[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.
More information about the time-nuts