[time-nuts] Sub Pico Second Phase logger

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Dec 11 21:43:52 UTC 2008


Brooke

The NIST papers concerned were about using mixers as phase detectors:

http://tf.nist.gov/timefreq/general/pdf/112.pdf


http://tf.nist.gov/timefreq/general/pdf/971.pdf

As is all too often the case with some NIST papers the operating conditions for which the stated phase detection characteristics are true are not specified.

Its probably a case of over familiarity with the subject and forgetting
that what's obvious to the author isn't necessarily obvious to the reader.

Since the Minicircuits phase detectors (RPD, MPD series etc) are
specified for operation with a 500 ohm resistive IF termination, and
they have relatively high RF port to RF port isolation changing the RF
port termination from 50 ohms (at least with low frequency mixers using
conventional transformers) doesn't necessarily degrade the port to port
isolation significantly.

Bruce


Brooke Clarke wrote:
> Hi Bruce:
>
> A general comments on mixers.
>
> Since they are very nonlinear devices the output consists of signals at:
> +/-m * RF +/-n * LO.
> The output will change if the termination on any of the ports at any of those 
> frequencies is changed.  How much it changes depends on how strong that signal 
> is.  Some mixers reflect the image frequency to improve the conversion loss of 
> the desired output.
>
> For the mixers I was working with the LO needed to be strong enough to drive 
> the diodes into saturation and the RF needed to be small enough to not effect 
> the LO power.
>
> More on that at:
> http://www.prc68.com/I/Diodes.html
>
> Have Fun,
>
> Brooke Clarke
> http://www.prc68.com
>
> Bruce Griffiths wrote:
>   
>> Joe
>>
>> I suspect that the phase detector characteristics stated in the NIST
>> papers only apply when the mixer RF port is saturated.
>> This is evident from the Kurtz application note:
>>
>> http://www.wj.com/archive/documents/Tech_Notes_Archived/Mixers_phase_detectors.pdf
>>
>>  which indicates (Figure 14 and Figure 15 plus surrounding text)  that
>> for a high impedance (resistive) IF port termination the phase detection
>> characteristic only approaches a triangular wave when the RF port is
>> saturated (Figure 15) whereas for an unsaturated RF port (Figure 14) the
>> phase detection characteristics still appears sinusoidal.
>> The Kurtz application note also indicates that the IF port signal
>> amplitude reaches a maximum when the IF port termination resistance
>> increases (for the particular mixer) above 400 ohms.
>>
>> There are no corresponding figures for the case of an IF port terminated
>> in a capacitor.
>> It would be interesting to check this.
>>
>> Saturating the RF port also degrades the isolation etc, thus another
>> interesting question is does capacitively terminating the IF port
>> degrade these parameters when the RF port is unsaturated?
>> This may well not be the case with 5MHz or 10MHz mixer input  frequencies.
>>
>>
>> Bruce
>>
>>
>>
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