[time-nuts] Re: uncertainty/SNR of IQ measurements

[Poul-Henning Kamp]

--------
Lux, Jim writes:

>I'm looking for a simplified treatment of the uncertainty of I/Q 
>measurements.  Say you've got some input signal with a given SNR and you 
>run it into a I/Q demodulator - you get a series of I and Q measurements 
>(which might, later, be turned into mag and phase).
>
>[...]
>
>I'm looking for a sort of not super quantitative and analytical 
>treatment that I can point folks to.

Good luck with that :-)

Some of the noise processes will be along the "vector" and distributed
between I & Q components depending on the phase, while other noise
processes affect the components individually.

To make matters worse, both kinds of noise processes may depend on the
phase, usually because of cross-talk and/or insufficient isolation.

Low-resolution ADC's are a particular nasty problem, because they add
+/-1 count jitter independent of the phase, and that causes very
large arctangent errors.

Counterintuitive as it may sound, it is easier to process the bits from
ADC's where the low two bits are pure noise, than ADC's where all bits
are good...

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