Multitone RPR 561, Technical Manual

Page: 39 / 109

RPR 561
TM1200 Issue 1 Page 3 - 4
Receiver
10. A block diagram of the receiver is shown in Figure 2.
11. Receiver antenna AE302/303 is tuned by CV3, CV5, C103 and C105. Signals from the
antenna are fed to the cascode amplifier TR35,TR55, to provide a high Q output.
Capacitor C106 decouples TR35 collector for stability. The output at TR55 collector is
tuned by L10, CV2, C192, and filtered further by L11, CV4 and C98, before application
to the first mixer TR32. Coupling between the two stages of filtering is provided by C1,
C2.
12. The local oscillator comprises TR31, XL11 and associated components. Frequency
adjustment is by CV1. The local oscillator frequency is calculated as follows:
fx = carrier fre 0.455MHz
where fx is the crystal frequency in MHz.
13. Output from TR31 emitter is fed to the second mixer TR32, where it mixes with the
incoming RF signal to provide intermediate frequency of 455kHz. Resistor R360 on
TR32 collector provides matching for the 455kHz ceramic filter FL3.
14. Output from FL3 is amplified by TR25 before application to the second 455kHz filter
FL4. Output from FL4 is then applied to the first amplifier in IC1 before application to
a third 455kHz filter FL2. The signal is then fed back into IC1 for further amplification
and application to the internal quadrature discriminator. A 90
E
phase shift to the
discriminator's second input is provided by ceramic discriminator FL1.
15. Demodulated audio is then output at pin 14 where it is split three ways thus:
a) Received audio (RXAUDIO)
b) Noise Squelch
c) Data Filter
16. The received audio is fed through de-emphasis network R339, C83, and then to the
high-pass filter TR29. This stage removes low frequency noise from the signal. From
this point the audio signal is fed to the Decoder Board for further amplification and
processing.