[time-nuts] minimalist sine to square

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jan 19 20:51:57 EST 2018


Even the modern PICs spec 50mA max input currents.
Simulation indicates 20mA peak diode currents without the 330 ohm resistors for a 2V pp input, even more for higher input signal levels. If one can guarantee that input is around 1V pp then the extra diodes and resistors aren't required. If its possible that an input of 16dBm or more may be used then the extra diodes and resistors are required. I simulated the circuit for inputs up to +22dBm.
Current flowing in the IC protection diodes can degrade the timing jitter substantially (tens of picosec for HCMOS).

Bruce 
> On 20 January 2018 at 14:34 Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> 
> Hi
> 
> 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
> 
> Some math:
> 
> 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.
> 
> Bob
> 
> > 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.
> > 
> > Bruce
> >> On 20 January 2018 at 12:37 Bob kb8tq <kb8tq at n1k.org> wrote:
> >> 
> >> 
> >> Bob
> >> 
> >> 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. 
> >> 
> >> Bob
> >> 
> >>> On Jan 19, 2018, at 5:40 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> >>> 
> >>>> Tom
> >>>> What's the input signal amplitude?
> >>>> What's the desired output signal (eg 5V CMOS, 3.3V CMOS etc)?
> >>>> Bruce
> >>> 
> >>> 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.
> >>> 
> >>> /tvb
> >>> 
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> > <PIC_CLK_Network.gif>
>



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