what's underneath it the other issue is that the part number is gone so I can't even tell what was there so that's going to be a problem that's probably maybe easily findable maybe you can find that online by looking at some of the 820 modules there is one of these modules available on eBay so technically force comes to worse we could try and invest in that but I think the first thing we should do is to take it apart and take a look at it okay it's a very nice design so this screws I actually don't fall out they are held in place I even after you remove them so they're tapped inside and here's the multiple layers of shielding you can see there's some we have a copper perforated copper plate there that ensures very good connection because it's got these points that stick out and make sure you have good connection to the chassis another layer on top they really really care about making sure that this thing is protected this discoloration is probably due to some thermal effects and some oxidation that happens over time but here's the board there's not that much on it actually it's quite a big section of it that is empty it's probably because this has to have a certain size so it doesn't need to be necessarily as big as everything else but here's our crystal here's the mixer section I can see if you wow that's actually quite a few barges on here so they spend a lot of time modifying these boards and it's kind of dirty I guess it hasn't been washed very very well I don't see any immediate sign that has been that tampered with well look at that there was a big grid of resistors here it looks like it may be a programmable attenuator perhaps unusual that this is not an ASIC I guess it's a fairly old instrument so you can see some stitch line here so there's clearly some signal that's being routed in a coplanar transport it's actually combination of strip line and coplanar waveguide that goes through the entire board now it looks nice I think we should take the other side apart as well so we can see what's on the other side of the board and here is the other side of the board and you can see on the other side with the resistors were there's looks like to be a grid of transistors which is probably consistent or at least pin diodes maybe I consistent with an attenuator that would enable and disable the switches that could be one of the things that's going on as we've quite a few amplifiers I see the amplifier sections here these these section seems to be almost entirely independent and here's the rest of that line that I was talking about and here's our crystal so as a there's a lot of stuff here of course with other nice chimeric or anything it's going to be very hard to find out now there's two things that can be happening here now the crystal I think is generating a signal the fact that is not getting warm is unusual two things could have gone wrong either crystal itself is bad but even if it were bad it's still generating ten megahertz the instrument doesn't know that this is not stable it has no way of knowing that it can it can vary by 10 percent and you wouldn't know it will still tell you that things are locked because the PLL will follow this go to VCO that summer on here would follow this so but the fact it doesn't get warm could be that there's a power supply problem because the power supply is running the coil inside the oven it's a different power supply than what's running the crystal itself you wouldn't want them to be the same anyway because the the power supply that runs this is going to turn on and off as the dip switches temperature you don't want the power supply of the crystal to be pulled so hard by it so that's not going to be the case so that maybe is a power supply problem now I can see a few power supply considerations here is that filtering here at the input you can see the thicker trace is directly coming from the connector are going to be filtered by this LC network Dave bill is a very deliberate LC network it seems like perhaps some particular frequencies were giving them trouble and they were trying to eliminate them by creating large filters on the power supply but I can see that power supply indeed is going through these large inductors and then decouple to the capacitors and then being distributed all over the place I'm looking for the VCO here I don't see one at the moment could be it could be this could be the VCO section actually you know what I think it was labeled yep I was right this is a VCO right here and the reason I could tell it's the VCO is by a few little kind of pointers here so we have an oscillation oscillator here and then we do have a very small reactor here I just a book a pastor there and that is probably to tune it and I think there's a hole there that you can reach this and I just is to bring the center frequency of this VCO to where you want it to be and then a signal from that is passed through these little guys that you see here these little things here these are essentially a coaxial connection from one side to the other order filters this is labeled L so this is probably a filter and it just filters signals going in all of these Faraday cage isolated sections here's some bars you see here they've added another board on top of it probably because it was much cheaper to produce this then to repin the board here's the array grid of resistors I was saying that which I was guessing is some kind of an attenuator here's a mixer over here know this and mixer is probably being fed very closely by probably that oscillator it's hard to tell though without I could x-ray this also but it's a fairly thick board so I don't know if my x-ray is actually strong enough to see through it but yeah well I think it might be a power supply issue now the fact that I turn the instrument and all of a sudden started working is kind of weird but the fact that the rest of the unit works I mean if the power supply let's say one of the main power supplies of the unit was dead you wouldn't expect only one problem you would expect many problems and it may not have even powered on correctly so it's worth investigating this board a little bit further starting with checking to see maybe this itself is pulling down the power supply because it was a short in the oven that can happen the oven can become a short circuit and then it would basically pull the power supply down and not allow a portion of this to be powered