2. 100 dB dynamic range of DSP. I'm not going to search for the exact
source but this is often quoted by manufacturers as the practical limit of
current DSPs using 24-bit ADCs. Theoretically it should be higher based
only on 24-bit ADC resolution but practically it's limited by other factors.
All hybird heterodyne/DSP rigs such as Orion, K3, FT-2000/9000 and
IC-7700/7800 have about the same limit. I suppose if we get 32-bit ADCs
this limit could increase.
We get about 6 dB per bit of ADC resolution in a perfect ADC. So a
24-bit ADC should give us 144 dB of dynamic range, 144 dB S/N, etc. In
practice, we have non-linearity, jitter and other forms of noise that
reduce the obtainable accuracy.
Pro-audio ADCs work the best. At the time the K3 was designed, the best
ADCs provided about 103 to 118 dB of S/N under laboratory conditions.
The device we chose is rated at 112 dB. To achieve this requires
careful design of all the circuitry that interfaces the ADC to the
analog signals.
Some ADCs now get S/N in the mid 120-dB range. Use of such devices
entails significant additional cost to the radio, but with *very* little
benefit.
Why?
Other important ADC characteristics -- distortion, dynamic range --
limit even these to something in the range of 102 to 108 dB. This is
exactly the same as the ADC we use in the K3. Note that these limits
are of the ADC itself, not limits of the radio passed on to the ADC.
Thanks to its modular construction, the K3 is designed to be able to
take advantage of advances in ADC technology.
73,
Lyle KK7P
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