[time-nuts] EFC divider resistors
GandalfG8 at aol.com
GandalfG8 at aol.com
Fri Jan 31 16:18:47 EST 2014
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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