User Guide

PAMS
System Module DB6
NHB–3
Technical Documentation
Original 26/97
Page 4–41
Functional Description
Receiver
The receiver is a triple conversion receiver.
The received RF signal from the antenna is fed via a duplex filter to the receiv-
er unit. The signal is amplified by a discrete low noise preamplifier. The gain of
the amplifier is controlled by the AGC control line (PDATA0). The nominal gain
of 14 dB is reduced in the strong field condition about 36 dB. After the pream-
plifier the signal is filtered by ceramic filter. The filter rejects spurious signals
coming from the antenna and spurious emissions coming from the receiver
unit.
The filtered RF– signal is down converted by a passive diode mixer. The fre-
quency of the first IF is 313 MHz. The first local signal is generated by the UHF
synthesizer. The IF signal is amplified and then it is filtered by a microstripline
filter. The filtered 1st IF is down converted by the second mixer which is also a
passive diode mixer. The 2nd IF frequency is 87 MHz. The 2nd local signal is
generated by the VHF synthesizer.
The IF signal 87 MHz is amplified and filtered by SAW filter. The filter rejects
adjacent channel signal, intermodulating signals and the last IF image signal.
The filtered IF signal is fed to the receiver part of the integrated RF circuit
CRFRT. In CRFRT the filtered IF signal is amplified by an AGC amplifier which
has gain control range of 57 dB. The gain is controlled by an analog signal via
TXC–line. The amplified IF signal is down converted to the last IF in the mixer
of CRFRT. The last local signal is generated from VHF VCO by dividing the
original signal by 4 in the dividers of CRFRT.
The last IF frequency is 13 MHz.
The last IF is filtered by a ceramic filter. The filter rejects signals of the adja-
cent channels. The filtered last IF is fed back to CRFRT where it is amplified
and fed out to RFI via RXI–line.
Frequency Synthesizers
The stable frequency source for the synthesizers and base band circuits is dis-
crete voltage controlled crystal oscillator, VCXO. The frequency of the oscilla-
tors is controlled by an AFC voltage, which is generated by the base band cir-
cuits.
The UHF PLL generates the down conversion signal for the receiver and the up
conversion signal for the transmitter. The UHF VCO is a discrete oscillator. PLL
circuit is from National: LMX2331ATM.
The VHF PLL signal ( divided by 4 in CRFRT) is used as a local for the last
mixer. Directly it is used as a second local in PCN. Also the VHF PLL signal (di-
vided by 2 in CRFRT) is used in the I/Q modulator of the transmitter chain.