[time-nuts] EFC tracking

[Didier Juges]

Steve,

You may want to check the "Analog Devices MiniKit for ADuC702x-series".

http://www.google.com/search?q=Analog+Devices+MiniKit+for+ADuC702x-series

This kit includes a 24 bit ADC and integrated ARM processor in a small PWB with all the tools and sample code to do what you want with very little code to write (you can probably use the sample code as-is).

The kit is $30 (or $35, depending on where you look...) and you will easily spend that much building something that will not work as well using your sound card.

Sound card ADCs are intended for audio, and I'll bet their linearity does not come close to that of the ADuC702x series, if you can even get the spec for it.

Didier 

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Steve Rooke
> Sent: Saturday, June 26, 2010 7:13 AM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] EFC tracking
> 
> I would like to track the EFC voltage in hardware using 
> something cheap and ready to hand. I was thinking of using a 
> sound card as it has good resolution but it's obviously only 
> AC coupled so it would not measure the DC of the EFC. I 
> thought about modifying a sound card to make it DC coupled 
> but most of them seem to reference the 0V point to some 
> internal reference voltage hence there is a DC shift there. 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 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 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). If you have any suggestions or ways 
> of doing this in a better way, I'd be very grateful for the advice.
> 
> Thanks,
> Steve
> --
> Steve Rooke - ZL3TUV & G8KVD
> The only reason for time is so that everything doesn't happen at once.
> - Einstein
>