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

Jim Harman j99harman at gmail.com
Tue Aug 5 15:16:59 EDT 2014

I was able to quiet things down a lot by putting a 100 ohm resistor in
series with the 74HC output. If these guys drive more than a few inches of
wire, they ring like crazy. Also make sure you pay close attention to the
bypassing of the 5V supply. Make your bypass cap lead lengths as short as

On Tue, Aug 5, 2014 at 2:36 PM, Bob Stewart <bob at evoria.net> wrote:

> I've run into a noise problem on the EFC line of my GPSDO engine at the
> frequency of the oscillator.  I've traced the source down to the 74HCT365
> I'm using to output the 1(or 5)MHz and 10MHz signals.  When I pull it, the
> EFC quietens down a lot.  I'm seeing about 50mv of 10MHz noise at the
> output of the op-amp that feeds the EFC voltage divider at the OCXO.  The
> voltage divider is corrected by the VRef from the OCXO with a simple
> circuit using temp-co'ed resistors.  On the 0.1uf cap at the OCXO's EFC
> pin, I'm seeing about 5mv of 10MHz signal.
> 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?
> I'm also wondering what the impact of this level of on-frequency noise
> will be?  Is the impact somewhat mitigated, since it's at oscillator
> frequency?  I don't have anything better than an HP 8558B to look at the
> output of the board.
> I'm not quite ready to generally share my schematic with the list, but I
> can make individual exceptions.
> Bob - AE6RV
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--Jim Harman

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