Re: QMX, one of op-amps self-oscillates at 25 MHz (or picks up the system clock?)
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Hi Adam
I watched your new video, thanks.?
I don't think it is possible for the IC202 output to produce that kind of signal, which looks so perfectly like 25MHz at one timebase then 833 Hz at the other. Note that the frequency measurement in the bottom right of your screen indicates 25MHz also, which would probably not be the case if there was some kind of interruption going on.?
I think Stan has it correctly identified as a red herring, an artefact of the measurement procedure (the 'scope). And also 833Hz doesn't match the symptomatic 100Hz noise either.?
I think I am inclined to suspect a partially damaged MS5351M chip. Not the 25MHz TCXO.
Hans Summers via <hans.summers=[email protected]> wrote:
> Downstream of IC202 there isn't any "almost", this is the very reason why
> IC202 is there. You will have a 3.3Vpp output at IC202 output.
I do. The scope shows some overshoots, but probably the probe needs to be
calibrated.
> Can you take a similar video please for the signal at IC202 output?
Yes -- posted below.
> I'm still having trouble trying to convince myself that the 25 MHz TCXO is
> at fault here rather than - sorry to say it - some weird artefact
> introduced by the measurement itself (at the TCXO output / gate input).
Don't be sorry -- in the realm of high frequencies I'm not sure of
anything :) HF never fails to find ways to surprise me.
BTW, I find this frequency weird as well. It's very low, compared to the
nominal frequency of the TCXO. It's probably way too low to result from
some stray capacitances or inductances.
It might be some weird artefact. But the oscilloscope never had such
artefacts before (I have it for 14-15 years), and symptoms (intermittent
CLK0 and CLK1 at 100 Hz, precisely the frequency of the buzzing both
audible in the speaker and visible in WSJT-X's spectrum view) fit.
Here's a new video with:
1. Output of the IC202 gate at various time bases
2. Signal from the TCXO with C203 (capacitor coupling TCXO with the gate)
removed -- so it's a signal on unloaded TCXO
3. VDD after L201 (so the power line of the clocking subsystem: TCXO,
gate, synth chip), showing some very minor ripple of the same frequency (I
suspect it's the effect of the problem, not the cause of it, but at this
point I'm not sure of anything)
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