SM 5 BSZ - Flat system bandwidth vs digital bandwidth.
(Jan 24 2001)
When the A/D converter samples at a frequency F (8kHz for example)
the digital signal will be the same if a frequency of F/2-x or
F/2+x is fed to the input. (3.5kHz and 4.5kHz give the same
digital output if F=8kHz). This is the well known alias problem.
Audio boards have built in filters to get rid of the alias
frequency but the suppression is far below what can be accepted
for a radio. The alias signal is a spur, and we want spurs to
be suppressed somewhere in the 60 to 100dB range.
Frequencies above the passband (4.5kHz in the above example) can
be suppressed in the IF filters or by adding low pass filters in
front of the A/D converter.
BFO mirror images
The audio boards work down to very low frequencies but in
case a single audio channel is used for each radio channel
it will not be possible to use very low frequencies.
If the IF passband is from say 10.7 to 10.703 one might place
the BFO 500Hz below 10.7 at 10.6995 in order to have enough suppression
by the IF filters for signals below 10.6990.
The useful passband will be 500Hz to 3500Hz or 75% of the
digital bandwidth when conventional filters are used.
Using many X-tals it is possible to get 90% bandwidth
with very good spur suppression as shown here
Analog hardware for the MSDOS dsp system
Using direct conversion, two channels for each radio channel
one will not need any X-tal filter.
It will be nessecary to use two low pass filters instead,
one for I (the in phase signal) and another for Q (the quadrature
signal, 90 degrees off).
Simple filters again give 75% bandwidth but it is possible to
get 90% and above as shown here:
Spur free dynamic range of
direct conversion receiver