System Specifications & Description
1-10
602-95555-01 Rev A
NXE1-20 Digital Radio
1.5.6
QAM Demodulator/IF Downconverter
IF Input
70 MHz
BPF
70 MHz
BPF
6.4 MHz
AGC Control
PLL
Loop
Filter
VCO
76.4 MHz PLL
Data
Clk
Enbl
Ref
IF Output
6.4 MHz
-10dBm
Synth Level
Synth
Lock
Figure 1-19. IF Downconverter Block Diagram
The QAM (Quadrature Amplitude Modulation) Demodulator is the receive portion of the QAM
Modem card. The QAM Modem also houses the IF Up/Down Converter. The QAM Demod
utilizes the downconverter portion of the IF daughter card.
The IF Downconverter receives the 70 MHz carrier from the receiver portion of the RF Module via
an external semi-rigid cable and directly converts the carrier to 6.4 MHz by mixing with a low-
noise phase-locked LO (see Figure 1-19). System selectivity is achieved through the use of a 70
MHz SAW filter.
The QAM Demod receives and demodulates the 6.4 MHz carrier (see Figure 1-16). The
demodulation process includes the FEC implementation and de-interleaving that matches the
QAM modulator in the transmitter, and the critical “data assisted recovery” of the clock. This
process requires an ultra-stable master clock provided by an OCXO (oven controlled crystal
oscillator).
The output is an aggregate data stream that is distributed to the trunk port for if the data
input/output is out of the Modem, or to the backplane for connection to the multiplexer connected
on the backplane.