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questions about VCXOs and square waves

I'm looking at the 19.44MHz VCXO 3.3V in our system.

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I replaced the tuning voltage with a resistive divider to eliminate a source of error. I used our scope (TDS2024) in FFT mode to sniff around. (Hanning, 64 averages) I expected to see the fundamental and its odd multiples at the output. Instead I see the second harmonic being higher in amplitude than the fundamental, and each spike has sidebands +-1.2MHz. And the craziest part is there is a spike at 1.2MHz. The power supply at the pins of the VCXO seems relatively clean. I expected to see the 19.44MHz but there's actually nothing. It's pretty quiet... So I took a VCXO chip from inventory, and connected it to a lab power supply by itself. The output has the same spectrum. So it's not some modulation caused by our board.

1) What is the output spectrum supposed to look like with a 19.44MHz square wave that can vary from 40-60% duty cycle? The 50-50 case is all odd harmonics, right? The second harmonic should be lower than the fundamental, right???
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This is what I used.

2) What is the significance of the 1.2MHz spike? That's the last thing I expected to see.

3) Is there something about VCXOs I'm overlooking? Yes it drifts by a few Hz every second, I'd hardly expect that to show up on a spectrum display where each division is 5MHz.

4) Probing technique. I usually use home brew Zo probes. A foot or two of RG-174, a feedthrough 50 ohm terminator at the scope end, and a 1K at the signal end so I don't load the signal too much. All soldered in. Is this OK? (I've been using this for years, but hey, I might be wrong)

I feel like ditching this ASVV part. The output signal is jittery enough that you can see the signal wobble back and forth by a pixel on the scope. I'm no expert here, but that seems like a lot of jitter. The output of my OCXO is rock solid on the screen.

Maybe another approach? Maybe a Pericom PI6CX100-27 and a 19.44 crystal? I know the Pericom part works above 27MHz, I see no reason why it shouldn't work at 19.44.

TIA

Reply to
a7yvm109gf5d1
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Is the spike very sharp or several bins wide?

It is not uncommon for crystals to have a wide peak in the noise 10 to

20% off the carrier. There is another mode in the crystal that can boost the noise.

Is there a humping great radio transmitter near your lab at 1.2MHz? What happens if you tune your AM radio to that spot on the dial? Does the 1.2MHz stay there if you switch off the power?

A clean square wave with 50:50 duty has no 2nd harmonic. Imagine flipping the signal over and sliding it by 1/2 a cycle. Would it look exactly the same or does the positive part have a ramp like thing on it that the negative doesn't.

Reply to
MooseFET

Sharp.

Oh hoh. Is the crystal slab being overdriven? Maybe I'll turn the Vcc down and watch.

Nah, it's from the output of the VCXO. The 1.2MHz spike isn't there on my other measurements.

I'll look at it tomorrow.

Reply to
a7yvm109gf5d1

Oscilloscopes are very bad spectrum analyzers.

What does the time-domain waveform look like?

John

Reply to
John Larkin

Yup. It's all I have right now.

I don't recall right now.... It didn't jump out at me when I checked last time. It merits looking at again. But I remember it jiggles noticeably. There's something deeply wrong somewhere. The 10MHz OCXO in my counter displays as a solid sine wave, it's not the scope. I'm preparing an email for the Abracon tech dude, who's actually pretty responsive.

Reply to
a7yvm109gf5d1

OK, any XO that jiggles on a scope time-domain waveform is seriously hurt... unless it's a triggering problem.

John

Reply to
John Larkin

1) what is the impedance of the divider on the control line, do you have a bypass cap there? 2) Control voltage in range? 3) Vcc bypassed? 4) Vcc in the specified range? 5) Output load in the specified range..

else the part is bad..

Mark

Reply to
Mark

There is something fishy here. Though the sidebands are likely real. What does the scope trace look like with a Mk I eyeball?

You could get a fair amount of second harmonic present if the xtal is hard overdriven in one direction creating a sawtooth type waveform with the higher harmonics strongly excited.

And 60% second harmonic content would look roughly like this

/ \\__ / \\__ / \\__ \\ / \\ /

Is it possible your scope leads or probe are enhancing or provoking the additional response to the second harmonic?

If there really is more second harmonic than main signal then the waveform will look like the second harmonic sat on a fluctuating baseline.

Regards, Martin Brown

Reply to
Martin Brown

With 1K loading this should be no problem but perhaps the problem lies in DC coupling the XO to your probe? AC coupling might be required.

Pere

Reply to
oopere

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