[time-nuts] Frequency Divider

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Apr 2 12:31:46 UTC 2009

John Ackermann N8UR wrote:
> 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.
> John
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One could also use a PECL to TTL level translator.

JPL have used ECL dividers throughout to produce 10MHz, 1MHz and 100KHz
outputs from the 100MHz signal derived from a Hydrogen maser:

Where TTL outputs are required an ECL to TTl translator followed by a
discrete amplifier to drive TTL levels in a 50 ohm load was used.


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