on anymore it is possible that this is just simply not getting power now if I look over here on this side I can see some of the lines coming here and could be that these are the power supply lines going in here getting buried in there and then God knows where they go from there so that's gonna be tough to find I could remove this and we can take a look at it by itself and then do some more testing around to see if we can figure out what's going on so I started looking around and trying to trace the power supply that connects directly to this crystal here and find out where the oven was and I was having some trouble and eventually I figured something out I measured something that was really surprising so let me show you so I was looking around looking at at the inputs and the outputs of these sections basically trying to find a direct connection so I was at some point basically doing at the measurements where I was looking either on this Saturday inductor or on the other side of the inductor now these inductors are going to have almost no resistance because they're they're very small and they're gonna have maybe less than a few ohms so I was measuring one and I was sometimes I would measure on this side sometimes I was measuring on this side because I was always taking for granted that these are all just inductors and then I I slowly was moving along and you can see I can measure them each individually and then I measured this one and there was nothing and I started pushing it around and every once in a while when I kind of pushed it and tapped it I could get it to should not of course it's not doing it right now but I don't know if I can get it to do it again but anyway I was pushing around and every once in a while I could get it to become a short circuit again but it's not easy I think I just I may have just turned again so there is it's an open circuit so this inductor is not even in circuit and it says it's a hundred micro Henry so it shouldn't have too much resistance but yeah that's interesting so let's go ahead and take a closer look at that I wonder if this could explain why this board was not fully functional maybe maybe this inductor is directly responsible for powering a section on this board and that's why this guy wasn't turning on so here's a closer look just so you can see what I'm talking about and you can see that indeed I get nothing I cross this or as I get perfect connections across these other ones so yeah this is a through-hole component should be really easy to remove so I'm gonna go ahead and remove it and I don't have a replacement but it looks like you may have it's a very fine wire it's a largest inductor of them all this 100 micro Henry and maybe the wires burnt out now it could be that this is still the short circuit that they've coil in the och so is indeed a short and what has happened is that the wiring here is just burnt out because of the short circuit in here so we have to be careful even if we can fix this that we don't I'll be making sure there's no short circuit across it well here's our inductor I mean it looks fine but of course it registers as an open circuit I'm going to take the casing off and examine the wires carefully under the microscope well I'll check this out I think I found that this doesn't seem to be quite connected now I think I may have I think I may have made it worse when I was trying to remove the casing of this but this is consistent with the instrument working all of a sudden because this probably must have been really really close to making contact maybe a cold solder joint or over time I don't know what must have happened to this but you must have been you know like it all over here basically and then sometimes it would make connections sometimes it wouldn't make connection and that explains it so I can unwrap it one round and then resolder it back and hopefully this will fix it it police inductance will change by a tiny amount but doesn't matter we can make it a full in doctors measurement to make sure it is still acceptable so here is our repaired inductor let's do a quick measurement to make sure it is still okay we're measuring this at one kilohertz which is reasonable and here we go so and let's see what do we have there we go a hundred and two micro Henry with the serious resistance of about point seven ohms so I think that's pretty good that's reasonable that's what I would expect to find out you can change the frequency to higher and lower frequencies it's at the 10 kilohertz we're going to see different value and we can go to 120 Hertz which is a little bit more realistic there we go you can see 0.1 and milli Henry and the resistance is about the same so I think that looks pretty good we should be able to put this back in the unit and see if it makes any difference so I put the unit back together and check it out there is no more ello unlevel error so that's a huge improvement of course it seems to be working fine when I initially turn it on it said you did say ocx oh and I think it's just was waiting for the other CXO to warm up and reach temperature it was warning the unit was cold laughter some time that also disappeared so I think it's working really well if I go on their info and look at let's see message system messages here you can see that these are the old messages so I can clear all the messages away and all messages update and there are no more new messages so it's not generating any error which is which is quite nice now what one of the other things we can do is to go and do a self-test on it and execute the softest I'm going to wait for the software's to run and see if it generates any errors and here we go self-test complete didn't generate any errors but of course we have to put some signal into it so which week I hit it is what I don't understand there is no it doesn't seem to be any button to get rid of this okay there's no there's no Enter key or anything like that so things were looking good while I was doing some initial measurements here you can see I have a tone at 7 gigahertz there's a marker on it the tone is at about 0 DBM which is showing up correctly but as soon as I change the frequency from 7 gigahertz to 7.1 gigahertz on the synthesizer you go I go up by point 1 and the signal completely disappears so it looks like that something in the front and switching part of the spectrum analyzer still isn't working and when it does work very