[time-nuts] DMTD for different input frequencies

[Magnus Danielson]

Dear time-nuts,

I do not know if someone has tested this or used it in anyway, but I 
just realized one thing which may be potentially useful for someone...

Consider that we have two very different frequencies, let's say 10 MHz 
and 64 MHz and we want to use a DMTD style measurement setup to compare 
them. However, a traditional DMTD setup would not be able to handle such 
large frequency differences...

But there is a method using a two-stage setup using two programmable 
offset oscillators. This is inspired by the DMTD setup from that 1964 
conference I referred to...

In the first stage, the input signals is mixed by the average frequency 
(37 MHz in this case) causing the beat frequencies to become roughly the 
same (27 MHz in this case). The second stage would then act as as the 
normal offset local oscillator and beat-frequency mix-down.

One has to recall that the noise gain factor will be different for the 
two sources.

Notice how the core aspect of common noise contribution to both channels 
is maintained.

This approach isn't all that hard to realize with todays DDS solutions 
and could extend existing DMTDs setups. Definitly within range for the 
hobbist with some spare mixers and generators...

Cheers,
Magnus