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
need.
Cheers,
Magnus
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