As a matter of interest just how bad were those OCXOs?
e.g. what was the ballpark ADEV for 1s, 10s etc.?
Bruce
Bob Camp wrote:
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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