RPR 550IS Series
TM1188 Issue 1 Page 3 - 9
RF Amplifier
23. The two-stage RF amplifier has the main effect in achieving the sensitivity of the
receiver. The signal passes to TR301 and TR302 before being fed to the Surface
Acoustic Wave (SAW) filter via C307. Capacitor C305 and L304 match TR301 to
TR302.
SAW Filter
24. The SAW filter, FL301, has a bandwidth of approximately 100kHz and provides a
image rejection 910kHz above the channel frequency. The signal passes to mixer
TR303.
Local Oscillator
25. Transistor TR304 and associated components form the self-tripling Colpitts crystal
oscillator, with XL301 being the third overtone quartz element. The oscillator is
trimmed to the correct frequency by C314 before the output is fed to TR305 via C322.
The non-linear action of TR305 triples the frequency and the third harmonic (i.e. the
ninth harmonic of XL301) is selected by a filter network composed of L309, C323,
R315, C326, C329 and L305. These components also provide impedance matching
for FL301. The local oscillator output, 455kHz above the RF input, is fed to the mixer.
The frequency of the crystal is calculated by the following formula:
Crystal (XL301) frequency (MHz) =
Receiver channel frequency (MHz) + 0.455MHz
9
Mixer
26. The RF and local oscillator frequencies are fed to the base of mixer TR303 to produce
an IF difference frequency of 455kHz across L306, C312 and R308. The IF signal is
fed, via SK301 pin 3, to the main board.
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