[time-nuts] About HP10544A

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Mar 10 19:48:42 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 circuit that is used to extract and buffer the signal from the
crystal oscillator can have a significant effect on the distortion.
In the case of the 10544A the cascode buffer contributes most of the
The emitter follower contribution to distortion with a 1k load is much
smaller, however it is sensitive to slew rate distortion with
significant capacitive loading.
Using a common base stage as in the 10811A can have lower distortion, as
the emitter current is filtered by the crystal.
If one uses RF transformers between stages then one can cascade common
base stages and increase the output signal current.
An output stage with a 50 ohm output impedance is useful in that the
connection to external circuitry can then be a 50 ohm transmission line.

Some of the early OCXOs even used a crystal filter to clean up the
oscillator output.
Modifying the 10544A circuit would require a new circuit board and the
mechanical space available would restrict what you can fit in unless you
you use some surface mount components.
You would need to reuse the trimmer capacitor.


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