[time-nuts] EFC tracking

[Steve Rooke]

On 27 June 2010 01:40, jimlux <jimlux at earthlink.net> wrote:
> J.D. Bakker wrote:
>>>
>>> I wonder if anyone has done something like this before and could share
>>> their experiences.
>>
>> The general principle should work. However, as you're interested in slow
>> changes, there are some error sources that might be unacceptable, including
>> the drift of (differential) channel resistances for the 4066 over
>> temperature, voltage and time. As shown the scheme is also sensitive to
>> impedance mismatch/drift on the two inputs. Charge injection is a bit on the
>> high side on a 4066; a more expensive (A)DG4xx-series chip may improve on
>> that.
>
> Or the traditional chopper approach of a mercury wetted reed relay?
> If you're processing with a sound card, you have the advantage that you
> don't need to process the samples coming from the time of transition (unlike
> a traditional analog chopper with synchronous detection), so a fairly crummy
> relay would probably work.  The key is that it can toggle at, say, 100Hz,
> forever.

I really hadn't thought of factoring in the resistance of the 4066 or
environmental effects on it I must admit. I wonder how long a mercury
wetted reed relay would last at 100Hz though. The sampling rate would
be much higher than this to discard the transition readings.

>>
>> I don't know if it qualifies as simple/cheap, but Analog Devices and
>> others have single chip low-rate sigma/delta converters with good to
>> excellent properties; these were meant for strain gauges but should be able
>> to track slow-moving control voltages just fine. Interfacing them to a
>> parallel port (or USB PP adapter) should be close to trivial. Do have a
>> close look at the data sheet: some parts have unbuffered inputs, and present
>> a fluctuating input impedance which might couple onto EFC lines. A simple
>> isolation amp with one or two precision op-amps should fix that.
>
>
> The eval board for the part may have a computer interface built into it.

So I need to locate this if possible, any pointer please?

Thanks for your help,
Steve

>
>
>
>
>>
>> JDB.
>> [had just been looking into this for a transistor matcher/noise test rig
>> I'm working on]
>
>
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-- 
Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein