HP 8664A Performance

[Ground_Loop]

I was goofing with my HP 8664A sig gen today and noticed a number of harmonics I don't recall being there. Please see attached.  Is it normal for the first harmonic to be only roughly 43 dB down from the fundamental?  Frequency output is 100MHz @ 0dBm. Center frequency is shifted a bit on the SA to get the trace off the grid line.

[bdunham7]

I don't think it would be normal.  I had one of those here for service a while back and I took some pictures (at 100MHz no less) but it was all close-in spurs and modulation stuff, no wide-span analysis.  I think the outputs were fairly clean and I'd be surprised to see harmonics that high.  I just peeked at the manual and I can't find specs right now.  What happens if you reduce the amplitude?

[TimFox]

I found a specification sheet.  Depending on the exact model and level, harmonic content is specified as -25 dBc or -30 dBc.  Other spurs are far lower.

[bdunham7]

OK, some stuff just trickled back into my brain--and I found some photos that I took of it in action (it went on eBay).  The manuals are not well organized for figuring this stuff out.  Maybe it's not broken after all.

There are controls for 'synthesis mode' on the lower right.  Only two of them work unless you have the option for enhanced spectral purity (don't remember the number).  I don't know how much, if any, effect this will have on harmonics.

Do you have Option 002 (Frequency doubler)?  I think the harmonics are worse with that option.

I think the harmonics will be higher at higher drive levels.  Drop down 5dB at a time and see what the harmonics do.

I still haven't found the spec sheet that TimFox mentioned, but -25dBc at 100MHz seems high, but maybe it doesn't matter for its intended uses.

EDIT:  Here it is, in black and white.  Guaranteed specs for harmonics are only  <30dBc @ <10dBm.

https://www.testequipmentdepot.com/usedequipment/pdf/8644b-8664a-8665b.pdf

[Ground_Loop]

I located the spec sheet as well.  Seems to be in spec.  Nonetheless, after appropriate warm up, I performed an internal recalibration and instrument test.  Returned result code zero for both.  The center frequency and amplitude tightened up a bit, but harmonics were unchanged.  This is one of the nicer instruments I own and would hate to think it's going South on me.  Thanks for the replies.

[TimFox]

Harmonics:  -40 dBc is 1% harmonic distortion, in other terms.  This is not atypical for signal generators, since the output stage is not tuned.