[time-nuts] DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Mike Monett]

You wrote:

> Mike,

> He was using an analog mixer, but your comment about XOR  mixer does
> not apply  to  analog mixers.  Your  oversimplification  that analog
> mixer and  XOR gates being the same thing does not  apply  here, and
> thus the  assigned  missbehavior does not carry over  to  the analog
> mixer case.

> Cheers, Magnus

Magnus,

Thanks for your comment. Here are some attached files:

1. DBMS.PNG

This shows the schematic of a double-balanced mixer. Note the mixer output
is on pin U24. A low pass filter is at R1C1.

2. DBMWFM.PNG

These are the waveforms in quadrature lock. The bottom waveform in red is
the signal at pin U24. It is a square wave at twice the signal frequency.
This signal is identical to an XOR, such as a 7486 logic ic, except the
amplitude is much lower at only 900 mV p-p.

The top waveform in green is the signal at the low pass filter. It is a
triangle wave, the same as you would get from adding a low pass filter to
any square wave. Thus my statement that a double-balanced mixer is an XOR
is accurate.

3. DUBLBA01.ASC

This is the double-balanced mixer schematic input for the LTspice simulator.

4. DUBLBA01.PLT

This is the output waveforms from LTspice.

Ordinarily, the triangle ripple output from a double balanced mixer would
add considerable jitter to any PLL. Eric's application avoids this problem
since his loop bandwidth is so low, at much less than 1 Hz. This makes it
extremely difficult for him to obtain lock, which is why I proposed using a
phase/frequency detector. 

The first block diagram I posted earlier, PNA.PNG, contained two errors. I
corrected them in PNA2.PNG, which I will post to Eric.

At first, I did not realize the significance of Eric's low loop bandwidth,
and I erroneously assumed the triangle wave ripple output would cause
significant jitter to his loop. It is now obvious the low loop bandwidth
will reduce the ripple amplitude to insignificance, and I now retract my
claim.

Thanks,

Mike
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