well you can see at the lower frequencies I can move the tone all the way back and it has no problems at all following it I can go back up I can go to as soon as I pass 7.1 gigahertz the signal disappears so it's typical that the front end of these spectrum analyzers switch at some point between one to another band and die flexors and filters and different kind of front-end if' stages are switching in and out so it could be that they're damaged as part of that inductor that had an issue or it just that it wasn't working at all and we just are finding it now well it looks like it's going to need more investigation so given that the transition between being able to measure a signal and not measure a signal was such a sharp point and basically you were getting something and all of us are not getting something right above seven gigahertz it is possible that the problem is in the front-end diplexer now there is a switch here that's the call the dialects sir I know if it's really fair to call this a diet lecture because it's not like it operates at two frequencies at the same time diplexer should really be reserved for frequency division systems but anyway so this one obviously switches either to one path or the other so the input goes through the front end a switch that I spoke about a ten year into the diplexer and then into what appears to be a uke tune filter and then after that goes out over here and into one of these boards the other path goes from underneath and to one of the other boards so there's obviously two if' sections that are involved here and depending on the frequency range a different one is active so what we can do is to basically open the cables connected to the diplexer switch and observe what's coming out of it from each output and see if it is present with the diplexer now before I do that I can do a simple measurement to make sure that the diplexer switch is actually switching so I'm going to go ahead and try that and turn this instrument on I'm gonna then wait until it boots up and I'm going to measure the voltage on the switch here so I expect to see two different voltages as the instrument sweeps past seven gigahertz so before and after seven giggles there should be two different voltages if that's working then we can go ahead and do the actual RF measurement so here I have the instrument set to sweep from DC to ten gigahertz with a span of ten seconds so it's going to take ten seconds to do those frequency sweeps it should be make it easy now to observe this voltage and see if it changes across the sweep so here's the voltage and there it is it at 14 volts there you go as he just went to minus 14 so it's right now is sweeping from DC it's going to hit seven giggles any moment now and then it should switch there it is you can see you switches to plus 14 volts and then it's going to then switch back down again and let's see if you go back again to - there it is yep so it is working the voltage is indeed being generated correctly so now that we have to check to see if anything is actually coming out of the diplexer or not so we know the instrument does work below seven gigarth so we know that one of the paths indeed is functional so there's no reason to test that path we're going to only check that the switch part above seven gears so I've separated the diplexer output from the egg tuned filter and I've separated the egg from the first converter so I've removed basically all those cables and I've connected the output of the switch directly to my keysight MX a that allows us to monitor the spectrum and see exactly what comes out of the switch as we change the state of the diplexer and that's easy to do because we can put an instrument in a zero span mode and set the frequency and that fixes the state of the switch so it won't be switching back and forth and then we can sweep the frequency at the input of the instrument and monitor the output and we can do the same thing with the e key as well and here the input is just simply coming from a synthesizer so it all makes sense so currently I'm applying an 8 gigahertz tone to the input and the switch is in the opposite direction so it's in the suite decide where you don't want it because to come through so it is doing that you can see that the signal is very very small it's at minus 60 DBM what I'm applying is at minus 20 DBM and there's some attenuation in the front end as well so let's go ahead and change that I'm going to change the center frequency of the instrument at 80 goes to 80 goes therefore you should see that signal go up and it does go up a little bit if I go do a peak search on that it's sitting at minus 40 DB M so let's think about if that is correct or not well there is some input attenuation in the front of the unit we can go ahead and change that if I change the input attenuator then that just that value completely goes away you can do manual attenuation and zero dB there we go so there's no attenuation now you can see when up 10db that makes sense because there was 10 DB front attenuation so -20 comes out and then minus 30 reaches the basically the impolitic i think that's okay there are some losses associated with that so it's probably working so the next step to check to see is the eke tone filter working or perhaps that's the part that is having a problem so we're going to change that now instead of taking this signal and applying it here what I'm going to do is I'm going to apply the front end input directly into the year and then we're going to take a look at the gig at the output the gig is controlled by this board as well so that's how the center frequency of the egg is determined that might be a little bit difficult you know what maybe we shouldn't do it that way maybe we should measure the eggs as parameter directly because we can do that and that will give us a complete response and we can change the frequency and see it live that's probably better it's gonna take a little bit longer to set up but I think it's worth it and here's the Eagles that are in an s-parameter setup just from its inputs to its output directly being measured on the hydrogen and 52:38 so I'm getting some really interesting measurements here I have to reposition the camera so you can see it but I'm going to explain what's going on and hopefully we can fix this and here is the result and by the way a lot of the instruments I use here to debug other equipment I have