Service Manual

MM-E 2072-09333-00
2-25
d. The first mixer converts the 45.1 MHz transmit IF signal to the final RF transmit frequency, by
mixing with the local oscillator signal, FIRST_INJ, provided by synthesizer 1 (para. 2-4.7.2).
e. In the transmit mode, the mixer output signal passes through the PIN diode CR1010 to the
exciter amplifier (CR1009 is reverse-biased).
f. The mixer output signal is filtered by a low-pass filter, and is then amplified by Q1004, Q1005.
Q1012 and Q1010 provide the bias current for Q1004, Q1005 (bias current is provided only in
the transmit mode, under the control of the 9R line).
g. The amplified RF drive signal, at the collector of Q1005, is coupled to the HI POWER module
via the TX line.
CR1001 provides a DC voltage proportional to the drive signal peak level, PWR_LEVEL,
which is monitored by U5008 via the PAD6/AN6 input of its internal A/D converter.
2-4.5 RF Front End
Refer to Figure A-1.H, Figure A-1.I, Figure A-1.N.
2-4.5.1 Preselector
a. The preselector receives the RF IN signal from the HI POWER module via J1. CR2008,
CR2009, VR4 and VR5 protect against excessive signal levels.
b. During receive path self-test, the BIT noise generator can be activated by a low level on the
NOISE_CONTROL line; to add noise to the signal appearing on the RF IN line.
c. The RF IN signal passes through a low-pass filter to the preselector. The preselector consists of
seven high-pass filters, and an additional non-selective branch. The desired filter is inserted in
the received signal path by two sets of PIN diode switches: CR1 to CR8 at the input, and CR9 to
CR16 at the output.
The pair of PIN diodes corresponding to the operating frequency band is turned on by control
signal received via the SPI interface, U1: U1 is a serial/parallel converter, which converts the
serial preselector control word received via the SPI_DATA_OUT line to discrete control signals
for the PIN diode drivers, Q1 to Q8 (a pair of PIN diodes is turned on when the corresponding
driver is turned on by a high level at the corresponding U1 output).
d. The preselector output signal, PS_OUT, is connected to the RAGC attenuator. The RAGC
attenuator consists of two sections: a 10-dB attenuator (series resistors R1016, R1017) and a
20-dB attenuator (the series resistors and the shunt resistor R1023), which can be inserted in the
signal path by the PIN diode switches CR1000 and CR1008. These PIN diode switches are
controlled by the STEP_AGC_10 and STEP_AGC_20 lines, via the drivers Q1000, Q1013 to
Q1015.
The lines used to control the AGC step attenuators are generated by a serial/parallel converter,
U1112, using the data received via the SR_DATA line (the data updates the control lines status
when the CONVST line assumes a low level).
e. The RF signal is applied to the front-end amplifier, Q1002, which operates only in the receive
mode (its bias voltage is connected by Q1007 only in the receive mode, under the control of the
9T line).
f. The amplified signal passes through a low pass filter, a 9-dB attenuator pad and the PIN diode
CR1009 to the first mixer, U1000 (CR1010 is reverse-biased via the 9R line).
U1000 converts the RF signal to the first IF, 45.1 MHz, by mixing with the FIRST_INJ signal
provided by synthesizer 1 (para. 2-4.7.2).