Magnus Danielson wrote:
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
One still has to ensure that close in spurs generated by the DDS sources
aren't an issue.
If the zero crossings of the beat frequency signals at the outputs of
the 2 final mixers arent closely aligned then such spurs can be problematic.
Bruce
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