hp 8590a "fm identification" tip/hunch? - Page 1
Forum section: electronics / rf-microwave

author: CopperCone
author_rank: Super Contributor
timestamp: July 13, 2017, 06:10:42 am
content: So I am unsure of spectrum analyzer specifications, but I am looking to find out how to better use this piece of equipment to identify signals.I have a active probe connected to nothing, thats acting like a high impedance antenna (good to 3GHz).I narrowed in on a frequency. I have the settings at VBW at 300Hz, the RBW @ 300Hz,Per scan, is there some reason a reading would jump around in peak frequency? I have a span of 15KHz. It appears to trigger +-400Hz.. in slide show mode. (1 second sweep) and like 200Hz in maximum sweep (20msec). So it kind of looks like frequency modulation.Now, I have stuff capable of demodulating FM (but I have not gotten to play with it yet).In the absense of an SDR with fast update rate or a demodulator (the hp8590a is the only "portable" instrument I have), i.e. not a mainframeIs there a way to kind of use it to determine if there might be a good chance of the signal being FM modulated? Or is what I am looking at likely something akin to 'trigger jitter' on a scope? On a slow sweep again, between sweeps the peak looks like it might move easily 200Hz.So if I decided to climb a tree with this thing, and a battery pack, could I write down in my notebook that this is possibly a FM signal? How much 'jitter' is allowed by the instrument before I would be wise to have a 'hunch' about a signal?Its CW btw.I intend to analyze it with a FM demodulator / SDR .. but I would like to be able to make a guess at things with this other instrument,especially since its rather expendable and I feel comfortable plugging it into the outlet (via a CR network fitted with gas discharge tubes), possibly in remote locations to monitor power noise/rfi etc (4x 11nF silver mica 10k resistor high pass filter with some gas discharge tubes to hopefully limit damage to the front end without actually blowing the thing up with violence (i also did not want any kind of semiconductor devices in the outlet adapter, hence the lack of protection diodes, MOV, etc).I suppose the best method of learning would be to setup some transmitters and study them. The hp8590a does have a AM pp measurement (which seems to vary highly based on the instrument settings) and 3rd order intercept analysis, along with FFT.. when I do the FFT of my signal (with the FFT button), it looks like it has a main body at -80dbm (0hz on the FFT) and two peaks 57and 64hz out, at -104dbm and -103dbm.Is there a way to tell from the FFT that the signal is probably FM (based on some kind of characteristics of a FM transmitter that translate into a characteristic 'signature' that a signal may have? (not necessarily dependant on taking multiple measurements, kind of like getting an idea if someone is speaking in anger by looking at a picture taken of them (without a recording or listening to a video, based on how their facial expression looks like).

author: AF6LJ
author_rank: Unknown
timestamp: August 03, 2017, 12:27:26 am
content: What frequency are you looking at??

author: CopperCone
author_rank: Super Contributor
timestamp: August 14, 2017, 07:54:11 am
content: I think it was like 180 or 230 MHz or something. I first found it when I plugged in my line-to-spectrum analyzer high pass filter/GDT protection network. I think it was like 1 or 2MHz off from what 'medical device' is on the frequency spectrum allocation chart for USA iirc.I was also able to detect with with a 3GHz high impedance probe from Agilent with the sheeth down in the airalso saw a 73 or 78 MHz signal on my outlet that was slowly drifting to a higher frequency. Seemed fairly stable in frequency (not FM) and amplitude (or AM), kind of weird. need to hunt that down. very slow FM perhaps.