[time-nuts] ADEV vs MDEV - using sound card

[Bob Camp]

Hi

Occasionally you also come across 5.000055 MHz OCXO's that have 5 MHz crystals in them. Then you discover just how much short term stability can degrade when they move the crystal 55 Hz. Same vendor crystal, same crystal spec., same oscillator circuit, not even close on short term stability....

Bob


On Feb 6, 2010, at 9:02 PM, Bruce Griffiths wrote:

> JPL resorted to using  a commercial synthesiser set for an offset of 123Hz (to minimise spurs and other artifacts) in their 100MHz N channel mixer system.
> 
> Occasionally one comes across 5.000055MHz OCXOs that use 10.000110MHz crystals internally.
> The resultant 55Hz (with 5MHz source) or 110Hz (with 10MHz source) beat frequencies are lie between the hamonincs of either 50Hz or 60Hz line frequencies.
> 
> Bruce
> 
> Bob Camp wrote:
>> Hi
>> 
>> Any approach that includes building a low noise synthesizer is opening up a whole new set of issues. I would much prefer to do my building at audio. Audio parts are cheap, and performance is usually a lot easier to check than at RF.
>> 
>> Bob
>> 
>> 
>> On Feb 6, 2010, at 8:30 PM, Bruce Griffiths wrote:
>> 
>>   
>>> Which just leaves the minor problem of the offset oscillator.
>>> 
>>> One option is to use a phase truncation spur free output frequency from a DDS.
>>> If one is using the Costas receiver approach the beat frequency need not be a nice round number like 1.0000KHz.
>>> 
>>> Another method is to use a crystal whose frequency is offset a few kHz from 10MHz.
>>> 
>>> Yet another is the classical method of dividing 10MHz by 100 and subtracting (using an LSB mixer) the resultant 100KHz from 10MHz to produce 9.9MHz, then divide the 9.9MHz signal by 100 and add (using a USB mixer) the resultant 99kHz signal to the 9.99Mhz signal to produce a 9.999MHz output.
>>> 
>>> Bruce
>>> 
>>> John Miles wrote:
>>>     
>>>> A sound-card back end has always seemed like a pretty reasonable approach to
>>>> me, if you're inclined to go the DMTD route.  I wouldn't send a 'baseband'
>>>> signal to the sound card, though -- I'd upconvert it to a few kHz to get
>>>> away from the numerous bad things that sound cards do near DC.
>>>> 
>>>> -- john, KE5FX
>>>> 
>>>> 
>>>> 
>>>>       
>>>>> Hi
>>>>> 
>>>>> My main concern with the low frequency pole in the sound card is
>>>>> the quality of the R/C used. You can certainly model what ever
>>>>> you have. If they used an aluminum electrolytic for the "C" it
>>>>> may not be the same next time you check it ....
>>>>> 
>>>>> On a 10 Hz system, a 1 Hz pole is probably not an issue. It might
>>>>> get in the way with a 1 Hz beat note.
>>>>> 
>>>>> Another thing I have only seen in passing: "Sigma Delta's have
>>>>> poor low frequency noise characteristics". I haven't dug into it
>>>>> to see if that's really true or not. If you buy your own ADC's,
>>>>> you certainly would not be restricted to a Sigma Delta.
>>>>> 
>>>>> Even with a cheap pre-built FPGA board, you could look into
>>>>> higher sample rates than a conventional sound card. You would
>>>>> drop back to 16 bits, but it might be worth it.
>>>>> 
>>>>> Bob
>>>>>         
> 
> 
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