[time-nuts] 10 MHz Oscillator comparison part II

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jan 12 23:33:56 EST 2011


On 12/01/11 19:24, Bob Camp wrote:
> Hi
>
> If you have a random frequency like 7.352 MHz that neither divides or
> multiplies to 10 MHz harmonic or sub harmonic, you can indeed mix the signal
> to 10 MHz.
>
> If you do so, you will need to filter the outputs, since the mixing spurs
> will mess up the input to the multiplier.
>
> If the generator you use for the mixing has more noise or jitter than the
> sources, that noise is likely to de-correlate unless the chains are
> absolutely identical. Since they multiply to two different frequencies, they
> really can't be identical. Net result is your measurement is messed up by
> the noise of the generator.

Well, some time back I proposed a DMTD style front-end which mixes two 
unequal frequencies with a common oscillator into a common frequency. In 
a second stage it is mixed down by a second LO in more traditional DMTD 
style.

LO1 = (f1 + f2)/2
IF1 = abs(f1 - LO1) = abs(f2 - LO2) = abs(f1 - f2)/2
LO2 = IF1 - IF2

The IF1 filtering needs to filter out the difference frequency and 
supress the sum frequency. However, since both sides of the chain will 
have same frequencies after first mixer, correlation between the sides 
will create smaller response differences unless built very different. 
The mixer oscillator contribution for LO1 and LO2 will correlate between 
the channels.

Cheers,
Magnus




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