Service Manual

MM-E 2072-09333-00
2-26
2-4.6 Receive IF Path
Refer to Figure A-1.M, Figure A-1.N.
2-4.6.1 First IF Processor
a. The 45.1 MHz IF signal generated by U1000 is amplified by Q1001 (the PIN diodes CR1011
and CR1012 are reverse-biased). The bias voltage for Q1001 is provided by Q1011, which is
turned on in the receive mode by the low level on the 9T line).
b. The amplified signal at the drain of Q1001 passes through an attenuator pad to the 45.1 MHz
bandpass crystal filters FL1 and FL2.
c. The filtered signal is amplified by the receive section, Q1003, of the IF bidirectional amplifier.
The resulting signal is applied to the second mixer, U1108, via the TRANS_IF line.
The STEP_AGC_30 control line is used to add an attenuation of 30 dB (when a high level is
provided by U1112) in the receive signal path, by reducing the input impedance and gain of the
amplifier stage built around Q1003.
2-4.6.2 Second IF Processor
a. The second mixer, U1108, converts the 45.1 MHz IF signal to the second receive IF frequency,
450 kHz, by mixing it with the TX_SEC_INJ signal provided by synthesizer 2 (para. 2-4.7.3).
b. In the receive mode, the PIN diode CR1904 is forward-biased, and CR1905 is reverse-biased.
This connects the 450 kHz IF signal to the second IF processor, comprising Q1021, the 450 kHz
bandpass crystal filter FL4, Q1022, the 450 kHz bandpass crystal filter FL3, and the output
amplifier Q1021.
R1832 is used to adjust the gain of the receive path to the prescribed value.
c. The gain of Q1021 can be reduced, as part of the operation of the 30-dB step attenuator, by
applying a low level on the STEP_AGC_30_2 line provided by U1112 (this turns off Q1020).
2-4.6.3 20 kHz IF Processor
a. The 450 kHz IF signal provided by Q1021 is applied to the third mixer, built around the quad
analog switch, U1111. U1111 operates as a balanced mixer, which converts the 450 kHz signal
to 20 kHz (nominal) by mixing it with 430.77 kHz. The 430.77 kHz signal is provided by the
divider by 39, U1113, U1114, via the duty cycle equalizer comprising the Schmitt trigger
inverters in U1109.
b. The two complementary waveforms generated by the two branches of the mixer U1111 are
combined by an amplifier in U1110, and then amplified by a variable-gain amplifier, also part
of U1110. The gain is controlled by the STEP_AGC_10_2 line provided by U1112 (a high level
reduces the gain).
c. The amplified 20 kHz passes through an antialiasing filter, part of U1110, and then it is
converted to a digital data stream by the A/D converter U1115 (para. 2-4.2.2.a).
2-4.7 Synthesizer Subsystem
Refer to Figure A-1.J, Figure A-1.K.d, Figure A-1.L.
2-4.7.1 Frequency Reference
Depending on the installed option, the reference frequency, 16.8 MHz, is generated either by the
DTCXO, U3001, or by the OCXO, U3002.
2-4.7.1.1 DTCXO Option
U3001 is a 16.8 MHz digitally temperature compensated crystal oscillator. Its frequency is controlled
via the SPI port: the temperature compensation data is received via the SPI_DATA_OUT line, and it is
latched into U3001 on the rising edge of the PENDULLUM_SEL line.