Service manual

3.4 TP0332/2
RECEIVER
Antenna Filter
Figure 3.3 on page 3.5 is a block diagram of the receiver and the circuit diagram is shown in
Figure 6.8.
The antenna low pass filter (L104 to RX_ANT on Figure 6.9) isolates the receiver input during
transmissions (D100 forward biased) whilst maintaining a low loss connection from the antenna in the
receive state.
First Tracking Filter
The first tracking band-pass filter limits the unwanted signals that can enter the receive path. This
filter also provides the attenuation required for image rejection and protection against IF spurious
response.
The filter is a conventional two section capacitively top coupled design, each section being tuned by
varicap diodes (D205 to D208 and D215 to D216). The required tune voltages are derived from an IIC
(Inter-Integrated Circuit bus) DAC (IC605), RX_TUNE1 and a DC level with a small amount of pulse width
modulation (RX_TUNE2). Both the DAC and the pulse width modulator have eight bit resolution. Alignment
of the filter is achieved by the control circuits recording the tune voltages required to peak the filter’s
response at the top, middle and bottom of the frequency band. The control circuitry uses quadratic
interpolation to predict the required operating point for all intermediate frequencies.
RF Amplifier
The RF amplifier (TR200 and associated components) is the first gain stage of the receiver and is
important in defining the overall performance of the receiver. The amplifier noise factor is less than 2dB
and gain is sufficient to overcome the losses of subsequent stages.
Second Tracking Filter
The second tracking filter, (varicap diodes D201 to D204, D217 to D218 and associated
components) supplies the remaining filtering required to meet image rejection and IF spurious response.
The required tuning voltages are derived from the Control section via RX_TUNE3, RX_TUNE4 inputs and
op amps IC200_B and IC200_C. RX_TUNE3 is controlled by a DAC and RX_TUNE4 by a pulse width
modulator.
The filter is of the same design as the first tracking filter.
First Mixer
The first mixer (D200 - T200) is a single balanced design implemented with a wound balun. The
local oscillator frequency is either 21,4MHz (E0 band) or 45MHz (AB and K bands) above the first IF
frequency or 45MHz below the first IF frequency (R1, Tk and U0 bands). The mixer requires 3dBm drive
from the local oscillator to achieve the required performance.
First IF Filters and IF Amplifier
The IF crystal filter (F200_A and F200_B) consists of two packages each providing two poles of
filtering. The IF amplifier (TR201) is placed between the mixer and filters. The choice of IF frequency is
band dependent as is the low/high side injection. This is summarized in Table 3.1.
Table 3.1 First IF Frequency for each Band
Band Injection IF Frequency
E0 High 21,4MHz
AB and K High 45MHz
R1, Tk, U0 Low 45MHz