[time-nuts] DMTD for different input frequencies

[Bruce Griffiths]

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