[time-nuts] LP filter on EFC line

Richard Moore richiem at hughes.net
Sat Nov 1 02:26:10 UTC 2008


Bruce, thanks for the tip on the anti-charge injection LP filter.  
Actually, early on, I put a 0.1uF cap across the EFC line on the osc,  
support board, so there is some LP filtering working there. But my  
old Tektronix "prayer wheel" Circuit Computer says that's only a 5ms  
TC -- prolly need quite a bit more or else buffer the EFC line from  
the DPM.

Best,
Dick Moore


On Oct 31, 2008, at 5:56 PM, time-nuts-request at febo.com wrote:

> ------------------------------
>
> Message: 6
> Date: Sat, 01 Nov 2008 11:00:59 +1300
> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> Subject: Re: [time-nuts] OFC OCXO
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Message-ID: <490B801B.2060900 at xtra.co.nz>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Richard Moore wrote:
>> Hi Ernie -- thx for the feedback. I have had all of those effects and
>> more with an HP10811A and an EG&G Cinox version of the 10811A, so I
>> won't be surprised to see such things with this one either. I'm going
>> to try it with Bert Zauhar's FLL controller to see how it runs. You
>> can visit my experience to date at:
>>   www.moorepage.net/gps2.html
>>
>> I'll update the site again after I run the OFC for a while.
>>
>> Best,
>> Dick Moore
>>
> Dick
>
> The usual way of reducing charge injection transients from a DVM into
> the circuit being measured is to use an RC low pass filter between the
> circuit and THE DVM.
> Charge injection can be somewhat larger for those autozeroing DVMs  
> that
> do not use a precharge amplifier to reduce the effect.
> panel mount DVMs do not usually have a precharge amplifier.
>
> With a low pass filter it may be possible to reduce the value of  
> the 50K
> isolation resistor that you used.
> The value is relatively high because it relies on the DVM and wiring
> capacitance as the capacitive part of the RC low pass filter.
>
> Bruce
>



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