[time-nuts] Trimble 34310-T EFC Question

[Bob Stewart]

Hi Dan,

I'm measuring with an HP 3456A using 1 meter of  unshielded twisted pair.  I do not have an LC filter on the EFC line.  The op-amp driver connects directly to the EFC port on the oscillator.  I spoke to someone else who is using the same board and he reports the same problem when attempting to measure the EFC.  His other equipment has been at least one generation newer than mine, so I assume the same for the voltmeter.

I've been giving some thought to putting an LC on it.  But since I can measure it without a problem at the input of the op-amp, that's gone to the bottom of my priorities list.

Bob





>________________________________
> From: Dan Kemppainen <dan at irtelemetrics.com>
>To: time-nuts at febo.com 
>Sent: Thursday, November 7, 2013 11:52 AM
>Subject: Re: [time-nuts] Trimble 34310-T EFC Question
> 
>
>FYI, if you are measuring the voltage with a fluke or similar meters,
>I've seen them throw a lot of noise onto a signal line. In many cases
>we've had to put an RC filter coming from the fluke to block the noise
>from the meter getting into the circuit!
>
>Also, you aren't running a KW Ham transmitter are you? Also, some CF
>bulbs are really noisy. I've also seen some PC's swamp out GPS receivers...
>
>Basically, I've learned the hard way to eliminate anything in the room
>first. Only then can you try to modify the circuit!
>
>Just some food for thought!
>
>Dan
>
>
>
>
>
>
>On 11/7/2013 10:41 AM, time-nuts-request at febo.com wrote:
>> Message: 1
>> Date: Wed, 6 Nov 2013 19:40:31 -0800
>> From: "John C. Westmoreland, P.E." <john at westmorelandengineering.com>
>> To: Bob Stewart <bob at evoria.net>,     Discussion of precise time and
>>     frequency measurement <time-nuts at febo.com>
>> Subject: Re: [time-nuts] Trimble 34310-T EFC Question
>> Message-ID:
>>     <CAFtYB4wY0QEFwc4G2E=F5fknP85EOfxijMAmvX8ir-LW9YHRcA at mail.gmail.com>
>> Content-Type: text/plain; charset=ISO-8859-1
>> 
>> Hello Bob,
>> 
>> If you think you are getting any high frequency into the EFC control - it
>> may be worth putting a(nother) low-pass filter in there.  Are you measuring
>> noise on that line?
>> 
>> Do you have a snap-shot of a schematic?  Or, can you take a picture of that
>> circuit?  I would hazard a guess you don't want to change the op-amp in
>> such a way the gain is too high - the circuit could be designed for unity
>> gain.  A different choice in resistors could still yield unity gain and
>> maybe knock down the noise a bit.
>> 
>> I would need to put an ammeter in the circuit to determine if there was any
>> meaningful current flow - without injecting noise of course... ;)
>> 
>> 73's,
>> John
>> AJ6BC
>> 
>> 
>> On Wed, Nov 6, 2013 at 7:26 PM, Bob Stewart <bob at evoria.net> wrote:
>> 
>>> > I think I may need to change the LPF feeding the EFC in my GPSDO to get
>>> > rid of dithering jitter.  Is there any point in adding a resistor and cap
>>> > in the EFC line, or do I need to go back and change the values in the
>>> > op-amp circuit feeding it?  Currently an op-amp directly feeds the EFC
>>> > pin.  I guess this boils down to something like: is there any meaningful
>>> > current flow in EFC circuit in this oscillator?  Or should I add them but
>>> > will I probably have to change the existing filter, as well?
>>> >
>>> > Bob - AE6RV
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