#270: Tune a Duplexer with a Spectrum Analyzer + Tracking Gen or VNA

in today's video we're going to talk about tuning a simple antenna duplexer with a spectrum analyzer and tracking generator as well as tuning it with a VNA or vector network analyzer but first let's talk about what a duplexer is the duplexer is a device that permits full duplex transmit and receive through a common antenna using relatively closely spaced signals a diplexer on the other hand typically will separate frequencies by band maybe a VHF and uhf but a duplexer typically art works within a common band where the frequencies can be very closely spaced together this can be this is often used in very common applications like cellular telephones and things like that as well as a public service radio an amateur radio of repeaters and because the transmissions occur simultaneously the duplexer must ensure that the transmit frequency doesn't get into the receiver and then the receive signal coming in also doesn't get into the transmitter and because these signals are very closely spaced that places some pretty strong requirements on the filter properties and the bandpass and band reject characteristics of the duplexer now there are different types of duplexer 'he's probably one of the simplest types is a band reject or notch type filter where essentially the transmit frequency is notched so there's a lot of rejection in the receive path and vice versa the receive frequency is notched in the transmit path so that ensures the transmit signal doesn't get into the receiver and the received signal doesn't get into the transmitter the problem with band reject filters is that the pass band characteristics are relatively broad so if you're working in a dense environment you might allow other signals to get into the receiver that might overload the receiver so the band reject filter while it's simple and it's the example that I have here that I'm going to demonstrate it's not the most desirable but it's certainly the easiest to tune so that's why we're going to use it in this video there's also been pass type filters bandpass type actually sets a bit a narrow band pass filter around both the transmit and receive paths with a high rejection per out of band signals and that's typically the preferred type and then also there a repeater or duplexer that buying both of them they're both a bandpass and band reject in the same device and they're more complex to tune and if I get ever get a hold of one here too to demonstrate I'll show tuning one of those combination type devices as well but for today we're going to just show tuning a band reject filter with both a spectrum analyzer and with a VNA or vector network analyzer this is the duplexer will be using for today's video it's a simple band reject or a notch type duplexer so we have an operating frequency pair a high upper frequency in the lower frequency and they go through to a common antenna port on the high side we will pass the upper of the frequency pairs with very little attenuation and will notch out the low frequency of the pair and just the opposite occurs on the low side the adjustments are done by essentially tuning each of these rectangular blocks which are kind of resonant cavity notch filters and there are little adjustment screws at the end of each of these the duplexer like this is a passive device so the response through a given path will be the same regardless of whether we're sending uh signal in that direction or in this direction so it almost doesn't matter really which side we hook the tracking generator up to and which side we hook the spectrum analyzer up to it is important though in many cases to ensure that you terminate the unused port when you're tuning one side we've got the tracking generator output connected to the high path port the antenna connected to the spectrum analyzer input and we've set up the sweep to encompass the notch that should appear in that high path now the sticker on the unit indicates that the low path or the low frequencies should be three hundred fourteen point seven five megahertz so I've got that set up as the center frequency of my sweep and I'm looking at a hundred megahertz span now what we can see on the analyzer is essentially the notch in this case there's three little notches because of the three cavities that are not properly tuned and right here at the center is where we actually want that all these things to line up so we'll just simply adjust each of these screws to make that happen it doesn't really matter where you start generally so we just start on the end here I'll start adjusting this one and we can see that the notch it's all the way over on the right is moving over so let's move that over until it kind of lines up right about in the middle and we're probably right about there let's adjust the second one here to see if we can make any improvement in that let's see if I move it this way you can see I'm kind of extending it out to the right so let's kind of bring that back now if we go too far we can kind of see we're extending it the wrong way so let's try to Center that back up and bring it into the center no grab the third slug here they give a screwdriver in there there we go and bring that one down so now I can see I've kind of adjusted that probably about as best as we can do we can see we're driving right down in here now the wideband of fuzz I see here indicates that I'm really driving down into the noise floor the analyzer here so let's see if we can do something about that so right now the resolution bandwidth on the analyzer is one megahertz so if we lower that let me go to a coupling menu and uncouple the resolution bandwidth and start lowering that down if I bring that down to say you know thirty kilohertz or even 10 kilohertz you can actually see now that I've been sweeping a whole lot slower but I've also driven that notch a little bit deeper down and if we actually take our span let's reduce that down to say fifty megahertz all to sweep quite quite as far now I get a little bit faster sweet but still taking about a second to do the full sweep but now I can actually see that noise is being dropped down quite a bit compared to the passband characteristic up here I'm down about 90 dB now I can see that the the bottom of this notch isn't perfectly flat so that tells me I probably don't have each of these cavities perfectly lined up so if we tweak on them a little bit more and I was able to kind of make that a little bit flatter so I'm probably a lot closer there now and see with that notch right there but I can't really do better than that with the noise floor where I have it I've wound up with a notch that is literally about 90 DB down from my pass ban characteristic up here so it's actually pretty good and and that's probably about as deep ears are gonna be able to go with most spectrum analyzers with a tracking generator I just not gonna have enough dynamic range to see anymore now the issue that I've got here is that I've got a relatively wide notch here and the reality is that the notch actually extends down below what I can actually see on this analyzer here so actually it can get more rejection and actually fine-tune each of these cavities even more to line them up to deepen that notch even further which would give me more and more isolation between the transmit receive path but I'm typically limited when I do that with a spectrum analyzer and tracking generator so let's take a look at how much better we can do with the VNA and the spectrum analyzer we left it with a 50 megahertz span and a resolution bandwidth at 10 kilohertz so I said at the same 50 megahertz frequency span on the VNA and at 10 kilohertz if' bandwidth now it might be tough for you to see well zoom in this in a moment but this line up right up here is 0 DB so there's my pass band characteristic like we saw on the top of the spectrum analyzer display and this line right here is my 90 to be 90 DB down we can see we're actually better than 10 DB and close to 20 DB better but now I can actually see I do have a little still a little bit of width here so we actually can make some improvements and tuning each of these cavities I'm winding them all up and drive that notch even deeper let's see if we can tweak these filters a little better and align them up and drive that notch a little bit deeper so we kind of bring these all over let's see we'll leave that one or about where it is so we can drive some of these other ones a little bit closer to it and bring that one down here even more all right I've tweaked used around a little bit and it's probably about as close as I'm gonna get but we take a look now we're kind of hitting the noise floor or getting closer the noise floor of the VNA here and I'm looking at Oh in the neighborhood of about 110 DB down so I've just gained myself another 20 DB in terms of improving that measured notch that we were seeing compared to the spectrum analyzer oh it made one small change here I reduced the if' bandwidth down to one kilohertz the sweep speeds gotten slower but now I can actually see that notch is actually closer to about a 120 DB and I see it's coming down pretty much to a point so which means I've got each of those cavities pretty much lined on top of each other probably not going to do a whole lot better than that but 120 DB of knotch you know of rejection versus you know the only thing we could really see on the spectrum analyzers was about 90 so that's that's a 30 DB difference of a factor of a thousand now in terms of the difference that we see in the rejection of one signal through one path or the other now of course a VNA brings a lot more to the party than just improved dynamic range in this particular case for a notch type duplexer you know the dynamic range improvement does really what we gain but we start getting into the more complex types of types of duplexes such as you know the bandpass type or those that include both band pass and band reject in the same cavity there's a lot more adjustments to make and you may want to look at more than just in this case like an s21 or through measurement you may also want to be looking at you know s11 or return loss for a given path or maybe the both Direction simultaneously because a lot of these other more complex duplexes will include adjustments not only for the best pen pass characteristic but for the notch characteristic and even for the insertion loss which will also adjust the cue of you know say the bandpass side or something like that so being able to look at both reflection and transmission properties at the same time with a VNA can be a lot more in fool and make it easier to tune these more complex duplexer types I hope this video is giving you a bit of insight into what a duplexer is and how you tune it with a spectrum analyzer and tracking generator as well as with a VNA if you like what you see give me a thumbs up on the video you haven't subscribed to my channel already please to do so and thanks again for watching I'll see you next time

