The OCXOs with 55Hz offset should still be adequate for a DMTD unless
one is comparing hydrogen masers or even lower noise sources.
For a Costas receiver the rejection of the offset oscillator noise can
be somewhat higher than for a DMTD without needing to keep the
corresponding zero crossings of the 2 beat frequencies approximately
aligned.
Bruce
Bob Camp wrote:
Hi
The 5 MHz stuff was down at or below 1.5x10^-12 at one second by our measure.
Others measured them a bit lower than that. We didn't do 100% testing at 10
sec, so I don't have a lot of data there. The ones 55 Hz higher often came at
or above 4x10^-12.
Bob
On Feb 6, 2010, at 9:40 PM, Bruce Griffiths wrote:
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
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