[time-nuts] About HP10544A

Esa Heikkinen scifiscifi at sci.fi
Tue Mar 10 16:48:09 UTC 2009

Hi Bruce...

> Try increasing C2 and C8 in the white emitter follower circuit schematic
> to 100nF.

Doesn't seems to change anything:

When doing this measurement I noticed that the 3rd and higher harmonics 
level are changing! First I thought that the capacitor change was some 
effect on harmonics but then those peaks come back...

Time domain analysis about 3rd harmonic level gave some explanation:

The EF circuit was turned on at start of sweep. Something is heating and 
raising the harmonic level. The bump and the end of sweep is a test 
where I momentary switch off the 12V feed to the EF circuit to see how 
it reacts.

Temperature sensitivity was also verified with cold spray:

Well, that's not an issue but makes measurements harder because even 
without any changes the results can differ. However the temperature 
effect on the 2nd harmonic frequency was very small.

 > The switcher sidebands will still be there, they are just buried in
 > the spectrum analyser noise floor.

Yes you're right. Just changed some settings and there they are again:

About 90 dB below carrier.. I would say that it's good enough! The LPRO 
for example, gives higher spuriouses, but far away from fundamental.

 > Does the board use the recommended LC filters and regulator for the
 > oscillator supply as depicted in Figure 3 on the 10544A data sheet?

I haven't analyzed it fully but it seems to be just the datasheet 
circuit having LM723, 10 uH coil etc. But it's layout is totally wrong 
because oven switcher current runs via wrong trace. It would also been 
possible to have other side as grouding copper but this was not done. 
The PCB is manufactured by Cubic western data.

 > The required parts shouldn't be too expensive, however you may need
 > to wind your own inductors for the series tuned LC circuits.
 > Air core or powdered iron core inductors should be OK as long as you
 > use shields between filter sections etc.

Sounds like hard. How it's possible that lower grade ocxo's (like in 
thunderbolt) output so much better spectrum? Is it all about ocxo output 
driver circuit? Would it be easier to modify the 10544A itself than 
trying to clean the distortion? Has anyone tried that kind of modification?


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