[time-nuts] reference oscillator input circuit

Magnus Danielson magnus at rubidium.dyndns.org
Wed Dec 8 15:20:29 EST 2010

On 12/08/2010 06:28 PM, Rick Karlquist wrote:
> jimlux wrote:
>> I'm looking for suggestions on a general circuit that can be used to
>> receive an external frequency reference (nominally a real clean sine
>> wave at, say, 10 MHz, although up to 100 MHz is possible) and turn it
>> into a "real clean" square wave.  Galvanic isolation is a plus (a
>> transformer or capacitor would probably do that).
> In the 5071A at 80 MHz, we capacitively coupled a sine wave into
> a 74AC series logic gate, that had DC bias resistors to hold
> it at half the supply voltage.

A feed-back resistor over an inverter and capacitive feed will self-bias 
such that PWM is 50%. The details of the gate being used may however 
prove lethal as you may end up with self-biasing into ring-modulation 
mode for some "inverters" being effectively three inverters in series.

The use of a long-tailed pair on the input to gain out of the problem, 
prior to a gate for final squaring up would be my recommendation. It 
would be in the spirit of the Dick and Collins papers. The needed 
slew-rate gain will not be that great for 10 MHz to 100 MHz sine.

You should not need to use very exotic setups to get the performance you 


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