[time-nuts] New topics (was Re: He is a Time-Nut Troublemaker....)

[Bruce Griffiths]

John Ackermann N8UR wrote:
> Hal Murray said the following on 12/23/2008 01:58 PM:
>   
>>> 2.  Several measurement techniques require a given phase relationship
>>> (e.g., quadrature) between DUT and reference.  For HF frequencies (ie,
>>> 5  or 10 MHz) is there a *practical* phase shifter design covering
>>> 180+  degrees that doesn't involve switching various lengths of coax
>>> in and  out of the line? 
>>>       
>> PLL up by a factor of N, use that to drive a DDS, then filter.  Maybe a pair 
>> of DDSes will get better tracking.
>>
>> For each possible phase offset, you need N slots in the table.  (N/4 with 
>> more work)
>>     
>
> If doing phase noise or short term stability measurements, wouldn't the
> noise of the DDS impact the results?
>
>   
John

Split the signal to be measured, use 2 independent DDS circuits, 2
mixers and measure the cross power spectrum of the 2 mixer outputs.
This should only measure the noise common to both channels.
Should be easy to do with a sound card for example.
The phase noise of other non common components such as isolation
amplifiers should also cancel.

Bruce