actually fixed on the side and you can go ahead and find a video there's a two-part episode on how I fix this one so definitely check it out so here's our filter we were looking at s21 you can see it has an incredible high quality factor which is exactly what you would expect from an egg filter and there is a video I've done on all about eggs and how they work and how you can make filters and ostlers Adam it's really interesting you can see the internals under the microscope definitely go check that out as well so if you look over here you can see the filters Center is 6.4 gigahertz but that's not what I have setting instrument to be the instrument is at 8 gigahertz so the filter is not moving with a sweep so that's the problem that's why you we're not getting anything on the unit when we measure because the square just absolutely filter everything out and we look at the floor here this is the noise floor of the instrument here at minus 80 DB so we have about 80 DB of rejection here so of course you're not going to see anything so that explains why the instrument wasn't working as the egg isn't moving so how do we fix that well there's a couple of things we can check we can check the control voltage of the egg it's it's moving or if there's something happened inside the year that's preventing it from being tuned the coil could be dead for example so we have to take a look and see what's going on but let me go ahead and put the instrument in sweep mode to see if it moves at all and here the instrument right now is sweeping from seven and a half gigahertz to 11 gigahertz and there's absolutely no movement on the egg at all now the fact that the egg is actually doing some filtering is interesting so part of it is probably working but there's not tuning so let's go and measure the tune voltage and see if this tuna boat edge is moving at all as the instrument is sweeping and the tuned voltage is readily accessible because the egg is controlled by the same a board that is controlling the diplexer switch which is nice so we can go and take a look so the instrument is sweeping slowly so we should be able to measure these voltages the four controls two of them are for a heater and two of them are for a coil so we can see what voltages we get so the first one we have 14 volts that's good second one we have 14 volts again okay then we have minus 14 volts and plus 13 volts okay but none of them are moving this is suspicious it should be moving around at least one of them should be moving around so they're all reference to to ground which is the chassis but it's most likely that they're applied plus and minus across the coil and the heater so interesting it's interesting that the heater is still going on I think there's two things I can do at this point I can take the egg out and see if we can write ourselves or I can take that board out so we can see where those cables go and perhaps there is a driver on there that's broken so let's pick one and go ahead and here is the a 20 millimeter wave converter unit this is the unit that allows the instrument to work up to 40 gigahertz and it's all essentially done in this alone multiplexing and this converter module here so we're not going to touch any of that because we don't need to worry about it we want to find out how why the egg doesn't seem to be working or at least doesn't seem to be tuning so the egg is controlled by this module over here by this little connector over here on this module and there are the way this connected is fairly straightforward there is a heater and a coil as I mentioned earlier the heater is permanently connected to plus and minus 15 volts and the yiq then controls how the heater is supposed to be operating internally the coil on one side is connected to plus 15 volts permanently so that's makes it much simpler which means that the other side of the court has to be controlled by this module in order to tune the ik now I traced it out it turns out that the negative side of the coil is actually controlled by this array of MOSFETs here and these are all in parallel there's a lot of them probably for current handling capability and they're essentially variable resistors in the way they're configured their source is connected to minus 15 volts their drain is connected directly to the ik and the gate is being controlled by some analog circuitry on the other side unfortunately we can turn this on when it's sitting on a bench like this because it needs a special connector and it has to be inside the instrument but we want to see is the MOSFET seem to be okay so I don't think the master saw the issue which means that the control voltage is either not present or not getting passed on to the MOSFETs so what I did is that I flipped it on the other side and I started looking at what's going on here there's a lot of stuff here obviously a lot of switches and op amps and so on to control various circuitry there's a DAC here which most likely is the course tuning of the igg and there's some other stuff and some analog voltage may be coming in I'm not sure but the last op-amp over here is what is connected to the gates of those MOSFETs so what I did is that I saw the three wires to the input plus input minus an output of this op-amp so we can now monitor all of these from outside of the unit and see if these voltages are changing at all and if they're not changing then we just continue working our way backwards until we find where the problem is if you're very lucky then the op-amp is the only problem and we would replace that I don't think the MOSFETs are an issue there's so many of them anyway so let's try that and see if these voltages change at all that would be a good start so I went ahead and did that measurement and the voltage just seemed to be stable it wasn't moving around there was a couple of additional things I think I should add here to this discussion keep in mind that a gig is tuned with current not with voltage that's because you're controlling a magnetic field through an inductor essentially so you want to make sure that electromagnet has the correct amount of current and the way to do that is to monitor how much current is passing through the egg which is done by looking at the voltage across these series resistor which are all in the sources of the MOSFETs and by taking that voltage and putting it into a current monitor then you can adjust precisely how much current