Hi If you are going to attenuate the EFC *and* center it up around 3V you will also need a voltage reference. Even a good one has a TC. You also have oven current running in the ground pin which contributes to voltage issues (oven and EFC share a common ground). Past that, you have thermocouple issues (resistors, wires, OCXO pins are different materials) ….
Bob On Jan 31, 2014, at 4:18 PM, [email protected] wrote: > Is the op amp just a unity gain buffer or does it have any gain setting > resistors that might be adjusted? > > I'm just wondering if you could tailor the gain to reduce the swing rather > than require the oscillator to effectively require a larger swing. > > The 344310-T is likely to have come from a Trimble Nortel NTGS50AA, that > might even be the only source although I don't know for sure, and the EFC > swing on those runs from 3 to 6 volts, or at least that's the hunt range, so > the 344310-T needs to tune to 10MHz with an EFC somewhere within that range > and that would have been a design or selection restraint on the oscillator > when new. > > Older units are likely to be closer to the top of that range and I know > some have exceeded 6 volts whilst still seeming to be otherwise ok, so it's > probably a fair bet, with that oscillator for example, that you wouldn't > need to drop below three volts anyway. > > Obviously other oscillators could have different requirements and the pot > arrangement certainly makes it a bit more versatile, but well worth leaving > that out if possible, and perhaps part of that "moderate effort" might be > for others to characterise their oscillator first and then adjust a few > values accordingly. > > Regards > > Nigel > GM8PZR > > > > In a message dated 31/01/2014 20:19:51 GMT Standard Time, [email protected] > writes: > > I'm trying to put something together that is generic enough for the guys > on the VE2ZAZ GPS Standard board, but that will be relatively free of > temperature issues. Yeah, I know: an impossible task. The precision of the > > GPSDO is probably not in the same league as what this board is used to. The > DAC is actually a 10-bit PWM with 4 bits of dither. The output from the > board comes from an op-amp. The OCXO (for me) is a Trimble 34310-T. Its > range across 0 to +5V is about 7.5Hz. I'm trying to reduce that to 2Hz. In > spite of those limitations, it works well. I'm just trying to wring every > last bit of performance out of it, but in a way that my results could be g > enerally repeatable by someone who was willing to make a moderate effort. > > I didn't have any problem with finding 5PPM/C resistors for the 20K and > 6.8k. The others were a different story. I found a 15PPM for the pot, and > 5PPM and 10PPM for the others in values that should be close enough. I > think I'll look around on Mouser a bit more before I press the buy button, > but > this is probably what I'll wind up getting. > > > Bob > > > > >> ________________________________ >> From: "[email protected]" <[email protected]> >> To: [email protected] >> Sent: Friday, January 31, 2014 1:18 PM >> Subject: Re: [time-nuts] EFC divider resistors >> >> >> It's hard to be sure without knowing the characteristics of the DAC > ouput >> and the oscillator EFC, but if you're only making a one off I wonder if > it >> might perhaps be possible to simplify that arrangement to just a > potential >> divider using selected values and being fed from the DAC and tapped off >> into the EFC? >> >> Regards >> >> Nigel >> GM8PZR >> >> >> In a message dated 31/01/2014 18:04:39 GMT Standard Time, [email protected] > >> writes: >> >> I just realized that I have a pot in the mix. I see one on Mouser with >> 25PPM/C, so I suppose I'll try that. Here's the circuit I'll use with > the >> replacements. What I have in there now has the values of R3, R4, and R5 >> multiplied by 10 and R2 is 3.9K. I suppose it won't matter so much for > the >> pot. >> >> http://www.evoria.net/AE6RV/GPSstd_PLL/EFC%20Divider/EFC2.png >> >> Bob >> >> >> >> >> >>> ________________________________ >>> From: Robert LaJeunesse <[email protected]> >>> To: Bob Stewart <[email protected]>; Discussion of precise time and >> frequency measurement <[email protected]> >>> Sent: Friday, January 31, 2014 11:40 AM >>> Subject: Re: [time-nuts] EFC divider resistors >>> >>> >>> >>> Jellybean resistors can have a tempco in the 100s of PPM per degree C. >> Some precision resistors are as low as 25 PPM/C while really good > resistors >> can be had at 5 PPM/C. Better yet look for a resistor array where part >> tracking is called out. Some arrays get below 5 PPM tracking so the > division >> ratio holds much better than the absolute value. (I find the DigiKey > part >> search tool helpful in finding the low PPM parts I want.) >>> >>> >>> Another possibility is an integrated resistive divider like the Maxim >> MAX549x series. The have pre-trimmed dividers in a number of ratios and >> values, typically with 35 PPM/C absolute tempcos but ratiotempcos as low > as 2 >> PPM/C. >>> >>> >>> Bob LaJeunesse >>> >>> >>> >>>> ________________________________ >>>> From: Bob Stewart <[email protected]> > >>>> To: Discussion of precise time and frequency measurement >> <[email protected]> >>>> Sent: Friday, January 31, 2014 12:01 PM >>>> Subject: [time-nuts] EFC divider resistors >>>> >>>> >>>> I put a divider network in the EFC line of my GPSDO to restrict the > OCXO >> range to 2Hz. Now I'm seeing heat-related drift that wasn't apparent >> before. I put a 20K resistor from the same strip on my 3456A, and the > warmth >> of holding it between fingers moves it by about 50 ohms. What type of >> resistors should I put in there? Or am I chasing a problem that doesn't > exist? >> Totally out of >> my league here. >>>> >>>> Bob - AE6RV >>>> >>> >>> >> _______________________________________________ >> 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. >> >> >> > _______________________________________________ > 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. _______________________________________________ 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.
