[time-nuts] Methods for comparing oscillators

[Bruce Griffiths]

John

John Miles wrote:
> The spec for the 10811A's trimmer range is +/- 10 Hz, actually.  Seems
> doable.
>
> There was a good thread on the list back in late 2006 where some of the
> pitfalls of the DMTD technique in general were discussed.  It seems that
> achieving optimal performance is a lot less trivial than anyone would like.
>
> If someone wanted to build a "minimum effort" system with a lower short-term
> floor than the 5370, my understanding is that it would require:
>
> - a 10811-class OXCO tuned to generate beat notes of several Hz
> - a mixer of some sort with good IF response down to DC
> - four good isolation amps (which thanks to Bruce's recent work is not a
> difficult or expensive issue)
> - some sort of delay compensation on one of the inputs may be needed to
> avoid decorrelation of the transfer oscillator's effect between the two
> channels
> - a sound card with good differential-phase stability
>
> Of course a 5370 or other two-channel TIC could be used instead of an FFT
> back end, but I'd be interested in seeing what level of performance could be
> expected with a sound card, too.  Bruce, you were looking at differential
> jitter and delay on stereo sound card channels at one point, weren't you?
>
>   
I haven't yet measured it but as far as I can determine from the
literature the sampling jitter is around 1ns or less (usually much less)
for high end cards.
Even as much sampling jitter as an unlikely 10ns is negligible (~10fs at
the mixer input) for an entry level system with a 10Hz beat note.
The biggest problem is likely to be the phase shift instability due to
the coupling caps in the sound card (its not always practical/feasible
to remove them).
With 10Hz beat note and a 1Hz single pole high pass RC input filter and
electrolytic coupling caps with a tempco of 1000ppm/C the phase shift
tempco will be around 1.6us/C (1.6ps/C at the mixer input). If the input
RC time constants for the 2 channels matching to within 20% the
differential phase shift tempco will be reduced to around 300fs/C at the
mixer input.
Replacing the input coupling caps with lower tempco plastic caps may be
worthwhile as the phase shift tempco will be reduced by a factor of at
least 4x even before the effect of closer matching is considered.

> If differential phase stability is a problem, then perhaps one of the
> channel IFs could be heterodyned up to a different carrier frequency that
> could share a single physical ADC channel.  Isolation and phase-delay
> compensation would again be a big deal in any such scheme.  Probably the
> heterodyne LO would need to be derived directly from the transfer
> oscillator.
>
> -- john, KE5FX
>
>
>   
Bruce
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>> Behalf Of Tom Van Baak
>> Sent: Sunday, August 02, 2009 9:14 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Methods for comparing oscillators
>>
>>
>>     
>>> A 1Hz offset may be convenient as its easy to adjust an 10811A or
>>> similar to achieve this.
>>> A 10Hz offset is harder to achieve.
>>>       
>> Would either of these be 10 Hz solutions?
>> 1) open the 10811 and change the xtal load capacitance, or
>> 2) change the oven temperature set point until the resonator
>> is 10 Hz off. You'd loose a little in tempco but...
>>
>> /tvb
>>
>>
>>     
>
>
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