is pushed to the yeegh and then if you have the tuning curve of the yiq you can then sweep that with the frequency of the spectrum analyzer itself so that characteristic obviously has to be pro calibrated into unit so I tried that in this once it seemed okay I traced some of the more signals and he obviously goes through all of these components over here and there's some other stuff that's been controlled also by this op-amp so it's a fairly complex structure here also the biasing of this entire allo multiplexer mixer down converter is controlled by all of this so you don't want to mess with it too much and that this is controlled by a digital interface coming from the unit itself the spectrum analyzer itself so this is all great now the only issue is that every time I want to monitor something I have to solder wires to it and put it in take it out and it's just such a hassle so what I'm going to do is I'm just gonna replace a few of these components nearby the yegg hoping that i'm gonna hit one of the op amps that most likely has gone bad or maybe the DAC has gone bad this is gonna take much less time it's gonna be cheaper than kind of putting it in taking it out of the unit it's gonna be pretty annoying very quickly so I'm just gonna try that and hopefully we'll hit the right component it's a not completely a brute-force method because I have traced out the critical components in the path of the Y it's not just randomly changing things I know that for example is the reference voltage here for controlling it or some op and some switches here that you can then switch this DAC in and out of various different components so it's you know I have a rough idea of what's going on here I wish there was a schematic then it would take a fraction of the time but anyway let's go and try this so I went ahead and replaced two of the op amps I ended up replacing these tube these are the two op amps that were involved in a transconductance configuration converting voltage to current to tune the yiq so I put everything back in there let's go ahead and turn the unit back on and then we can monitor once again the s-parameters of the year to see if medium you made any difference and if not we can go back and do some additional changes so the instrument is going to boot up now and then we're going to take a look at it and check it out it sweeps this is fantastic so the gig is actually moving obviously is jumping all over the place because the sweep is pretty fast but I can slow down the sweep and I can sweep between two frequencies that fits within them network analyzer range so let's start from a frequency of 8 gigahertz and then I'm going to stop it at a frequency of let's say 12 gigahertz and I will change the sweep time make it really slow and that will allow us to observe it sweeping slowly and check it out isn't that beautiful look at that very nice sharp bandpass filter is going to go all the way to 12 gigahertz which is at the edge of the frequency of this network analyst and jumps right back so it is indeed working we can try doing a zero spam situation and the 0s panel allows us to find out how for example accurate the whole thing is so here zero is man I'm going to set the frequency to let's say 10 gigahertz let's say put it at 10 gigahertz and then we can put a marker here and see how good we are at 10 megahertz where's the marker here we go and check it out I don't know if you can read this or not but 10 point 0 0 3 5 7 8 and of course the reference between this instrument in this room and are not locked so you're not gonna get it for size measurement but it is exactly at angers this is great now we can put it back together and test that but before we put it back together I do want to discuss a little bit why this in each unit is even needed why do we need a gig filter in front of this instrument at you know at angers but we don't have it at 7 gigahertz so let me go through that theory because I think it's important for you to understand that and then we gonna test the unit so here's an example of what a front end or the typical spectrum analyzer looks like this is obviously heavily simplified but they generally have 2 bands of operation they have a low frequency band this is a very simple example of that and you just low-pass the filter to ensure no high frequency components exist in the use of downconvert mixer and then your bandpass filter it and then you down come read it again this is to I'll remove some DC and some images and then you process that and you eventually display it now this is fine and it works really well but what happens when you want to look at really high frequencies let's say I want to look at a signal at 15 Biggers well this mixer here when you're looking at low frequency operates in fundamental mode so you want to down convert 35 gigahertz signal you will down convert let's say with an L oh that's let's say 4.8 giggles that will give you a 200 megahertz if' which can easily be handled by the rest of the components and you down convert again and so on but if you want to look at a signal at 10 or 20 or 30 gigahertz then you would have to generate an Aloha signal that's also a 30 gigahertz which would mean that this other mixer would have to operate a third Eagers allow you to generate that you have to tune it in the entire band nobody wants to do that that's really expensive it's going to be difficult to generate harmonics are going to be there so instead what people do is that this high band path can operate in harmonic down-conversion so it will operate let's say at 10 gigahertz to the unconverted angular signal but it will also be fed with the tangles alone if you want to down convert a 20 gigahertz signal it will just simply operate in the second harmonic the disadvantage of that is that the conversion gain of this mixer is going to be significantly affected operating in higher order harmonics is linearity will also not be as good and worst of all is noise figure would be quite a bit worse because this conversion gain is quite a bit worse so but that's a price to pay that would enable you to down convert very high-frequency signals generating only a fraction of the elo signal to do so but then there is confusion because the images are going to be in the band and more importantly you don't know which harmonic is being done converted so if you have a signal at 10 gigahertz 20 acres 30 gigahertz they're all going