Channel: w2aew

Views: 97,890

Keywords: W2AEW, Tek, Tektronix, TTR500, TTR506A, VNA, spectrum analyzer, tracking generator, duplexer, tuning, measurement, radio, RF, antenna, transmitter, receiver, repeater

Id: tf5h_wz9G2o

Length: 10min 58sec (658 seconds)

Published: Fri Nov 03 2017

Reddit Comments

[u/wpar]

Great video, thanks for sharing!

[u/zap_p25]

I hate tuning mobile duplexers...much easier when you can physically separate the cavities and go from there.

Go to the Repeater Builder website and look up the Wacom BPBR tuning directions. Simple and well written. Also, if you have access to either the Quantar Manual or GTR8000 manual, they have decent write-ups as well (using a R2001, so there isn't a tracking generator involved).

Also, good points bringing up the limitations at crowded sites with notchplexers. I've also found they are fairly limited for use with power. Most of the ones I've had (even factory tuned) may be rated for 50W but will generally start desensing around 35W.

Also, if you need a set of BPBR I've got some Motorola T1500's laying around and I'd be happy to loan you one.

[u/[deleted]]

Great video, for all of us that still tune by ear! haha

[u/rem1473]

Ever try tuning with the SA + TG and a VSWR bridge? I'm told that can get a better match. As the match is closer to 50 ohms. My vSWR bridge should get here on Monday so I can give it a try.

As a plus, I think I'll be able to sweep antennas as well, with the vSWR bridge.