[time-nuts] HP10514B Mixer Terminations
Brian Kirby
kilodelta4foxmike at gmail.com
Tue Mar 30 02:06:10 UTC 2010
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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