## Comments on the RF Space SDR-14 Receiver

### by Pieter Ibelings N4IP

The SDR-14 uses a 14bit ADC with a SNR of about 74 dB. This SNR is equivalent
to about 12.3 bits. This SNR is based on the whole Nyquist band of 33.333 MHz
when sampling at 66.666 MHz. In other words, the equivalent noise power is about
-150 dBc/Hz , or 74+10*log_{10}(33333333 Hz).

The SDR-14 uses a digital down-converter
(DDC)
processor chip that tunes a narrower bandwidth within this 33.333 MHz band. When
the DDC processor is set to 100 kHz, for example, there is a
"processing gain" of 10 log_{10} (33.333/0.1) = 25.2 dB, yielding a total of
99dB. Of course, one can reduce this bandwidth even more and get more dynamic
range. The improvement is limited to the internal math precision of the
post-processor which is 23 bits (138 dB).

A Google search on "digital down conversion processing gain" will yield more
information.

So for the case of a 1 GHz ADC that has 2 effective bits, we end up with an SNR
of 12dB over 500 MHz. If we go in with a DDC processor and filter 100 kHz of
bandwidth, we end up with only -10 log (500/0.1) = -37 dB of that noise. If we
keep our math precision high, we will have 12 + 37 = 49dB of SNR. It would be
the same as having a 200 kHz ADC with 8 bits.

Copyright © 2005 Pieter Ibelings N4IP. Last updated:03/01/08