User Guide

PAMS
Technical Documentation
NSB–1
System Module
Original 06/98
Page 3 – 30
Receiver
The receiver is a triple conversion receiver consisting of two ASICs;
CRFU2a (N402) and PLUSSA (N401). CRFU2a contains LNA bias circuit-
ry with an external transistor which provides step gain depending on the
incoming RF level and the first and second mixers. PLUSSA contains the
third mixer. All filtering is external.
The received RF signal from the antenna is fed via the duplex filter (3
pole bandpass filter; Z401) to the LNA. Biasing and the AGC step circuit-
ry are integrated into CRFU2a but the RF transistor, input and output
matching networks are external. The LNA gain step is controlled by
MAD2 (FRAC, D200). Gain step in LNA is activated when the receive RF
level is below –48 dBm. Following the LNA, the signal is fed to a 3 pole
ceramic bandpass filter (Z604). The combination of the duplex filter and
the bandpass filter define, the blocking characteristics of the receiver.
The bandpass filtered signal is fed back to CRFU2a, where the signal is
down converted with a double balanced active mixer (Gilbert cell) to 487
MHz. The local oscillator signal for this down conversion is generated by
the UHF VCO (G701) and buffered in CRFU2a. The first IF signal is
bandpass filtered with an 487 MHz SAW filter 7621. This filter attenuates
the intermodulating and image frequencies. The second down conver-
sion (occurs in CRFU2a) results in a balanced IF of 87 MHz which is fil-
tered using an 87 MHz SAW filter (Z605). This filter provides selectivity
for channels greater than +/– 200 kHz, and attenuates the image frequen-
cy of the third mixer and intermodulating signals. The local oscillator sig-
nal for this down conversion is 400 MHz which is generated by the 800
MHz VHF VCO module (G702). The VHF VCO signal is buffered and di-
vided in PLUSSA and the 400 MHz resulting signal is again buffered in
CFRU2a before the mixer.
After the 87 MHz filter, the signal is fed into the AGC amplifier which has
been integrated into PLUSSA. The AGC amplifier contains analog gain
control which provides accurate gain control (minimum 57 dB) for the re-
ceiver. Control voltage for the AGC is generated by the D/A–converter in
COBBA_GJ (N300). The final mixing stage occurs in PLUSSA with a lo-
cal oscillator signal of 100 MHz generated by dividing the VHF–synthesiz-
er output (800 MHz) by eight.
The third (final) IF filter (Z606) is a ceramic bandpass filter with a centre
frequency of 13 MHz. This filter attenuates adjacent channels with very
little attenuation for +/– 200 kHz. The +/– 200 kHz interferers are filtered
digitally by DSP. The 13 MHz bandpass signal is converted to a balanced
signal with a buffer circuit in PLUSSA. This buffer circuit has a voltage
gain of 36 dB. This balanced signal is then fed to COBBA_GJ. The PGA
stage in COBBA_GJ has a gain setting of either 0 dB or 9.5 dB which is
controlled via the COBBA_GJ control bus. For HD950 the PGA gain will
be set to 0dB.