BEC BEC
SPEECHEN
RPR 550IS Series
TM1188 Issue 1 Page 3 - 11
IF Amplifier, Filters and Discriminator
28. The 455kHz IF signal passes from the radio board to PL1 pin 3 and then to the first
455kHz bandpass filter FL1. The filtered signal enters IC1 via pin 1, where it is
amplified before passing to the second 455kHz filter FL2, via pin 4 and C13. The
signal re-enters IC1 at pin 5 for further amplification and limiting before passing from
pin 13 to the quadrature discriminator circuit L2, R23. Capacitors C11, C12, C14, C15
and C42 act as decoupling capacitors to IC1.
Audio and Data Processing
29. The audio signal leaves IC1 at pin 14 and goes to the de-emphasis circuit R26, C16
from where it splits into the following paths:
a) Data
To the input of the data filter. The data filter TR11a, is enabled by analogue gates
TR31 and TR11b when operates. A low signal turns TR31b off, causing
its collector to go high. This enables TR31a whose collector goes low enabling
TR11b. This collector goes high, enabling data filter TR11a and allowing data to
pass to TR29 which is also enabled. Data then passes to pin 15 of IC1 for
squaring. On leaving IC1 via pin 16 the data is inverted by TR12a. This ensures
that the overall data sense of the receiver is the same as the non-IS product.
b) Speech
To the input of the speech amplifier. The audio signal passes via C57, R32 and
C46 to the audio power amplifier stage which consists of the DC coupled amplifier
TR10a, TR1 and TR2. Transistor TR10a is enabled by a signal from
pin 8 of IC3. TR1 and TR2 subsequently conduct and speech is generated from
the loudspeaker. R93 and R94 are IS components and provide infallible isolation
of the power available from R61 to the speaker and driver from the rest of the
circuitry. Capacitor C62 allows a small amount of beep signal to be passed to the
speaker to increase the beep volume.
DC Feedback
30. The DC operating point of the output is set to half B+ by TR10b, R52, R55 and C36.
The varying collector voltage of TR2 alters the bias on the base of TR10a through the
DC feedback circuit R57, C40 and R56. The degree of conduction of TR10a changes,
keeping the DC level constant and stabilising the operating point.
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