[time-nuts] EFC tracking

Steve Rooke sar10538 at gmail.com
Sat Jun 26 13:36:27 UTC 2010


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 next thought about turning the DC into AC by chopping it, IE.
>> inverting 50% of the voltage via an oscillator. This way I could pass
>> the square wave directly into an unmodified sound card, take
>> measurements and then do an RMS calculation on them (really just need
>> to flip the sign on, say, the negative readings).
> 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.

>   2. Buffer the input so that your waveform is not so dependent on
>      source impedance.

Good idea, thanks.

>   3. Make the input buffer differential so that you can get some small
>      amount of ground isolation and CMRR

If you look closely at it you can see that it is a differential input.

>   4. look at the 4053 mux, it might make your interconnect life easier.

Thanks, will do that.

>   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.

>   6. If you mix (in the RF receiver sense, not sum in the audio studio
>      sense) rather than chop the DC offset becomes a phase shift,
>      generally pretty easy to calibrate for and decode from the output
>      samples of a sound card. See
>      http://en.wikipedia.org/wiki/Frequency_mixer

Interesting idea, hadn't thought of doing it that way but it's a good
idea, thanks.

>> I wonder if anyone has done something like this before and could share
>> their experiences. I've attached a diagram image (hope it is accepted
>> by the list) which is my first go with Eagle so I'm not exactly very
>> familiar with it, sorry. The R's and C's in the astable would be set
>> to a clock frequency that enables this to work without bias given the
>> sampling frequency. I'm not sure if this clock should be slower than
>> the sampling frequency or higher, just haven't got my head around that
>> yet.
> The clock needs to be much higher than the highest frequency of the
> input waveform to keep Nyquist happy and things simple.  You can do this
> inband, but you don't want to.
> If you chop very close to half the soundcard sample rate I suspect
> you'll get no output because you'll be in the roofing filter cutoff and
> your waveform will integrate to zero.  I suspect you want to be 5 - 10X
> below that to make waveform recovery easier, and even lower is better.
> So, if you use a 44.1 ksps default rate, Nyquist is 22.05.  I'd run the
> chopper at less than 1 kHz. The good news is that your input waveform
> period is hours (maybe ~100 microhertz) and chopping at 1 Khz will make
> 100 Hz response easy and 500 Hz possible with great care and some effort.

Right, that makes sense, thanks.

>> The R's around the op-amp would need to be set in a ratio that
>> transforms the EFC voltage into the range that the sound card can
>> handle (that is yet to be calculated by measuring the limits).
> Most sound cards I've seen are ~ 1V pk to peak, though some are MUCH
> higher.

Gives me a ball-park to start work with, thanks.

>> If you
>> have any suggestions or ways of doing this in a better way, I'd be
>> very grateful for the advice.
> It's worth exactly what you've paid for it...

And worth every penny :)


>> Thanks,
>> Steve
> Oz
> --
> mailto:oz at ozindfw.net
> Oz
> POB 93167
> Southlake, TX 76092 (Near DFW Airport)
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Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein

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