[time-nuts] Unified VCXO Carrier Board

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Oct 23 20:11:57 EDT 2015

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, 
> 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 
> 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

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.


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