[time-nuts] Another question about PHASE and DMTD

[Bob Camp]

Hi

To further simplify:

1) Get rid of the second mixer Fdut (it's a dupe of the first mixer Fdut)
2) Get rid of the second mixer, it's not really doing anything

You now have a single mixer system doing exactly the same thing your dual
mixer system was doing. The dual mixer setup *only* makes sense if you have
three fully independent oscillators.

What do you have?

It's the front end on a quadrate phase noise setup. Feed the (amplified)
audio out of the phase detector (mixer) into an audio spectrum analyzer. Do
a little calibration math. You are looking at phase noise.

What comes with all this?

1) DC offset in the mixer output that's temperature sensitive.
2) Output levels that are drive, load, and temperature sensitive

For phase noise, DC offsets don't matter. You calibrate out the level stuff
each time you do a reading. For long term stability, offsets and levels
matter very much. AC coupled measurements at zero crossings are insensitive
to this stuff. 

Bob



-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Paul A. Cianciolo
Sent: Tuesday, August 23, 2011 10:31 PM
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] Another question about PHASE and DMTD

Folks,

I know I am going to regret asking this but you only live once.

Please accept my assumptions during this question 


Say we have a DMTD system that uses the same freq. for the common LO and the
F Ref and the FDUT 10 Mhz.
Further ... these oscillators are all synchronous, and in phase because they
are all derived from the same source.
This is all in a perfect world of course. No phase delays anywhere.

There should be 4 outputs from the mixers:

Fref
Fdut
Fref+ Fdut
Fref- Fdut

Perfect mixer so Fref and Fdut never make out of the mixer and cancel
perfectly.
Leaving:

Fref + Fdut   20 Mhz  at 2 times the level I assume?

And now ... all phases being equal...

Fref - Fdut should equal zero.

Now introduce a 90 phase shift in Fdut using a coax cable or some stable
lumped device.
I did this in excel and it yields a voltage of approx..1.5 times the mixer
output level and still at 10 MHz.

SO if.. and I say if... that is true and we return the Fdut back to an
oscillator in question in other words an actual dut, would associated phase
shifts be detectable and record able in an analog fashion?

I suppose in the real world the wave forms and phase delay's in the circuit
would be too high to make something like this worth while.

And the only advantage I can see does really not needing an offset LO...

I guess I am just living in the analog domain...   Edges are probably far
easier to count and more reliable.




Paul A. Cianciolo
W1VLF
http://www.rescueelectronics.com/
Our business computer network is  powered exclusively by solar and wind
power.
Converting Photons to Electrons for over 20 years









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