Rockwell SoniCrafter BT8960 - Manual (Page 29)

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2.0 Functional Description
2.2 Receive Section
Bt8960
Single-Chip 2B1Q Transceiver
N8960DSB
2.2 Receive Section
Like the transmit section, the receive section consists of both analog and digital
circuitry. The VGA provides the interface to the analog signals received from the
line and the hybrid. The Analog-to-Digital Converter (ADC) then digitizes the
analog signal so it can be further processed in the digital signal Processing (DSP)
section of the receiver. The receiver DSP section includes: front-end processing,
echo cancellation, equalization, and symbol detection.
2.2.1 Variable Gain Amplifier
The Variable Gain Amplifier (VGA) has two purposes. The first is to provide a
dual-differential analog input so the pseudo-transmit signal created by the hybrid
can be subtracted from the signal from the line transformer. This subtraction pro-
vides first-order echo cancellation, which results in a first-order approximation of
the signal received from the line. Figure 2-1 illustrates the recommended sug-
gested echo-cancellation circuit interconnections. All off-chip circuitry, including
the hybrid and anti-alias filters, consists entirely of passive components. Further
echo cancellation occurs in the receiver DSP.
The second purpose of the VGA is to provide programmable gain of the
received signal prior to passing it to the ADC. This reduces the resolution
required for the ADC. There are six gain settings ranging from 0 dB to 15 dB. The
gain is controlled via the gain[2:0] control bits in the ADC Control Register
[adc_control; 0x21]. See the Registers section of this datasheet for a more
detailed description of the gain[2:0] control bits.
Figure 2-2. First-Order Echo Cancellation Using the Variable Gain Amplifier
RXP
RXN
RXBP
RXBN
TXP
TXN
Line
(Twisted Pair)
To
ADC
On-Chip Circuitry Off-Chip Circuitry
Line
Impedance
Matching Gain[2:0]
Anti-Alias
Filter
Hybrid
–
+
Anti-alias
Filter
+
–
+
–
+
–
+
–
Line
Driver
Line
Transformer
+
–
Resistors