[time-nuts] SE880 GPSDO

[Bob Camp]

Hi

Well, based on about a hundred breadboards, it always comes out this way
regardless of who does the layout or what caps are used for a buffer amp. 
Consider that this is *not* an oscillator, so the only cap that is sure to be there is
a blocking cap.

Bob



> On Apr 26, 2016, at 5:22 AM, Attila Kinali <attila at kinali.ch> wrote:
> 
> 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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