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SFDR and SNR

Hi, I want to relate the required SFDR (spurious free
dynamic range) of my Analog to Digital Converter
(ADC) to the required SNR of my system.

For example, if I want to communicate using 6-bit PAM
(64 point constellation) I would need about 40dB SNR
at the receiver. If the entire digital system is perfect
(eg floating point, or perfectly designed fixed point),
there is no receiver noise, and the only impairment
is the imperfect ADC, what SFDR would I need to
guarantee me this 40dB SNR at the decision device?

I understand SNR, but I dont understand SFDR so well.
 
A

Anonymous

Jan 1, 1970
0
SFDR and SNR really aren't related much. SNR has to do with the signal
energy versus quantization noise. Some ADCs are noisier than others so they
have an effective number of bits that is less than the actual number of bits
per sample to represent that the noise is higher than what one would expect
for just quantization, e.g. an 8 bit ADC may only have 7.2 ENOB. The SNR
then should be given by ENOB*6 or 43.2 in this case.

SFDR however has to do with spurs or images that are generated from a
sinusoid at the input of the ADC. You see when a sinusoid is input, the
quantization of some ADCs isn't really random it has a periodic structure
that creates another sinusoid at a different frequency and much lower
amplitude. In other words the quantization noise isn't spread across the
whole bandwidth it collects in tiny little "spurs" that track the frequency
and level of the input sinusoid. The SFDR then is the difference between the
input sinusoid level and the highest spur, e.g. an 8-bit ADC may have a
sinusoid input at -1 dBFS and a spur at -60 dBFS so the SFDR would be 59.

The energy in these spurs is very tiny and rarely matters for single signal
systems. The two cases when SFDR does matter is wideband digitization and
spectrum monitoring. If you want to take you ADC and digitize the entire HF
band for example such that you want to be able to tune within the ADC data
to pull our large and small signals the SFDR is important because the big
signals in the band will produce spurs that you will not be able to seperate
from real (but weak) signals. The other case is spectrum monitoring where
you want to see the entire band on an FFT display, for example. Again the
spurs created by the strong signals will appear like real but weak signals.
In that case you have to decide that every signal that is less than -SFDR
full scale is not reliable. Some systems even add wideband noise to wash out
everything below the SFDR just to make sure the operator can't confuse spurs
with real signals.

-Clark
 
OK, I think I got that. I am interested in wideband
digitisation. In that case we wont have a single spur,
we will have noise that is spread across the band.
Is that noise flat in frequency? In that case is the
SFDR not just the same as the SNR? If everything
below the SFDR is washed out, then this would
seem to me to be equal to the SNR.
 
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