[time-nuts] 10 MHz Oscillator comparison part II

Bob Camp lists at rtty.us
Thu Jan 13 12:35:20 EST 2011


Hi

The gotcha as I see them:

1) One input gets multiplied to say 100 MHz. The other goes to 110 MHz. The
20 log N thing is going to get you there.  

2) The multiplication process will have issues. With a PLL, bandwidth and
gain are the source. For discrete multipliers the limiting process is an
issue. Either way, the common noise between the two will de-correlate at
some offset frequencies.

In both cases, the issue may seem small. Since you are trying suppress the
noise by subtraction, a fractional db error will reduce your suppression by
quite a bit. The same is true of a small phase error. 

Bob


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Magnus Danielson
Sent: Wednesday, January 12, 2011 11:34 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] 10 MHz Oscillator comparison part II

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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