On Friday, October 23, 2015 03:27:39 PM Charles Steinmetz wrote: > Gerhard wrote: > >What do you consider a run-of-the-mill comparator? LM139, LMV7219, > >AD8561, ADCMP580? > > For squaring 1-10MHz sine waves, the LT1719 and 1720 are the best > that I've found. This is a matter of how much internal hysteresis > the comparator has, how smoothly the internal hysteresis acts, how > much gain the part has and how it is structured, how fast it is (both > risetime and propagation delay), whether it suffers from thermal > feedback from the output stage to the input stage, how much internal > ground bounce it has, and a host of other die-level issues. > > The LT1719 uses bipolar input supplies, so ground can be the > reference voltage and also be in the center of the input common-mode > range. This is always quieter than biasing the inputs to the middle > of a single supply, which is usually done with the 1720 and many > other comparators. (Like other single-supply comparators, the 1720 > will work referenced to ground with only a positive supply -- but (i) > the inputs are then at the very edge of the input common-mode range, > and (ii) you can only drive the input 100mV below ground, so you have > to figure out how to clamp the input signal. It is much easier to > just use an LT1719 with +/-5v on the input stage, and it works better, too.) > > Page 22 of the LT1719 datasheet shows the simplest possible circuit > and discusses its performance. > > >What optimizations? I have seen the "Wenzel" circuit in cheapish > >frequency counter inputs in > >the late seventies, maybe with a diode bridge added as input protection.. > > Use medium-speed transistors, bias both bases from the same low-noise > voltage reference such as an LM329, capacitively couple the emitters, > and use a higher supply voltage, for starters. I use MPSH81/MMBTH81s > and a power supply of around 20v (see attached schematic) for a > reasonably optimized implementation. Other transistors can be used, > but I've found that the H81s work better for squaring 1-10MHz sine > waves than anything else I've tried -- they hit the sweet spot of the > bandwidth/gain tradeoff and have a nice flat gain vs. current > characteristic. > >so it shows that one can replace filtering by signal power :-) > > It's a matter of the slew rate of the input sine wave at > zero-cross. Once you reach the critical slew rate for any particular > input architecture, the comparator is hard-switched fast enough that > it doesn't spend significant time in the linear region generating noise. > > >Any objections against the AD9901 phase comparator? > > That should work fine. > > Best regards, > > Charles Charles
A resistor from point A to ground in the Wenzel style shaper you attached has little effect on the output symmetry due to C4. However it does allow the output amplitude to be adjusted. Bruce _______________________________________________ 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.
