[time-nuts] SR620/PM66xx/CNT-90 input stages

Li Ang 379998 at qq.com
Tue Jun 20 06:01:25 EDT 2017

 I think the purpose is to get high input impedance for high frequency signal. The input impedance of comparator is high at low frequency range only.

Li Ang / BI7LNQ
From: "Attila Kinali"<attila at kinali.ch>
Date: 2017/6/20 15:20:44
To: "Discussion of precise time and frequency measurement"<time-nuts at febo.com>;
Subject: [time-nuts] SR620/PM66xx/CNT-90 input stages


I had a look at the PM668x[1] and CNT-90[2] schematics yesterday and
noticed one thing: The input stages are strikingly similar to the
SR620[3] (down to the parts used) and all of them have a gain 1 amplifier
infront of the comparator. The PM668x service manual explicitly calls it
"impedance converter", even. Now, if the downstream circuit would be low
impedance, I could understand that, but the sink is a comparator with a
high impedance input (only a few µA input current). I am sure the engineers
had a good reason to add those amplifiers, but I cannot guess why. Would
someone be so kind and enlighten me?

Attila Kinali

[1] http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/Fluke_PM6681_Service_Manual.pdf
[2] http://assets.fluke.com/manuals/6690____smeng0000.pdf
[3] http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Stanford_Research_Systems/SR620_Universal_Time_Interval_Counter_Schematics.pdf

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