Hi In most cases it's a T section (two coils, one cap) low pass filter tacked on to the output of a logic gate (or FPGA output). Net result is a reasonable sine wave *if* you terminate it correctly. Based on the observations posted on the list, the filter in the FE-5680 seems to be set up for a 50 ohm resistive load and is mis-terminated when run into a scope probe. Not to surprisingly, you get a lot less output voltage into the correct load than into an open circuit. With a filter, the difference can be significantly more than 2:1. The logic gate / FPGA is putting out about 3 volts p-p, so that will be the "filter went away" output level. The matched value will be quite a bit lower to keep the gate / FPGA from going a bit nuts power wise.
Bob On Jan 2, 2012, at 9:52 AM, ehydra wrote: > Is it possible to sketch the circuit? I can SPICE it. > > Symmetry limiting is the holy grail and it is questionable if a discrete > design is way better than one of the chips. > > > Here is another limiter circuit (by Chris Trask): > http://ehydra.dyndns.info/NG/LTspice/Negative%20Impedance%20LO%20Driver.pdf > > > - Henry > > > David schrieb: >> What kind of performance would you expect in this application? Low >> jitter? 50 ohm output? TTL or better signal levels? Fast rise and >> fall times? Duty cycle correction? >> After reading your post I was thinking about how to go about it and >> ended up with an 8 transistor discrete design using a differential >> amplifier input and pair of current mirror transconductance amplifiers >> for the output. I have been looking into designing a pulse generator >> for oscilloscope calibration and have an interest in GPSDOs so maybe I >> will prototype this as well just to see what kind of performance a >> bunch of 2N3904 and 2N4401 jelly bean transistors can provide. >> On Thu, 29 Dec 2011 21:14:30 -0800, John Beale <[email protected]> >> wrote: >>> In case it's useful... there are many ways to get a square wave out from a >>> sine wave in, but one straightforward way is with a comparator. Some work >>> better than others. The slow ones won't work at all at 10 MHz, and the very >>> fast comparators (MAX999, ADCMP600, LT1116 etc.) are more expensive, and >>> perhaps harder to work with. I tried a MAX9013 in SO-8 package and it works >>> well for the job. You can see my schematic, circuit and scope plots at the >>> bottom of this page: > > > -- > ehydra.dyndns.info > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
