[time-nuts] EFC tracking
lists at ozindfw.net
Sat Jun 26 13:49:21 UTC 2010
On 6/26/2010 8:36 AM, Steve Rooke wrote:
> On 27 June 2010 01:09, Oz-in-DFW <lists at ozindfw.net> wrote:
> On 6/26/2010 7:12 AM, Steve Rooke wrote:
>> 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
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