[hpsdr] WG: New Hermes 2M Board

[Helmut Oeller]

Hi Isaac,

thanks for your comments. This gives the chance to cancel some myths about
ADCs. I'll try to do this comparing (my favorite) 75 USD dual 14 Bit, 125
Msps AD9645 with the 300 USD dual 16 Bit, 310 Msps AD9652 you mentioned.
Both ADCs are produced by Analog Device and the following data are gathered
from their data sheet.
It's not enough or tolerable to compare only the nominal number of bit of
the ADC. You have to compare the effective number of bits (ENOB) specified
in the data sheet. We'll find ENOB of AD9645 @70 Mhz 12,0 bits while the
expensive AD9652 offers @70 MHz 12,1 bits, i.e. 0,1 x 6.02 dB = 0.6 dB more
DR is no significant benefit.
Let's compare the spurious free dynamic range SFDR at this frequency (middle
of HF and considered undersampling bands 70/144 MHz): AD9645 is specified
with 91 dBc, while the 16 bit AD9656 can only come up with 89 dBc.
Same story occurs with SNR (74 dB both) and crosstalk between the two
channels (-97 dB vs. only -90 dB of the AD9652!). Moreover the difference in
power consumption is dramatically: the 14 bit AD9645 needs 0,24 W, while the
fast AD9652 consumes 2,16 W.

It's evident that there is absolutely no reason for that fast, hot and
expensive 16 Bit ADC.

73, Helmut, DC6NY