[time-nuts] About HP10544A

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Mar 10 21:05:00 UTC 2009

Esa Heikkinen wrote:
> Hi Bruce...
>> Try increasing C2 and C8 in the white emitter follower circuit schematic
>> to 100nF.
> Doesn't seems to change anything:
> http://www.amigazone.fi/files/gpsdo/544-15.png
> http://www.amigazone.fi/files/gpsdo/544-16.png
> 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:
> http://www.amigazone.fi/files/gpsdo/544-17.png
> 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:
> http://www.amigazone.fi/files/gpsdo/544-18.png
> 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:
> http://www.amigazone.fi/files/gpsdo/544-19.png
> 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?

The attached circuit schematic is for a slightly modified white emitter
follower that reduces the temperature dependence of the input transistor
collector current.
It also increases the loop gain slightly.

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