[time-nuts] EFC divider resistors

Bob Camp lists at rtty.us
Fri Jan 31 21:50:43 UTC 2014 

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, GandalfG8 at aol.com 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, bob at evoria.net  
> 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: "GandalfG8 at aol.com" <GandalfG8 at aol.com>
>> To:  time-nuts at febo.com 
>> 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, bob at evoria.net  
> 
>> 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 <rlajeunesse at sbcglobal.net>
>>> To:  Bob  Stewart <bob at evoria.net>; Discussion of precise time and  
>> frequency  measurement <time-nuts at febo.com>  
>>> 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 <bob at evoria.net>
> 
>>>> To: Discussion of  precise  time and frequency measurement  
>> <time-nuts at febo.com> 
>>>> 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
>>>> 
>>> 
>>> 
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