Digital Signal Processing Reference
In-Depth Information
Note:
In these Illustrations high precision bandpass filters in the form of purely digital
(computer-based) filters are used. The signal is transformed via an FT into the
frequency domain where all the unwanted frequency data are set at zero. After that
a re-transformation IFT into the time domain is carried out.
Mixing
As can be seen in the formation of pre-groups or basic primary groups the highest
demands are made on filter technology in the case of single sideband modulation. This
was always a problem in the history of radio technology, that is to filter out a station
accurately from the frequency band of the closely packed stations.
In actual fact
adjustable
filters would be necessary. They cannot, however, - also for
theoretical reasons - be realised with a constant bandwidth and quality. This is only
successful in certain frequency ranges with filters (bandpass filters) given a constant
conducting state region.
In traditional radio and television technology - everything will be different with digital
radio and TV technology - a trick is used which leads indirectly to an adjustable filter.
This works in principle as follows:
• By multiplication (of the whole frequency band by all transmitters) by an adjustable
oscillator frequency (carrier) the frequency band (difference formation) can be
converted to a lower range at will (intermediate frequency range)
• At the same time a second complete frequency band “ at the very top” arises by sum
formation which, however, is disregarded.
• In this intermediate frequency band (IF domain) there is a relatively high quality band-
pass filter installed. This is usually a quartz crystal or ceramic filter. With a certain
fine adjustment of the whole intermediate frequency band the desired transmitter lies
precisely in the conducting state region of the IF filter and is thus selected. It is then
demodulated and amplified.
• This controllable frequency conversion to an intermediate frequency range is called
“mixing”. It is, of course, also multiplication by an (adjustable) carrier, e.g. nothing
but the conversion process of a modulated signal into a different frequency range (IF
domain) and has, therefore, been given a name in radio and television technology.
Illustration 168 shows the simulation of a complete AM tuner as it is used in the medium
wave range. For the sake of simplicity a noise signal is used here as an aerial signal which
contains all the frequencies.
