Distorted Sine Wave

Lots of 2nd harmonic. Got a schematic of the clock generator?

If so, where are you probing?

Looks like reflections in the cable. Try the 50 Ohm termination.

Arie

If the drive is a sine wave, a cable can't generate that 2nd harmonic.

Our boxes output a 10 MHz square wave. Our clock inputs have a 10 MHz bandpass filter, so they accept most anything.

I don't understand how a reflection can account for it either. THe cable's only 4' long! However, with the 50 ohm input enabled, the 2nd harmonic disappears. It's just one of those inexplicable mysteries that no one knows the answer to. :)

What is the nature of the load?

Joe Gwinn

If this oscillator is made to drive 50 Ohms and you don't provide that, internal buffer stages may saturate or do other weird things. If it works OK *with* the 50 Ohm load, then your problem is solved, no?

Jeroen Belleman

Unfortunately not. the suspicion that there was something wrong with that oscillator was my main hope of an easy fix for this analyzer. Now I have to go back to the drawing board and start trouble-shooting all over again. And it's a complex beast!

That’s pretty diagnostic. There must be an LC filter on the output—mis-terminating it will cause all sorts of frequency-response whoopdedoos.

Cheers

Phil Hobbs

It also occurs to me that if there is a diode in series with a resistor somewhere, the impedance presented to the feed coax may be 50 ohms for positive input voltage, and say 10 Kohm for negative. At the very least one could get an inverted reflection on negative.

Joe Gwinn

Weird but I'm not surprised that 4 feet if coax, unloaded at 10 MHz gives a strange waveform. Can simulate this, I believe, in LTspice using the transmission line element(s).

Learned something here though.

boB AZ

No txline can create frequency components that are not in the source.

(Well, a NLTL can, but 4 feet of coax isn't a shock line.)

But the problem, as usual, is underspecified. Maybe some driver is going nonlinear. A schematic would help.

I was thinking the same thing. Non-linearity.

But he said that when he loaded it with 50 Ohms, it went back to a sinewave. Maybe that non-linearity went away when it was loaded ? Could be some kind of termination non-linearity or even coming from the source with the higher levels of reflections. Can't explain it otherwise.

boB

maybe the oscillator is source only so without a 50R load there's no sink to do the low side of the sine

Yeah, or an emitter follower. Good point.

Cheers

Phil Hobbs

Plus some LC filter wiggles, to distort and smooth things.

I dug around and found a copy of the HP 8566B spectrum analyzer service manual. The 10 MHz ref input is an amplifier driving a mixer, with not hint of for instance a TTL input. So, the problem must be elsewhere. Or, it's just busted.

.<chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/

Joe Gwinn

The pagination was unclear, so I didn't see a way to say. How I found it was to look at the schematics. Look for module A22. Don't think search works on such images, but visual search didn't take that long.

Joe Gwinn

Too much work for free consulting.

I took another look. There is a page reference on the right edge, near the bottom, which is not marked as a page X of Y, but is.

Anyway, look at pages 59 and 77. At 77, look for A22 in the lower left region. Just above, look for INT and EXT jacks. The EXT jack is where an external reference enters.

Joe Gwinn