[time-nuts] HP10514B Mixer Terminations
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Tue Mar 30 02:52:25 UTC 2010
Your poor results at 1kHz have more to do with the 1K +0.1uF low pass
filter which has a cutoff frequency of about 1.6KHz.
This will attenuate the beat frequency harmonics required for high slew
rate at the beat frequency zero crossings.
A filter cutoff of 16kHz (1K + 10nF) should improve the slew rate at 1KHz.
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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