Рубрика:
26
R.F. OSCILLATOR MODULATOR
MODULATING
SIGNAL
ARIAL
Fig. 1. Basic radio transmitter block-diagram
The basic туре of a receiver is a tuned radio frequency, however, this is
rarely used. The standard receiver configuration is the superheterodyne
(superhet) shown in fig. 2.
MIXER DETECTOR OUTPUT
LOCAL OSCILLATOR
Fig. 2. Basic superheterodyne receiver block-diagram
The desired radio frequency (r. f. ) is converted to a constant intermediate
frequency by taking the difference frequency after mixing the received signal
with the output from a local oscillator. Since most of the amplification and
selectivity is provided by constant frequency and bandwidth stages, the design
problem is eased.
In both the transmitter and the receiver, r. f. oscillators have to be tuned to
different frequencies. In the transmitter it is the master oscillator, while in the
receiver it is the local oscillator. Modern practice is to use a frequency synthesizer
with a single crystal to provide stability and accuracy.
Text 2 (D). Current and Future Developments of Radio Systems
Changes in aircraft radio systems occur more and more frequently. The
first airborne radio equipment
1
used thermionic devices
2
, cat’s whisker detectors
3
and large parallel plate tuning capacitors
4
; power, weight and size were
restrictions on the development of such equipment. In the 1950s transistorized
equipment began to appear although not completely transistorized. Even now
thermionic devices are still with us in the shape of the magnetron and the
cathode-ray tube. Claims concerning all solid-state weather radars were made
about mid-1979, and commercially available equipment appeared in 1980.
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