[time-nuts] HP10514B Mixer Terminations

[Brian Kirby]

I have been working on a Dual-Mixer Time Difference system.  In the 
first "design type/experiment", I was using HP10514B mixers and a LT1037 
preamp and a OP27 zero crossing amplifier/limiter - all a very basic 
setup.  I obtained some fair measurements;

Using 10 MHz sources, a 9.9999 MHz offset for a 100 hertz beat, the 
"floor" of the system looked like this:
0.01 second = 1x10-10
0.1 second = 1x10-11
1 second = 1x10-12
10 second = 1x10-13
100 second = 1x10-14
1000 second = 1x10-15
10,000 second = 1x10-16
this was three days of data

Running it again, with a 10 hertz beat; it looked like this;
0.1 second = 4x10-12
1 second = 4x10-13
10 second = 4x10-14
100 second = 4x10-15
1000 second = 4x10-16

I also had a lot of good suggestions From Ulrich Bangert, Bob Camp and 
Bruce Griffins, who I will call my mentors and thank for all the help.

I went back and did some basic experiments this evening.  Looking at 
mixer terminations.  I have attached two photos - low res.

The first photo named mixer_10db, is the mixer driven with +10 dbm on 
both ports.  The o'scope is looking thru a basic RC filter of 1 kilo-ohm 
resistor in series with the mixer output, and on the output of the 
resistor is a 0.1 uF capacitor to ground.  This is a mixer that is 
intentionally over driven to use as a phase detector.  The mixer is 
rated +13 dbm maximum, and about everybody I have talked with (NIST and 
BIPM) about these mixers ran them at +10 dbm on both LO and RF ports.  
As these mixers are hard to find, and they are not made anymore, I would 
not over-drive them any further.  These mixers also have some of the 
lowest phase noise measurements on record.

The second photo named mixer_330 pF, is the same setup, except I have 
put a 330 pF capacitor across the mixer output.  By capacitive 
terminating the mixer, it squares up the output of the mixer - which 
makes it easier to be converted to a high slew rate signal.

What I found, is you want to run the minimum capacitance value for the 
highest beat frequency you plan to run.  That way the signal stays 
"squared up" from the highest to the lowest beat frequency.

I got this value by playing around by looking at the mixer filtered (RC) 
output at 1 hz, 10 hz, and 100 hz.  When I was using 0.1 and 1 uF 
terminations, The 1 and 10 hertz beat was OK, but the 100 hertz beat was 
still a sine wave.  That may be why the results above shows a difference.

For a test, at 330 pF, I did try it at 1 KHz, it was back to a sine 
wave.  So 330 pF looks good for trying to get a "squared" wave out of 
the mixer for 1, 10 and 100 hertz beats.....I tried 36 pF for 1 KHz, it 
did not present enough capacitance to give the "squared" wave at 1, 10 
and 100 hertz beat.

We have been running email outside of Time-Nuts group as I am not sure 
if any of you wanted to see the project I am working on.  I did not want 
to clutter up the forum......but if there is an interest, I can bring it 
back.  My next plans are to start over building a new system using a 
much lower noise op amp, the LT1028.  If the mixer terminations are OK 
with my mentors, I will use a LT1028 preamp set for about x15 gain and 
it will dump into the first set of limiter diodes.  And I believe that 
will call for 1.6 KHz low pass filtering on the first limiter diodes.

Comments ?

Brian - KD4FM

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