Multitone RPR 583, Technical Manual

Page: 37 / 91

RPR 583
TM1212 Issue 1 Page 3 - 5
Output from the RF amplifier passes through an image filter which provides these
functions:
a) Attenuation of the first image frequency.
b) Attenuation of signals outside the required band.
c) Matches the output impedance of the RF amplifier to the input impedance of
the mixer stage.
C406, L401 and CV407 form a parallel tuned circuit with the resonant frequency
adjusted for minimum loss at the frequency of operation.
C428, CV406 and L407 also form a parallel tuned circuit adjusted for minimum loss
at the frequency of operation. In addition, C428 and L407 form a series resonant
circuit with the resonant frequency close to the first image frequency. The complete
filter network gives approximately 40dB of attenuation at the image frequency.
The local oscillator comprises transistor TR410, crystal XL402 and associated
components. Frequency adjustment is by CV405. The local oscillator frequency is
calculated as follows:
carrier frequency - 45MHz
fx =
3
where fx is the crystal frequency in MHz.
The collector of TR410 is tuned to the third harmonic of XL402 by L411 and C459.
The circuit provides the correct Local Oscillator frequency and reduces the level of
2nd, 4th and higher harmonics. The two tuned circuits, L413, C463 and L415, C464
give further filtering of unwanted harmonics.
The mixer circuit, TR403 and associated components, receives the output of the
image filter. This is mixed with the local oscillator output to produce a 45MHz
product which is selected by the resonant circuit L406, C451. The 45MHz output
passes through a monolithic crystal filter, FL401, which has a bandwidth of ±7.5kHz
at the -3dB points.
The output from FL401 is fed to IC401 pin 24. IC401 is a combined mixer/ local
oscillator/IF/detector.
The second oscillator runs at 44.545MHz (45.455MHz for Band 4) and, with the
exception of XL401 and its associated components, is built into the IC. As the
second oscillator is not trimmed, small frequency errors are absorbed when tuning
the first local oscillator.
The input at pin 24 is applied to the internal second mixer, and is mixed with the
second oscillator output to produce a 455 kHz 2nd IF signal at pin 3. The IF signal
then passes through two ceramic filters FL402 and FL403 which remove unwanted
mixer products and provide further selectivity.