[time-nuts] EFC tracking

Oz-in-DFW lists at ozindfw.net
Sat Jun 26 13:49:21 UTC 2010

On 6/26/2010 8:36 AM, Steve Rooke wrote:
> Oz,
> On 27 June 2010 01:09, Oz-in-DFW <lists at ozindfw.net> wrote:
> On 6/26/2010 7:12 AM, Steve Rooke wrote:
>>> <Deletia>
>> I've done similar stuff in work projects, but never written code.  I've
>> thought about this some as well.  I'd consider a few things;
>>   1. Use the sound card output as the chopper control signal instead of
>>      the discrete unit.   You'll have more control and phase sync will
>>      be easier.
>>          * I'd be temped to take the sound card output and run it
>>            through a comparator to square it up, but I'm almost certain
>>            this isn't needed.
> Sorry, not sure what you mean here. Are you saying that I should
> derive the chopper frequency directly from a connection to the sound
> card? I was hoping not to modify the sound card in any way so as to
> keep it simple.
Soundcards have inputs and outputs.  You can feed the output with a
series of samples that represent your control waveform.  The PC becomes
the oscillator and you know it's frequency and relative phase track. 
>>   5. The probelm with chopping is that signal levels around zero don't
>>      have much amplitude and are a challenge to extract from noise.
> I was under the impression that this was the idea that is used to
> amplify very low level signals like the output from the likes of
> strain-gauges. It would surely seem to me to be a problem to amplify
> small signals around zero due to offsets in the amp unless you do this
> sort of thing.
Chopping is used to cancel DC offsets in imperfect amplifiers, it adds
no gain. If there is a DC component and you filter with a cutoff
frequency below the chop rate, the offsets of the amplifier can be
effectively canceled. 

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