[time-nuts] Low cost alternate to Dual Mixer/DMTD

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 2 22:28:19 UTC 2009


WarrenS wrote:
>
> Bruce
>
> Yes, there are a few disadvantages using this simple low cost
> configuration
> along with all of its advantages.
> You can not get everything for nothing, but you can get higher speed,
> better
> resolution and less noise from this.
>
>> This can not be used to evaluate the stability of an offset oscillator.
> Correct,  It can not test osc frequencies that are much different than
> the
> ref osc such as an offset Osc,
> BUT one of the many advantages of this configuration is that an offset
> Osc
> is not needed.
> This will give freq offset data down to under 1 ms (>1 kHz) without Any
> offset osc.
>
You also need to measure the EFC slope at the operating point as the EFC
transfer characteristic can be highly nonlinear.

>> You need to ensure that the isolation between the 2 sources is
>> sufficient
> I have no problem with signal injection at 1e-12 resolution levels, even
> with 10811s without buffers
> using 10db attenuators and ground Isolation transformers at each Osc
> output.
> Maybe due to the fact that they are locked.

That is the worst possible case for injection locking.


> This is verified by adding a slow low level freq modulation on the DUT
> osc
> and plotting the freq change on the reference's EFC.

Probably not a good test for injection locking as a small shift in
frequency from equality rapidly attenuates the effective injection
locking signal.
Better test is to insert a very high reverse isolation amplifier between
each ocxo and the mixer and see if that makes any difference.
Also need to ensure that injection locking doesn't occur through
injection via the EFC input.


> The phase error can also be changed by adding large or small offsets
> at the Loop amp's input to check for any pulling tendency.
>
> ws
> ***********
Bruce






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