[time-nuts] Odd-order multiplication of CMOS-output OCXO

[Mark Haun]

Hi time nuts,

I'm looking for a 5x frequency multiplication scheme to let me use a
16-MHz square-wave OCXO for an ADC encode clock at 80 MHz.

Constraints in order of importance:

1. Don't degrade the nice phase noise of the OCXO (-90 @ 1, -120 @10,
-140 @ 100, -160 @ 1k) any more than necessary; at the very least, it
should not impact the ADC noise floor in the primary 0-40 MHz image. 
(This should give quite a bit of leeway, but better is better :)

2. OCXO power consumption (~150 mW) should still dominate total
clock-system power.  Would like to keep the multiplier/buffer under 50 mW.

3. No supply rail above 3.3V.

This "ought to be" (?) easy, because the OCXO output is already rich in
odd harmonics.  All that's needed is to isolate and perhaps buffer the
right one without screwing up my noise spec.  This is where I could use
some help...

The ADC (AD9266) wants a differential clock, sinusoidal or square
doesn't matter.  The datasheet recommends transformer coupling with
antiparallel diodes to keep the swing ~ 1.5Vp-p.  (The min/max spec says
anything between 0.2 and 3.6V.)   The 3.3V OCXO should give me 0.8Vp-p
at the 5th harmonic without any amplification, so in theory I guess I
could just filter and transformer couple and be on my way.  But perhaps
some amplification is in order to increase the slew rate?

I looked at the Wenzel tech notes for ideas, e.g. this one using logic
gates and tuned circuits:
http://www.techlib.com/files/hcmos.pdf
but I lack the background to evaluate the pros and cons of introducing
extra CMOS logic.

I also found this common-base amp circuit in the archives:
https://www.febo.com/pipermail/time-nuts/2016-January/095683.html  and
https://www.febo.com/pipermail/time-nuts/attachments/20160126/ae3b4be8/attachment-0001.pdf

I've read that I should avoid high-Q tuned circuits, because they will
introduce more noise with temperature variation.  Are there any rules of
thumb for how much Q is too much?

Any pointers would be most appreciated!

Thanks,
Mark