noob spectrum analyzer question - Page 1
Forum section: electronics / testgear

author: Dan Moos
author_rank: Frequent Contributor
timestamp: August 27, 2020, 02:11:08 am
content: Been playing with the FFT on my newly acquired HP 8590d. Using the manual, I learned how to use to see a specific am modulation frequency of a carrier.I modulated a 10 MHz carrier with a 60 Hz signal. It showed up as expected.My question is, how does my machine that has a lower bandwidth limit of 9 Khz get this information? How does it get through the front end? I tried just sending a plain 60 Hz signal in, and viewing it using the FFT, and as I expected, it doesn't show up.The specifics of how an FFT is computed is likey out of my depth, so this puzzles me greatly.Does this mean that if I wanted to explore some lower frequency signal, I just need to modulate a carrier with it and do the FFT?Also, this FFT looks 1000 times better than the janky business my 1054z shows.

author: 0culus
author_rank: Super Contributor
timestamp: August 27, 2020, 02:28:49 am
content: The 9 kHz information is modulated on the RF, it's not at baseband. Take a closer look at the signal at a narrower span and note the spacing of the AM peaks. You can also use zero span mode (the SA is essentially a receiver, tuned by the center freq control, when set to zero span) to demod AM. FM, too, with slope detection.

author: Circlotron
author_rank: Super Contributor
timestamp: August 27, 2020, 02:31:12 am
content: Quote from: Dan Moos on August 27, 2020, 02:11:08 amI expect the FFT will always be of the carrier, not the modulating signal.Disclaimer - I don't have a SA to play with.

author: JohnPi
author_rank: Regular Contributor
timestamp: August 27, 2020, 02:35:31 am
content: If you AM (amplitude modulated) the 10 MHz with 60 Hz, you'll end up with signals at 10M-60, 10M, and 10M+60 Hz. This arises from the AM process (multiplication) which (trigonometry) generates frequencies of the sum and difference of the inputs.Yes, to look at a lower frequency, 'just' modulate it onto a carrier. That's how you hear radio stations -- it's modulated onto a carrier (which propagates well as a higher frequency radio wave), and the receiver demodulates it...

author: 0culus
author_rank: Super Contributor
timestamp: August 27, 2020, 02:45:35 am
content: Case in point, I threw a 500 MHz carrier on the spectrum analyzer. It's modulated with a 1 kHz sinusoid.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: August 27, 2020, 02:47:54 am
content: Ok, that actually makes sense. Basically, by looking at the new frequencies created by multiplying the carrier and the modulating signal, I get get high frequency products that mathematically infer what the modulating freq actually is.So when I modulated 20 MHz with 60 Hz, I created signals at 20.00006 Mhz, and 19.99994 Mhz. That's enough info for the FFT to know what to do.I get it right?

author: bdunham7
author_rank: Super Contributor
timestamp: August 27, 2020, 03:14:09 am
content: Two things. The spec that matters is Resolution Bandwidth, not the lower limit bandwidth of the input (9kHz). RBW determines how far apart two frequencies have to be for the SA to distinguish them. Also, unless you've engaged an optional feature, the display is a swept spectrum analysis that 'scans' each frequency using an oscillator and a filter, not an FFT.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: August 27, 2020, 03:34:22 am
content: Quote from: bdunham7 on August 27, 2020, 03:14:09 amUnless I'm using the FFT function of my SA (as mentioned earlier)

author: srb1954
author_rank: Super Contributor
timestamp: August 27, 2020, 08:18:35 am
content: Pretty sure the 8590 series is only a swept spectrum analyser.When I was using an 8590 back in the 90's I never found any FFT functions on the menus and there is no mention of any FFT function in the catalogs up until the time it was discontinued.

author: rf-loop
author_rank: Super Contributor
timestamp: August 27, 2020, 10:06:26 am
content: Quote from: srb1954 on August 27, 2020, 08:18:35 amWrong, if you add one letter after 8590Sorry it was nessessary...'You have never seen this...Hewlett-Packard (destroyed by "Lady" Agilent):"This Product Note explainshow to use the FFT function in 8590C/E/L-series Spectrum Analyzers with Fast Fourier Transform (FFT). FFT simplifies AM analysis by providing a smart user interface....."I have previously owned and used several of these, also repaired and adjusted these... time ago... so long time ago that I do not remember just details about using FFT but this I know sure it is there like water is in sea.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: August 27, 2020, 01:29:55 pm
content: This is silly. Under user measure, I can turn FFT on and off. It has it.

author: rf-loop
author_rank: Super Contributor
timestamp: August 27, 2020, 03:07:57 pm
content: Quote from: Dan Moos on August 27, 2020, 01:29:55 pm'Yes because it is normal feature as told just in previous message you did not read.More here.Manual page 4-2https://doc.xdevs.com/doc/HP_Agilent_Keysight/HP%208590D,E%20Series%20User.pdf

author: Dan Moos
author_rank: Frequent Contributor
timestamp: August 27, 2020, 06:58:48 pm
content: RF-Loop, I read everything. Perhaps I was responding to the same person you corrected