Question about Hewlett Packard 3580A Spectrum Analyzer - Page 1
Forum section: electronics / repair

author: electronix27
author_rank: Contributor
timestamp: April 07, 2025, 12:43:06 am
content: Hi allAfter a bit of warm up, calibrated and set frequency to 0 Hz start. Wanted to look at the secondary 6V, of a 120v primary, 60 Hz standard linear transformer. After connection, sweep showed 60 Hz fundamental plus several odd harmonics. Didn't seem right so I checked terminations, checked for ground loops, no other equipment energized. Metered out plug end ground to rear panel ground of machine, and to the input ground, all < .2 ohm. With no input termination and input sensitivity set to -40, sweep showed same 60 Hz fundamental and odd harmonic spurs, with the same relative amplitude to one another as test with transformer. Appears to be generated from the Analyzer. Does anyone have one of these, is this standard behavior. Possibly generated from IF filter problem, Power supply issue? Appreciate your thoughts.ThanksRussell

author: George Edmonds
author_rank: Frequent Contributor
timestamp: April 07, 2025, 02:31:43 am
content: HiI have seen strange problems with these analyzers due to poor contact by the holder with the internal fusesG Edmonds

author: electronix27
author_rank: Contributor
timestamp: April 07, 2025, 12:39:11 pm
content: Are you refering to the A6, low voltage power supply board, soldered in fuses on the +11 and -11 supply rail.Thanks

author: Perrold
author_rank: Contributor
timestamp: April 08, 2025, 06:47:36 pm
content: Try feeding it with a clean good signal generator or function generator. Harmonic distortion with 60Hz out of the wall is plainbad. Outright visible with a scope so a spectrum analyzer showing odd products some -20dB below the fundamental is not unreasonable.

author: electronix27
author_rank: Contributor
timestamp: April 09, 2025, 06:19:01 am
content: Fair enough. Had already done this sort of test, along with many other waveforms and frequencies from a signal generator when I was able to make the repairs to get this analyzer working again. Connected to a signal generator @ 60 Hz sine, these spurs were less dominant. That being said, real world measurements don’t come from a somewhat perfect source like a signal generator, and it doesn’t explain the harmonics with no termination. I asked the original question to find if the power supply or something else within the analyzer was actually generating and amplifying these line voltage harmonics. I've read a number of posts where the line voltage frequency and following odd harmonics were present in spectrum analyzers while making test measurements. Some tested this by moving the power supply away from the analyzer and those spurs vanished. The A6, low voltage power board, while not SMPS, uses transistor switching to regulate the + - 10 VDC rail and + - 11 VDC rail from an 18.5 VAC linear transformer. The A6 board is right next to the A5, 5 stage IF filter board, maybe no correlation. This Analyzer is a dinosaur, completely manual, no markers or start stop frequency, but it's just a great piece of vintage CRT gear and I like it. I haven't had much experience using spectrum analyzers, but like any other piece of test gear, knowing how it works and it's limitations, the work arounds and correct setup, are most important in getting a true measurement. So if anyone has one of these and set up as I did, do you have the same result? The input is a 1 MEG impedance, and maybe the input, unterminated, is still sensitive enough to act as an antenna.