[time-nuts] SE880 GPSDO

Attila Kinali attila at kinali.ch
Tue Apr 26 05:22:53 EDT 2016


On Mon, 25 Apr 2016 20:45:53 -0400
Bob Camp <kb8tq at n1k.org> wrote:

> > To get to CMOS levels, you usually add a simple inverter, with an
> > capacitor in front and a 1M resistor across the inverter (from input
> > to output).
> 
> If you set up a modern (74AC or faster) inverter with a resistor from input to 
> output, you are very likely to get it running as an oscillator all on it’s own. That
> oscillation may or may not add to the desired output. 
> 
> Stick with a two resistor bias on the input … it’s a lot safer.

Not really. In order to prevent oscillation you have to ensure
that the lowpass filter formed by the input capacitor and the
bridging resistor has a low corner frequency. Putting the corner
frequency in the range of 10kHz-1MHz is usually good enough, the lower
the better. Though there are of course limits to this:

* from a certain point on, the surface resistance of the PCB
  dominates the resistor. A normal FR4+solder stop board has a
  surface resistivity of 1M-10G depending on the actual solder stop,
  humidity and dirt/grease on the PCB and the geometry of the wires.
  (I usually go with 10M for close wires when i'm too lazy to calculate)

* The larger the capacitor the lower its self-resonance frequency
  becomes. Ie from that poin on the capcitor behaves more like an
  inductor than a capacitor. Rule of thumb: a 4.7uF 0603 X5R has a
  self resonance frequency around 1-3MHz

For my squaring gates, I usually use 100nF+1M (~100kHz) on a
single gate inverter (usually LVC, but not always) and have
not seen any oscillation yet.


BTW: Does someone know about actual measurements of the small
signal transfer characteristics of a single CMOS gate?
It would be nice to put the above rule of thumb onto a more
solid foundation.

			Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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