Brian
You should get even better results if you replace the 330pF cap with a
1/4 wave (at the 20MHz sum frequency) coax cable open circuited stub.
Thats around 2.5m of RG 58 coax for example.
Connecting a series tuned circuit (at the sum frequency) across the
mixer IF output should also work well.
Bruce
Brian Kirby wrote:
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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