Multitone BSR 113, Техническое руководство, Страница: 18 / 63

BSR 113
TM1194A Issue B Page 3 - 4
First Mixer
6. The first mixer comprises TR6 and associated components. The injection frequency
from the first local oscillator is fed to the first mixer via the helical filter FL7 and
inductor L9. The RF signal is input via the inductor L13. The difference signal of
45MHz is selected by the collector load of L12, R18 and C29. The 45MHz is then
filtered by FL1 and FL2 to remove unwanted signals.
Second Local Oscillator and Mixer
7. The second local oscillator is formed by TR4, XL2 and associated components. It
runs at a fixed frequency of 44.545MHz. The signal is selected from the collector
of TR4 and input, along with the RF from FL2, onto the base of mixer TR5. The
difference signal of 455kHz then passes to FL5, with matching provided by R31.
Second IF Stage
8. The second IF chain comprises three ceramic filters of channel bandwidth, FL5, FL6
and FL4, each followed by an IF amplifier. After FL5 the 455kHz IF signal is
amplified by the low noise transistor TR25 and then passed to FL6. The signal is
then fed to IC1 (pin 1) where the remaining IF amplification and amplitude limiting
are provided. Another stage of filtering is carried out by FL4. Demodulation is
carried out using the ceramic discriminator FL3 as the quadrature network for the
on-chip demodulator. After demodulation the signal is output from IC1 (pin 14).
Audio Splitting
9. The demodulated audio signal leaves IC1 (pin 14) and divides three ways:
a) To a de-emphasis circuit (comprising R339 and C104) and the Rx Audio path.
b) To a de-emphasis circuit (comprising R365 and C154) and the Noise Squelch
path.
c) To an active filter (comprising TR24, C82, C83, R341 and R344) where it then
divides two ways:
i) To the Sub-Audio Tone path.
ii) To the Data path.