Gary J FollettDukes HiFi wrote:
> How is it that a 16 bit A to D can now handle a
> dynamic range of 132 dB (in band)?
A perfect 16-bit ADC would have a difference of 97.8 dB
between the average noise level and the level of the full-scale
sinusoid. This is a basic fact, and it is NOT violated by ADCs
used in SDR receivers. In fact, a real-life 16-bit RF ADC
has substantially less than 97.8 dB.
Under certain conditions, not very difficult to fulfill,
ADC noise spectrum density is uniform, i.e., there is
the same average noise level in each Hz of the ADC output.
Now, if the ADC output goes through a filter with
the bandwidth smaller than half of the sampling
frequency, the average noise power AFTER the filter
will be decreased in proportion to bandwidth reduction.
For example, if the bandwidth gets reduced from 50 MHz
to 500 Hz, i.e. 100,000 times, the resulting average noise
level will be decreased by 50 dB.
So, one might say that the ADC dynamic range got increased
from 97.8 dB to 147.8 dB (minus imperfections).
Such an interpretation would not be correct if the essential
factor of bandwidth reduction is omitted.
73,
Sinisa YT1NT, VE3EA
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