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RC6505
PRODUCT SPECIFICATION
6
P
Application Discussion
The RC6505 is specially suited for use in set-top boxes and
cable modems for decoding QAM modulated signals based
on IF sub-sampling techniques. The RC6505 simplifies
the front-end design and makes it more cost effective by inte-
grating in a single chip all the analog processing functions
needed between the standard tuner and high performance
A/Ds. The other major components required for the front-
end of the modem are the tuner, a SAW filter, crystal and the
appropriate DSP demodulator/decoder.
DVB Set-top Application
Figure 2 shows the application of RC6505 in IF bandpass
sampling decoder for 256QAM cable transmissions. Here,
the sampling clock for the A/D conversion can be generated
using the crystal oscillator operating in the 3rd overtone
mode at 57.6MHz and an external divided by 2 prescaler.
The reference signals for A/D are the VRT and OUT outputs.
The application is shown with the Fairchild Semiconductor
Division’s 10-bit ADC TMC1185. Other high performance
A/Ds needing fully differential input can also be used. The
A/D inputs are referenced to be in the mid-scale using the
output from TMC1185. The filtered and buffered IF outputs
can be a.c. coupled to the A/D inputs. In this application an
external differential band-pass roofing filter is used to band-
limit the signals before conversion.
Figure 3 shows details of circuits used to evaluate the perfor-
mance of RC6505 with the TMC1185 A/D.
Figure 2. RC6505 Application in a Sub-sampling Digital Receiver for 256 QAM
Tuner
SAW
36MHz
RC6505
2
5V
XTL1
XTL2
57.6MHz
28.8Msps
CLK
RF_IN
TMC1185
INP
REFT
OUT
REFB
Roofing Filter
I
I
B
B
BUF_OUT+
BUF_OUT-
VCC_RF
12V
VCC_BUF
IF_IN+
IF_IN-
V
12V
/2
VCM
XOSCOA
2V
INP
65-6505-04