I received my sva1015 last week and have been having fun getting to know it. However I'm having a bit of trouble with the analog modulation analysis option and I'm wondering if anyone can provide some help/guidance. I'm not sure if the issue is with my setup, theory, expectations or some combination thereof.
I want to be able to do some FM deviation measurements, so I wanted to start by making sure I could get (what I believe to be) the correct values with a controlled setup.
I've tried several different setups and I'm getting similar results from each of them, so I'll describe just one of the setups. I have an amateur radio VHF/UHF transceiver connected to a Bird 4431 wattmeter which has an RF sample port. The output of the bird is connected to a 50Ohm dummy load and the sample port is connected to the SVA1015. For the modulating signal, I'm using the waveform generator on my SDS2204X scope, connected to the audio line-in on the data port of my transceiver.
The waveform generator is sending a 1.2kHz sine wave onto a carrier at 144.41Mhz, and I used the spectrum analyzer functionality to adjust the waveform generator's amplitude until I hit the 1st order Bessel null. This should be a modulation index of 2.4 and thus a frequency deviation of +/- 2.88kHz. I also double-checked the deviation using the spectrum analyzer via the min/max trace differential and got the expected 2.88kHz deviation. So I'm pretty sure the signal's FM deviation is indeed 2.88kHz.
But when I enable the analog FM modulation mode, I can't seem to get it to give me that result. I've tried every value of IF Bandwidth available, as well as every EqLPF. I've tried both with and without averaging. None of the values (min, max or average) are showing a deviation of 2.88kHz. And the signal on screen is a bit more wonky than I'd expect. It does sort of, mostly, show what looks like the 1.2kHz signal but it's not clean or consistent at all. I've tried moving the SVA1015 away from the transceiver and the dummy load in case there was some noise getting into the machine from them, but that didn't seem to have any effect.
Is there something I'm doing wrong? Any advice/suggestions?
BTW - I want to give some kudos to Siglent's support staff. There were some issues getting my option licenses installed, which was exacerbated by the fact that the analog modulation analysis option was not enabled under trial license. The tech support stayed late on friday waiting for the corrected codes to send to me so that I could do the work I was expecting to do over the weekend.
One other side note for anyone looking at using the vector analysis functionality, their calibration kit is back-ordered and won't be available for roughly another week. So unless you already have an appropriate calibration kit, you won't really be able to use the VA functionality for a little while. Of course you can play around with it, but you can't *really* use it without proper calibration.