
RC6508
PRODUCT SPECIFICATION
2
Functional Description
The RC6508 shown in the block diagram performs all the IF
and baseband signal conversion with the minimal external
components. It consists of three general sections:
IF Gain Section
IF Down Conversion and Frequency Synthesis
Reference Voltage
IF Gain Section
This is the first stage of the IF-to-Baseband conversion
IF input signal is fed into a variable gain control amplifier
that is capacitively coupled to the subsequent stages. The
gain control amplifier has stabilized gain over temperature
and supply variations. The amplifier gain is directly propor-
tional to the IF_AGC voltage. The gain in various stages is
not reduced at the same time in order to minimize the noise
figure degradation. The transition point is set by the voltage
on T_Strt pin. T_strt sets the T_AGC trigger to control the
front end tuner gain.
.
The
IF Down Conversion and Frequency Synthesis
This is the second stage of the IF-to-Baseband conversion. It
consists of a double balanced linear mixer. The output of the
front end gain stage is capacitively coupled to the input (RF
port) of the mixer. The LO signal for the mixer can be
directly driven or synthesized with the Voltage Controlled
Oscillator (VCO). This section has also a crystal oscillator
that can be used to generate a master clock for the frequency
synthesis. The mixer translates the signal to a second IF fre-
quency equal to the symbol rate and passes through an
amplifier. The final output is a baseband signal, BB_Out.
This signal can be further filtered externally before connect-
ing it to an external ADC and QAM demodulator.
Reference Voltage
The RC6508 has a built-in 2.0V reference with capability of
driving 10mA load and can be used to set up A/D reference.
Pin Assignments
VCC_XTL
GND_XTL
XTL_Tnk
XTL_
Out
RFGND
VCC_RF
VCO_In
VCO_Fs
BB_GND
NC
BB_Out
VCC_BB
T_Strt
IF_AGC
T_AGC
VCC_HF
HFGND
IF_IN–
IF_IN+
SGND
NC
VCC2
VRT
NC
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
65-6508-02