User's Manual

7. THEORY OF OPERATION
7.1 DUPLEXER
The duplexer allows for simultaneous transmission and reception of RF signals. There is a receive path and
a transmit path which connect to the RF input and output of the main PCB and a common point at the
antenna connector.
In the receive path is a two-cavity notch filter. This filter allows the receive signal from the antenna
connector to pass through with about 1.5 dB loss, but notches the transmitter RF output signal by 70 dB or
more to prevent overloading the receiver. The transmit path also contains a two-cavity notch filter, this one
tuned to the receive frequency. This filter removes transmitter noise which exists at the receive frequency
and which would interfere with the desired receive signal, if not removed.
7.2 RECEIVER
7.2.1 Front-End
The receiver front-end comprises the RF amplifier stages with associated filters and the first mixer. The
input signal from the J100 (RX IN) passes through a two-pole bandpass filter formed around L100 and
L101. This filter is narrow enough at 10 MHz to protect the receive RF amplifier, but wide enough to keep
its insertion loss low at 2.3 dB. This filter stage is followed by the RF amplifier. The RF amplifier stage
has a gain of about 12 dB and a noise figure of 1.5 dB. This stage, along with the first filter, are largely
responsible for the sensitivity of the Repeater. The RF amplifier is followed by a three-pole bandpass filter
formed around L102 through L104. This filter is somewhat narrower than the two-pole filter and thus has a
higher insertion loss at 4 dB. The total filtering action is to limit the number of off-channel interfering
signals that the receiver might be exposed to and to remove signals at the spurious response frequencies of
the mixer which follows.
The job of the 1
st
mixer, U100 is to convert signals at the receive frequency to that of the 1
st
IF at 43.65
MHz. It does this by mixing the 1
st
local oscillator signal at a frequency 43.65 MHz below that of the
desired receive frequency to the IF frequency of 43.65 MHz. The 43.65 MHz output of U100 exits at pin 4
and is routed to the 1
st
IF filter. U100 has a conversion gain of 12 dB and a noise figure of 6 dB. This
performance in combination with the front-end RF amplifier stage determines the sensitivity of the
receiver.
7.2.2 1
st
IF
The 1
st
IF stage consists of two two-pole monolithic crystal filters separated by an amplifier. The two
filters are YF101 and YF102. These filters are bandpass in response with a bandwidth of 12 kHz and an
insertion loss of 1.5 dB. To overcome the losses of the filters and to prevent interaction between them, they
are separated by an amplifier formed around Q102.
7.2.3 2
nd
IF Sub-System
IC U101 is responsible for converting signals at the 1
st
IF to the 2
nd
IF where additional filtering and
amplification take place. The 43.65 MHz signals enters U101 at pin 16. It is then mixed with the 2
nd
local
oscillator signal at pin 1 to product a signal at the 2
nd
IF of 450 kHz at pin 3. The 2
nd
IF signal at pin 4 is
filtered by the YF103, the 2
nd
IF filter. This filter is a bandpass filter with a bandwidth of 7.5 kHz and an
insertion loss of 5 dB. The output of the filter re-enters U101 at pin 5 where it is amplified and then limited
to remove amplitude modulation components. The limited signal is then mixed against a 90° phase shifted
version of itself and filtered by Y102. This acts as an FM demodulator. The demodulated output exits
U101 at pin 9.