to react to a 10 gig RSL oh and are going to mix with different harmonics of it so you don't want that because then you don't know which signal it is that you're looking at so instead what they do is that they put a pre scalar in it which is what we have a yick tuned filter which sweeps the entire band of the input frequency and filters a narrow signal so that you don't have image problems and you don't have sub harmonic conversion problems so this will solve it the disadvantage of the prescaler is of course it itself is quite expensive it has to be tuned very carefully it can drift away and prescaler will have lost if you pay attention to the s21 we just measure of the egg filter you will see that it has quite a bit of loss so it eats into the dynamic range of the system yet again so not only do you have a worse noise figure because you're using a sub harmonic or a harmonic mixer you're also going to have an additional noise figure in the front end of your instrument because now you have to go through the gig filter the prescaler but that's the way it is and if you look at very high frequency spectrum analyzers you see that the noise continues to go up as you go higher in frequency and you pay attention you'll often see that this noise is in those sections it's got those jumps that happens and those jumps happen when harmonic of the mixer changes so this characteristic can be seen in the displayed average noise level of any spectrum analyzer now people obviously can make a spectrum analyzer that goes from DC or to let's say 67 or 110 gigahertz and never use a prescaler and even never use harmonic mixers it's just not very practical but that's the reason why this is here if I were to remove this from this instrument let's say was broken and you would just remove it instead of trying to fix it then you would never be able to get a clean spectrum all the harmonics and all of them other versions of the signal you're looking at will show up and you would never know what it is that you're looking at so that's the reason why it is there but I'm glad that now we have I think we've managed to fix it so we should go ahead and close up the instrument so we can measure some high frequency signals with it okay let's be able to try now we knew it was working at 5 gigahertz I'm gonna put a 5 bigger signal in it we're gonna do a search here there are this 5 gigahertz you can see this measuring of a -2 DBM roughly correct we have a 0 DB signal coming out through DBM signal coming and there's some losses in the cable okay so that works fine but obviously wasn't working above 7 gigahertz so let's give it a try and let's triangulate and tada check it out 10 gigahertz - 2 DBM it's a little more power there there's some calibration obviously this this thing is a bit of calibration but I think it's working fine you can go up to 20 gigahertz with the source that I have right now here's 20 gigahertz we'll do a big search again there you go - 4 DBM it still makes perfect sense so he seems to be fine and I think it is working just fine now I want to test it at 40 gigahertz also just in case even though this path is the same path that is shared all the way up to 40 gig hours here's one of the jumps I was talking about on the harmonic of the mixer changes so let's go ahead and try again and see if I can replace that with a millimeter wave source and then we can go all the way up to 40 gigahertz and here is our setup for generating signals all the way up to 40 gigahertz I've already showed you these modules I've even open them and show you what's exactly inside of it this is a fascinating design done by HP quite a long time ago so I'd definitely recommend that you watch that video but we're basically generating a 1/4 of the frequency of interest going into the source controller amplifier and back into the millimeter-wave module multiplier and then from that we go to waveguide this is actually a wave got output and then from we that go to coax and back into the unit so with this we should be able to measure up until the edge of the frequency which is 40 gigahertz so let's give it a try so let's look at the frequency in in the upper band of the down conversion to make sure it's working we can look at let's say 32 gigahertz here we go turn this on and do a peak search here there it is check it out 32 gigahertz again remember the references are not locked to each other so they're not going to be exactly matching and you're looking at an enormous span here so of course it's not going to be precise but what I can do is they can let's say Center this instrument to 32 gig or so you can take a closer look and make the span smaller let's make this band let's say 100 megahertz and we should be able to see our signal right there so and I can do a little bit lower resolution bandwidth let's say we doing a manual resolution bandwidth and let's go down to there are this 50 kilohertz and we can do another peak search here and we can see the frequency now it's much closer of course but they're still unlocked from each other the power this is reporting is about minus 3 DBM the power reported from the multiplier is very close to that so it's it's good it I think it matches there might be some minor inaccuracies it may need a fool the calibration if you want to get it back to factory but it's really close it's definitely close enough for something that was built you know in the 90s and you know we just repaired basically and there you have it I hope you enjoyed this repair of this instrument it took a while to fix it there was many many things wrong with it I also I changed the battery that was inside it just wasn't very exciting it wasn't keeping time but now it does so that that's definitely working I also backed up the hard drive that was inside of that in case it ever breaks that we can replace it I have an image of that now and that image is of course available for the fsck 30 and with all the things that are built into it it looks good and I want to thank everyone from patreon you're the reason why this is possible instruments like this are really expensive that's the only reason I can do this with these broken instruments because you guys are supporting the channel I really appreciate that so let's discuss it in the comment section any other suggestions I would love to hear if there are some specific video types you want to see let me know I'll see you next time [Music]

