[time-nuts] Frequency Divider

John Ackermann N8UR jra at febo.com
Thu Apr 2 12:19:12 UTC 2009

Hal Murray wrote:
>> Start with a buffer amp and then a decent Schmidt trigger.
> If you have a clean input signal, a Schmitt trigger doesn't solve any 
> problems.  It does help if you have a slowly rising signal such that noise 
> might be significant while the signal is near threshold.  A 10 MHz sine wave 
> is slow relative to AC logic.
> Since we were recently speaking of LPROs, their user manual has a section on 
> how to convert 10 MHz sine waves into TTL signals.  None of their suggestions 
> used Schmitt triggers.
> This feels like the sort of thing that should have been hashed out here by 
> now.  Is it time to start a FAQ?

The TADD-2 uses an input circuit published by Wenzel in their "Waveform 
Conversion" document at http://www.wenzel.com/documents/waveform.html.

I haven't measured its standalone jitter, but its input sensitivity is 
great --  it will reliably trigger a CMOS gate from an input at least 
down to -10 dBm, maybe lower (I don't recall the exact limits I found 
when I tested).  If you build this, note one thing -- with the 100 ohm 
emitter resistor specified, the square wave output is more like 6V than 
5V p-p.  I use 120 ohms instead to get a 5 volt output.

While the Wenzel circuit requires a modest handful of discrete 
components, I think it's the most useful solution by a pretty clear 
margin for our typical requirement of driving a single-ended logic gate 
from an HF source.


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