Sweep test of HP 423A Detector - Page 1
Forum section: electronics / testgear

author: George_Race
author_rank: Regular Contributor
timestamp: September 08, 2015, 07:08:26 pm
content: I can use my Wavetek 2002A to sweep test an HP Detector using a two channel TEK scope to see the actual detected response of the Detector.If I do the same test with my HP 8595E Spectrum Analyzer with TG, by simply making the Detector the Device Under Test, the results do not look the same.I am using a good HP 423A detector for the test.Are these both valid tests to see the response of the Detector, or am I missing something here?

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 08, 2015, 07:47:04 pm
content: Quote from: George_Race on September 08, 2015, 07:08:26 pmThe detector is supposed to convert the RF into a DC voltage, so what exactly are you expecting to see on the SA?I guess all you are going to see is RF leakage, and that is something completely different from what you see on the scope.

author: George_Race
author_rank: Regular Contributor
timestamp: September 08, 2015, 08:36:38 pm
content: That was my thinking as well, the output is a DC voltage, in this case negative going, so there would not be any output that would show on the SA.So if I were to use the ramp output on the rear of the SA as the horizontal sweep signal for the scope, and let the SA and TG output feed the Detector input, then feed the detector output into the vertical input of the scope, I should see the same signal that I would see from the Sweep Generator?Guess I will give that a try!

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 08, 2015, 08:55:22 pm
content: Yes, that should be more or less the same, although the output of the SA's TG will be different from the Wavetek so the amplitude will not be the same.

author: George_Race
author_rank: Regular Contributor
timestamp: September 09, 2015, 12:19:44 am
content: After spending some time with the SA this afternoon, I am pleased to say that the SA out performs the Sweep Generator in several of ways. I have much better control of the sweep frequency start and finish, and can set any level of signal that I want from the TG output port. \There are two things that the Sweep Generator can do that I have not figured out how to do on the SA & TG. One is to add a marker, but I may be able to do that with my comb generator.The other is that I need to find a way to blank the return trace on the scope, if that is possible. It is of a different intensity then the detected trace being shown, so really does not bother, but the fact that it is there is a challenge to try and figure out a way to blank it during retrace time. If I can figure out a way to trigger the trace on the sweep level, I may be able to do it that way. Will do some more experimenting soon and report back.If I can do those two things, the SA will be a much better source then the Sweep Generator for doing receiver IF alignment, and looking at the bandpass frequencies of various kinds of devices when connected within operating systems, like various kinds of receivers.And someone told me that it could not be done using the SA & TG!

author: MarkL
author_rank: Unknown
timestamp: September 10, 2015, 09:24:27 pm
content: For an ad-hoc marker, here's something to try:Turn on the "Marker Pause" function on the SA and set it to something minimal, like 10ms, and position the marker to the frequency of interest. The sweep will dwell at the frequency of the marker, which in turn causes the scope to show a brighter trace at that position. (This is assuming you're using XY mode, and on an analog scope or DSO with intensity grading.)Because the SA/TG uses an analog sweep generator, the dwell frequency isn't exact (but it is very close). If you need to know the precise frequency, you can temporarily increase the pause time to a few seconds, and read the actual frequency on a frequency counter during the pause.And for the retrace issue:You could use the SA "High Sweep In/Out" and feed that to the Z-axis input on the scope. The output is high when sweeping.On my 2465 scope (and most scopes) the Z-axis input is a blank-when-high signal, so it's the opposite of what's needed. A simple inverter IC can flip the signal for you. Or some pieces of test equipment can do it, such as an HP 8116A function gen in E.Wid mode.But unfortunately, the above marker and retrace suggestions aren't compatible with eachother. When the SA marker causes a pause, it also drops the "High Sweep" output, which in turn would cause that part of the scope trace to blank. However, in actual practice there seems to be enough slop in coordinating the blanking and the sweep that it does work to an extent on the equipment I tried (2465 scope and 8495E SA).If the retrace bothers you enough, a slightly more sophisticated circuit could be built that controls the scope Z axis based on the rising edge of "High Sweep In/Out" (unblank), and the falling edge of "Sweep Output" (blank).Attached is a screen shot of the SA "Sweep Output" (yellow), and "High Sweep In/Out" (blue). Sweep time is 100ms and marker pause time is 10ms. You can see the actual dwell time is not very accurate (it's about 22ms), and there's a little bit of glitchiness in the "High Sweep" output. Something to ponder if you're considering building a blanking a circuit.

author: George_Race
author_rank: Regular Contributor
timestamp: September 12, 2015, 01:32:17 pm
content: Good Morning Mark:Thanks so very much for the detailed information. All of that will certainly get me on the right path to making my SA act like a very functional SG.I am using the HP 8595E and the TEK 2465 so my results can be very similar, given your instructions.I am working on a design for a Boonton 92 type detector probe that can be built and sold for a reasonable price. There are a lot of 92's out there, at a good price, but usually no probe so it makes them quite useless. Being able to sweep the probe project and determine the best placement of the chip components, is what I am working on. I am using the HP 423A as a test standard, so have a nice "Flat Line" source to work with.I do have a nice Wavetek 2002A sweep generator that does the trick, but always wanting to do things differently seems to be my interest!Thanks again for the very detailed information, greatly appreciated.