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PRODUCT SPECIFICATION
RC6564
9
P
Typical Performance Characteristics
(Continued)
Figure 3. Noise Figure over Gain Control Range
Figure 4. IF Input Bandwidth
0V
0
5
10
15
20
25
30
35
40
1V
2V
3V
IF_AGC
N
4V
5V
6
0
10
20
30
40
50
60
70
60MHz
40MHz
80MHz
100MHz
G
6
Applications
The RC6564 is specially suited for use in set-top boxes and
cable modems for decoding QAM modulated signals based
on IF sampling techniques. The RC6564 simplifies the front-
end design and makes it more cost effective by integrating in
a single chip the IF-to-Digital functionality. The other major
components required for the front-end of the modem are the
tuner, a SAW filter and the appropriate DSP demodulator/
decoder.
Modem Applications
Figure 5 below shows the application of RC6564 in IF band-
pass sampling decoding for cable access. Here, the master
clock for the Digital Demodulator can be generated using the
crystal oscillator that is configured as a VCXO. The mixer
down conversion clock can also be pulled in using the varac-
tor control circuitry shown below. The on-chip VCO
provides low-phase noise characteristics. The VCO and
VCXO oscillator frequencies can be pulled by the voltage
control on VCO_CNTRL and VCXO_CNTRL lines respec-
tively. The sampling clock for the A/D conversion can be
derived from the master clock through external frequency
synthesis. The full scale reference signal for A/D is conve-
niently derived from the VRT output. The baseband output is
referenced such that the filtered output is automatically in the
mid-scale of the A/D input. The filtered output can be a.c.
coupled to the AD_In. In the application below an external
low-pass filter is used to bandlimit the signals before conver-
sion. The gain is adjusted by the average voltage on the
IF_AGC line to keep the signal in the optimum range of the
A/D input. The T_AGC output is used to control the tuner
gain when the input levels into the RC6564 are too high.