[time-nuts] EFC tracking

Didier Juges didier at cox.net
Sat Jun 26 20:04:10 EDT 2010


I am not sure how to translate the IMD specs into integral or differential non-linearity, but from what I have seen, IMD specs are not significantly better for 24 bit sound cards than for the older high-end 16 bit models, when high-end 16 bit models were available. Noise specs are better, and that's about it.

In sound cards as in many other consumer products, customers equate more bits to higher quality and today you cannot buy anything but 24 bit cards, regardless of the actual improvement obtained. I am not sure the actual specs reflect the higher number of bits. I believe Bruce has quite a bit of data on that.

What I know is that the standalone A/D converters like the AD parts referenced in the other emails have outstanding linearity specs (at low bandwidth of course) at a price well below that of a "high quality" sound card, so even if the specs were the same, it would still make more sense to use the external device to measure a slow moving DC voltage.

Didier


------------------------ Sent from my BlackBerry Wireless thingy while I do other things... 

-----Original Message-----
From: "John Miles" <jmiles at pop.net>
Sender: time-nuts-bounces at febo.com
Date: Sat, 26 Jun 2010 15:25:28 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] EFC tracking


> >sound card ADCs, the high end 24 bit ones, are pretty darn linear [...]
>
> That is actually a very debatable proposition, a lot of them are
> tracking types that conveniently cover up any lack of linearity
> on the analog side of the fence.

Can you elaborate on that?  Linearity is linearity - wouldn't it ultimately
show up in the card's IMD specs?

> The major problem with using sound ADC's is that their references
> has absolutely no long term stability, so you will see your EFC
> wiggle and wander all over the place, even when it stays perfectly
> still.

One idea: the sound card gives you at least two input channels to work with,
and it isn't much of a stretch to assume they share a common reference.
Perhaps you could use one channel to digitize the oscillator's supply
voltage or (ideally) EFC reference voltage output, using the same
chopper/mixer/VFC/whatever approach, and have the software take the
difference out.

> I would find one of those cheap-ish DVM's with a serial or USB port...

I'd be surprised if a cheap DVM outperformed a decent sound-card ADC in any
respect, other than perhaps DC baseline drift.

-- john, KE5FX


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