[time-nuts] Effects of noise on EFC line?

Chris Albertson albertson.chris at gmail.com
Tue Aug 5 16:00:17 EDT 2014


On Tue, Aug 5, 2014 at 11:36 AM, Bob Stewart <bob at evoria.net> wrote:

> I've considered switching the HCT out for a 74LS365, assuming my drive levels are compatible.  Unfortunately, I don't have one "in stock", and "I'm way out of my pay grade", as they say.  I've also thought about putting a 100uh inductor in series with the EFC line.  I wonder if I'll have to isolate the 74xx365 chip's VCC through an inductor?  Any thoughts?

Not just an inductor but a "low pass filter" (maybe CLC) designed
reject anything outside the bandwidth of your control signal.  How
often do you update the EFC?  Once per second? then your bandwidth is
0.5 Hz.     That EFC is very close to DC if your uP is only updating
EFC once per second. Your filter should be physically close to the
OCXO and have a very large time constant.  In addition to this I have
a 0.1uF cap soldered directly to the EFC and Ground pins of the OCXO.

You say you have detected 10MHz noise, but you can bet there is some 60Hz too.

About Vcc, I assume each chip has a bypass that is physically right on
the Vcc pin.  That is enough.
Well almost.  You need a digital and an analog power supply, sort of.
Don't power both off the same voltage regulator.  They can share a
transformer But make sure the digital stuff has it's own power rail
and it's own voltage reg.  Then you do the same with the grounds,
digital and analog and tie them together at one place near the power
supply.

Tuns out these GPSDOs are way-simple now. Just a few parts needed.
You can use a serial interface DAC to drive EFC.

Chris Albertson
Redondo Beach, California



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