Hi You can get a *much* more squared output from the mixer than the photos you show on the scope. The waveform looks a lit like a triangle wave with the tips chopped off. Normally the fastest edge happens into a capacitive load at RF that's below about 0.5 J ohms for a "50 ohm" mixer.
Bob On Mar 29, 2010, at 10:06 PM, 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 > > <mixer_330pf.jpg><mixer_10dbm.jpg>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
