Using a FG and a RTSA for filter characterization - Page 1
Forum section: electronics / testgear

author: ValhalQoholic
author_rank: Newbie
timestamp: February 24, 2021, 05:19:54 pm
content: I have a two in one project where I'm trying to characterize a Bessel filter that has a cutoff frequency of 120kHz and while doing so learn how to use a Siglent SSA3075X-R real time spectrum analyzer. The problem is that the tracking generator of the RTSA only goes down to 100kHz. What I do have is an Agilent 33500b 30MHz waveform generator. My idea is to use both of these pieces of equipment to characterize my Bessel filter from 0-200kHz. To start out I setup the RTSA to operate like a traditional swept-tuned spectrum analyzer because I'm familiar with how those operate. I like starting out replicating something I know should work before moving on to more advanced features of a piece of equipment. A "print hello world" if you will. I did this by settting the "mode" of the RTSA to "spectrum analyzer" (the options for mode are spectrum analyzer mode, VNA mode, RTSA mode) . Then I set the "sweep mode" to "sweep" (the options are Auto, Sweep, and FFT). I set the trace type to "clear write". This erases the data after each sweep. The signal gen and the spectrum analyzer was set to sweep at 1ms intervals. I set the signal generator to use an external clock input and hooked that input the the 10MHz ref out of the spectrum analyzer, so now they are on the same clock. Then I set the signal generator to external triggering and hooked the external trigger connection up the the sync connection of the function generator. The goal of this was to have them both on the same clock, start their sweeps at the same time, and sweep for the same duration. I was expecting the spectrum analyzer to plot the dB magnitude of the function generator as a function of frequency. And I was expecting the frequency response of the signal generator's sweep to be a fairly flat curve. However, what I'm getting doesn't look like that at all. The plot of the SA is mostly close to the noise floor with random blips of signal. There doesn't seem to be any pattern for when signal is displayed or not. There is a message at the top right that says "waiting for trigger" that appears and disappears. It seems like it is correlated with signal appearing and disappearing. I think what I have is a triggering issue but I'm not really sure how to fix it. Is there something obvious that I'm missing?

author: egonotto
author_rank: Super Contributor
timestamp: February 24, 2021, 05:52:42 pm
content: Hello,try:Set trace type to Max Hold and set the sweep speed at the Agilent 33500b to 1sBest regardsegonotto

author: tautech
author_rank: Super Contributor
timestamp: February 24, 2021, 07:28:49 pm
content: Quote from: egonotto on February 24, 2021, 05:52:42 pmMaybe that is not required so much as just setting both instruments to the same sweep time.For the SSA X-R sweep times can be set in the Sweep menu.

author: TurboTom
author_rank: Super Contributor
timestamp: February 24, 2021, 08:02:28 pm
content: You didn't tell what particular model of the 33500b you've got. If it's got a built-in arbitrary noise waveform, you may want to use this and have your filter (DUT) and the SA do the work...

author: ValhalQoholic
author_rank: Newbie
timestamp: February 24, 2021, 08:27:48 pm
content: When both the SSA and SG are set to the same sweep time, you can occasionly see the signal sweep across the screen. It's hard to capture it with a screen shot though. This is what the signal looks like most of the time.This is what the signal looks like when I have the SG set to 10ms and the SSA 100ms. I'm not really sure what's going on here. If I set the sweep time to 1s like egonotto suggested, it just adds more peaksThis is what the signal looks like when I have the SG and SSA set to the same sweep time (10ms) with MAX HOLD on. I can see how this would be useful, because it does seem to show the frequency response of the of SG's sweep. But this is highest measured value of each frequency after just a couple sweeps. It no longer updates after all the highest values have been reached. I'm not really sure if this is a true representation of the output of the SG sweep

author: colorado.rob
author_rank: Frequent Contributor
timestamp: February 25, 2021, 06:35:37 pm
content: Dave has a video on using a FG and oscilloscope to do Bode plots. Seems much more appropriate for this freq range than an RTSA. The magnitude scale is linear instead of the more useful dBV, but I think it would work better than this.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2021, 06:57:44 pm
content: Yes for the absolute bottom end frequency although the SA in question can resolve down to 9KHz but the TG doesn't go that low.Interesting problem that if I wasn't so busy I investigate more further than my current knowledge.