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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