On 6/26/2010 8:36 AM, Steve Rooke wrote: > Oz, > > On 27 June 2010 01:09, Oz-in-DFW <li...@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.
-- mailto:o...@ozindfw.net Oz POB 93167 Southlake, TX 76092 (Near DFW Airport) _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.