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

Bob Stewart bob at evoria.net
Wed Aug 6 19:18:56 EDT 2014

Thanks to everyone for their comments, and especially to Bill, WB6BNQ for cast a jaundiced eye on my schematic.

The problem turned out to be in the design of my 5V rail on the PCB.  There is a 2.5 inch run from the 5V regulator over to the hex driver (74HCT365) that outputs two signals of either 1MHz or 5MHz, as well as four 10MHz outputs.  About .5" before the hex driver, I tapped off 5V to power both a thermistor and the output op-amp.  I hadn't counted on the impedance of about 2.5" of trace at 10MHz, and the noise the hex driver would put on it.

What I did was cut the tee at both ends, and put a  jumper from  the thermistor & op-amp directly to the 5V regulator.  The 10MHz noise is now gone.  There is some noise from the DAC output that looks to be at the PIC's clock frequency, but that was filtered out by the voltage divider and .1uf cap at the EFC pin of the OCXO.

I did learn something interesting, though.  The Vref output of the OCXO (Trimble 34310-T) has about 4mv of 10MHz on it.  That was also cleaned up by the voltage divider/corrector.  So, I may throw another cap on it, but it seems to be clean down to what I can measure at the OCXO on my old Tek 455 with an X10 probe.


 From: Bob Stewart <bob at evoria.net>
To: Time Nuts <time-nuts at febo.com> 
Sent: Tuesday, August 5, 2014 1:36 PM
Subject: [time-nuts] Effects of noise on EFC line?

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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