Hi The 10811 is very unusual in that the EFC is not bypassed. It’s still got a modulation bandwidth. The important part is that the EFC has an inherent “lowpass" due to FM to PM conversion.
The EFC line on the 10811 should be bypassed to ground at the socket with at least a 1000 pf cap. Yes that’s not in the little manual. It’s the only way to knock out all of the RF tune effect on the EFC line. Of course if you are driving it with an op-amp that will terminate. Bob On Aug 7, 2014, at 10:08 PM, Bob Stewart <[email protected]> wrote: > 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. > > > Bob > > > > ________________________________ > From: Bob Camp <[email protected]> > To: Discussion of precise time and frequency measurement <[email protected]> > Sent: Thursday, August 7, 2014 6:11 PM > Subject: Re: [time-nuts] Effects of noise on EFC line? - Resolved > > > Hi > > 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. > > Bob > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
