MDS 05-2415A01, Rev. A 5-1
CHAPTER 5—THEORY OF
OPERATION
For the following discussion, see Figure 5-1, the radio’s block diagram at the end of this
chapter.
RECEIVE FRONT END
Connector J4 on the main PC board conducts the RF signal from the front panel ANTENNA
connector to the antenna switch network. In the receive mode, one port of the antenna switch
conducts the receive signal to the input of helical filter Z1.
The output of Z1 is fed to RF amplifier Q1. The output of Q1 goes to helical filter Z2. The
output of Z2 goes to M1, a double-balanced mixer whose local oscillator injection signal is
derived from the VCO output.
HIGH IF
The 45 MHz High IF signal from M1 enters the IF amplifier. The output of IF amplifier
connects to L25 as an input matching transformer. The receive signal is then passed to FL1A
and FL1B. FL1A and FL1B are components of a 4-pole crystal filter that provides some of
the IF selectivity of the receiver. The output of FL1B is conducted to U32, which contains
the low IF amplifier and other functions.
LOW IF
U32 contains several circuit sections: mixer, oscillator, IF amplifier/limiter, quadrature detec-
tor and received signal strength indication driver. The second 44.55 MHz LO for U32 is
derived from the TCXO. A frequency tripling circuit multiplies the 14.85 MHz TCXO signal
and triples it up to 44.55 MHz. This is consequently mixed with the 45 MHz IF to form the
450 kHz IF.
The 450 kHz output of the second mixer is fed to a ceramic filter set consisting of FL2 and
FL3. This filter set provides the main adjacent channel selectivity of the receiver.
The output of FL3 is fed to the limiter amplifier input pin of U32. The limiter output is fed to
a quadrature detector circuit tuned by discriminator coil T1; recovered audio is passed to
U3A audio buffer.
A secondary output of the IF subsystem of U32 gives a received signal strength indication
(RSSI) voltage. The RSSI signal is used by the remote maintenance module and is available
at the
INTERFACE connector J1-21 through a buffer and scaler, U3C.
RECEIVE AUDIO
The unfiltered recovered audio from the IF discriminator passes through amplifiers U3A,
U3B and configuration jumper J14.
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