On Thu, 30 Aug 2007 22:46:46 GMT, Owen Duffy wrote:

It is interesting in marketing hype that reference is made to 2 digit and
3 digit instruments, which implies a log based metric (10*log
(MaxReading)) when you assume a 'full count', and the same hype refers to
the upper digit if it can only have values of 0 or 1 as half a digit,
whereas it probably has a weight of log(0.5) or 0.3... so in utility
terms, a 2 1/2 digit instrument is really a 2.3 digit instrument.

Hi Owen,

There are also 3000 count meters.

In my case, I was making the measurements straddling 200mV, so I needed a
bit of headroom for outliers, say 1dB or 225mV fsd, so it was effectively
2.35 digit instrument if you followed that argument.

Certainly, but I abandoned multimeters to general utility long ago and
went straight to my own designs for known precision. The common sound
card will give you 65000 count readings; and there is a world of
higher ADCs up to at least 16 million count readings.

Nevertheless, the error introduced by the resolution issue and instrument
accuracy does not explain the experimental results... something else is
happening, and one needs to look beyond the instrument itself to form a
realistic view of measurement uncertainty when measuring narrowband
noise.

You get non-monotonicity, quantization error, sample hold time errors,
codec error, issues of conversion errors through flash, successive
approximation, or single/dual-slope methods.

It would be simpler to handle the noise power in the linear domain,
and do the conversion to digital late in the chain (if at all).
Getting out into the hundredths of dB resolution (outside of the
standard 1KHz product lines) drives you into building your own
solution.

Linear circuits in the AF arena have long managed 6, 7 and sometimes 8
place resolution. You might have to twist as many knobs to get the
reading, but you also control the variables. Bolometery solves a lot
of complexities (but that is where this topic started - after a
fashion; and in that regard, optical pyrometry might be summoned up).

73's
Richard Clark, KB7QHC