[time-nuts] About HP10544A

[Bruce Griffiths]

Bruce Griffiths wrote:
> 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?
>>
>>   
>>     
> Esa
>
> 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
> distortion.
> 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.
>
> Bruce
>
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>   
Esa

Attached circuit schematic for the While emitter follower optimises the
bias circuit frequency response.
The bias circuit has a bandwidth of around 40kHz so that will reduce the
effect of flicker noise should RF transistors be substituted for the 3
transistors comprising the white emitter follower.
The modifications also remove peaking in the bias circuit frequency
response.

Bruce

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