It looks like I got the squarest wave at 150 pF. Lesser capacitance, give a peaked sinewave, like maybe a second harmonic. Past 200 pF, it starts rounding. 150pf= XC of 53 ohms....
This DMTD system, will only go to 100 hertz maximum beat for my design...The scope is set to 100 mV per div, 1ms per division, I intentionally mis-triggered it to show rise and fall times. 100 hz beat signal. ANd Bruce told me how to calculate slew rate, but it has to be beated in my thick tough head.....
And Bruce, I'll try some coax tomorrow, if I get back home early enough - my work project may keep me late. And I plan to only use this system to 100 hertz beat, I was just playing around at 1K.....I like learning.
Good night.....Brian....KD4FM Bob Camp wrote:
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.
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