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

Bob Stewart bob at evoria.net
Thu Aug 7 22:08:08 EDT 2014

Hi Bob,

I hadn't even considered a filter in the OCXO.  This isn't a 10811, but that's the OCXO I have a schematic of, so I'll assume that's the benchmark.  Following the EFC in, it looks like it goes to a 100K resistor and then tees to the 100pf varicap and a 15pf to the xtal.  Other caps are attached as well, but it doesn't look like it's bypassed to ground anywhere along the EFC line.  I see that there's another 100K to a 6.4V reference with a 6.8uf cap.  So, that means that the EFC line ranges from +6.4 to -6.4?  I haven't worked out the time constant, but that wouldn't seem to apply for a 10MHz signal riding on the EFC voltage.

Like I said, I don't have a 10811 on my GPSDO.  It's my faithful Trimble 34310-T.  Still, I would imagine that they at least looked at HP's design.

As to Hal's comment about probe pickup.  I was careful to specify the X10 position of the probe.  In the X1 position there was a signal that wasn't visible in X10, but should have been.  So, I assumed that was some sort of induced signal.  I'm using a generic cheap Chinese probe available on ebay.


 From: Bob Camp <kb8tq at n1k.org>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Thursday, August 7, 2014 6:11 PM
Subject: Re: [time-nuts] Effects of noise on EFC line? - Resolved


Your EFC line is probably bypassed internally to the OCXO. A 3db modulation bandwidth beyond 1 KHz is unlikely. A modulation bandwidth below 100 Hz is quite possible. 

Next thing to consider is that the EFC does FM on the OCXO. Phase noise is PM modulation. FM is 1/Fmod relative to PM. If I go up a decade in frequency with constant FM, my PM sideband will go down by 20 db. Yes that’s for small modulation indexes. That’s very likely the case if we are dealing with noise. 

You can calculate exactly what PM sideband in dbc you will get from a 1 Hz tone at 1 mV p-p on your EFC. From that you can pretty quickly work out what this or that number of microvolts will do at this or that frequency.  The answer is normally that the noise you have from a reasonable regulator or op-amp isn’t a big deal. 


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