1-2 GHz Tracking Generator or VNA

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Sam Wetterlin
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Joined: Sat Apr 28, 2018 6:48 pm

1-2 GHz Tracking Generator or VNA

Post by Sam Wetterlin » Mon Jul 06, 2009 9:05 pm

There is a simple method to extend the range of the VNA (or tracking generator) beyond 2 GHz. It involves connecting the output of Mixer 1 directly to the input of the IF2 amp, bypassing Mixer 2 and the cavity filter. For people working on getting the basic unit operational, this may seem like something to deal with in the future. But the point I want to make is that when you decide where to put connectors and where to direct-solder the coax, it is a good idea to arrange the cabling so Mixer 1 output, which normally goes to the cavity filter, can instead connect to the IF2 amp input (either the first or second half of that amp), which normally is connected to the Mixer 2 output.

For anyone with a completed VNA (or SA with tracking generator), to try this all you need to do is connect the cable as described above, and operate as normal. Run the TG output to some device with an interesting response in the 1-2 GHz area--maybe just run it through a Tee with a tiny coax stub attached to the third arm of the tee. Do a scan from 0 to 1000 MHz. The MSA thinks it is scanning that range, but what is now happening is that the substantial LO3 contained in the TG output becomes the operative TG signal. When the MSA thinks it is tuned to 0, LO3 is 1024 MHz, so you are really tuned to 1024 MHz. So you are scanning 1024-2024 MHz, even though the graph says 0-1000 MHz. Any LO3 that gets through the DUT gets mixed with LO1 in Mixer 1; since these signals are separated by 10.7 MHz (assuming that is your IF2), Mixer 1 produces a 10.7 MHz output proportional to the amount of LO3 that got through the DUT. And the new cabling scheme runs that signal directly to the IF2 amp for processing.

I did a scan of a 1.13 MHz cavity filter, with very nice results. The dynamic range is not as good as the normal VNA, due to a low level interference signal, probably created by LO3 leakage into LO1. But on mine that signal is generally less than -80 dBm. So there is some very usable range there.

This illustrates the process, though some refinements could be made, such as filtering out the normal TG signal, leaving mostly the LO2 and LO3 components, and then amplifying the remaining components.

As I said at the beginning, my main point is to recommend that you arrange your cabling plan so that you will be able to do this in the future if you desire. The key is to allow connecting Mixer 1 output to IF2 amp input. It is possible that the IF2 amp has too much amplification for this purpose, so you may want to be able to skip the first half of the amp and connect directly to the second half input. So place connectors as needed.

Sam W.

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