[time-nuts] minimalist sine to square
kb8tq at n1k.org
Fri Jan 19 20:34:20 EST 2018
Unless you really beat on the thing for days on end, you can do without the 330 ohm and 100 ohm
resistors (along with the two diodes). Most modern gates have pretty robust protection diodes. The
source impedance is high enough after the transform that the available current is pretty low. On a
NC7SZ125 the negative diode is rated for 50 ma max and the positive diode is rated for 20 ma
If the two 1K’s properly terminate the circuit, you have a 250 ohm source. (500 ohm load and 500 ohm
transformed from the sine input). A 1V overdrive (1/2 V + and 1/2 V -) will put 2 ma into the diodes on the
peaks. The more likely case is that the negative is hit a bit harder. The bias is most likely a bit below
1/2 Vcc for best symmetry.
None of this is to say you *should* hit the diodes. No matter what sort they are, the performance will
degrade a bit when you do. How much is of course a “that depends”. Most of us are not driving the
gate with a -180 dbc/Hz source and expecting -177 out of the gate.
> On Jan 19, 2018, at 8:14 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> Something like the attached circuit is suitable for driving the MCU clock input directly.
> The diodes should be schottky signal diodes like the 1N5711 series. The series resistors limit the diode peak current and the CLK input protection network current. It should work with inputs from 1V pp to 8Vpp. If SMT components were used it should all fit on a DIP compatible daughter board.
>> On 20 January 2018 at 12:37 Bob kb8tq <kb8tq at n1k.org> wrote:
>> With a 1V p-p sort of output, a simple matching network will get you into the 4 to 6V p-p range.
>> Drive that into a 5V compatible CMOS gate and move on …. If you have a super hot output, put
>> a 3 db pad on it.
>>> On Jan 19, 2018, at 5:40 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>>>> What's the input signal amplitude?
>>>> What's the desired output signal (eg 5V CMOS, 3.3V CMOS etc)?
>>> It's for a typical 5 or 10 MHz OCXO / Rb / Cs with sinewave output; say, 1 Vpp. The output should be 3.3 or 5 V depending on what the MCU needs. It doesn't have to have stunning performance: think breadboard, PIC, Arduino sort of stuff. I was looking for something in a PDIP-8 package; the same as all the picDIV or picPET chips I use. That's why older parts like µA9637 / DS9637 came to mind.
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