SM 5 BSZ - Flat system bandwidth vs digital bandwidth.
(Jan 24 2001)

Alias signals.

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