Channel: The Signal Path

Views: 32,292

Rating: 4.9579287 out of 5

Keywords: spectrum analyzer, rohde & schwarz, power supply, YIG, Filter, Inductor, LCD LED, ccfl replacement, LED, Backligh, Pre-Selector, OpAmp, Operational Amplifier, Mixer, Frequency Converter, OCXO, Crystal, Broken, Repair, Teardown, Multiplier, Shield

Id: oi4ipg9qHR4

Length: 51min 12sec (3072 seconds)

Published: Sun Aug 25 2019

Reddit Comments

[u/YOUREABOT]

have u tried just asking R&S for a block diagram? I'd expect that quite a few people who work there know this channel.

[u/poggy39]

I haven’t used this equipment for 30+ years. I flew as a Flight Engineer on P-3 Antisubmarine Aircraft for the US Navy and had to repair a lot of radar, radio & sensor equipment in some pretty remote locations. We sure could have used your expertise then. I can tell you that our band widths changed from equipment to equipment and we relied heavily on an earlier version of this unit. Received a lot of additional education at Moffett Field, Ames Research Center, Silicon Vally California after advanced electronics. You Sir have a unbelievable understanding of the flow and usage of electronics. Bravo Sir.

[u/AyekerambA]

https://giphy.com/gifs/good-burger-i-know-some-of-these-words-zXA5VEmXr7OUg

[u/ClashOfTheEnder]

Did they try turning it off and turning it back on again before doing all that?

[u/Public_Fucking_Media]

Well that certainly beats the shit out of the 30+ year old spectrum analyzer I've used...