参数资料
型号: 73M1903-IGV
厂商: TERIDIAN SEMICONDUCTOR CORP
元件分类: 消费家电
英文描述: SPECIALTY CONSUMER CIRCUIT, PQFP32
封装: TQFP-32
文件页数: 11/45页
文件大小: 449K
代理商: 73M1903-IGV
73M1903
Modem Analog Front End
DATA SHEET
Page: 19 of 45
2005 TERIDIAN Semiconductor Corporation
Rev 1.4
MODEM RECEIVER
A differential receive signal applied at the RXAP and RXAN pins or the output signal at TXAP and TXAN
pass through a multiplexer, which selects the inputs to the ADC. In normal mode, RXAP/RXAN are
selected. In analog loopback mode, TXAP/TXAN are selected. The DC bias for the RXAP/RXAN inputs
is supplied from TXAP/TXAN thru the external DAA in normal conditions. (See Appendix) It can be
supplied internally, in the absence of the external DAA, by setting RXPULL bit in Control Register 2.
The output of the multiplexer goes into a second-order continuous time, Sallen-Key, low-pass filter (AAF)
with a 3dB point at approximately 40kHz. The filtered output signal is the input to an analog sigma-delta
modulator (ASDM), clocked at an over sampling frequency of 1.536MHz for Fs = 8kHz, which converts
the analog signal to a serial bit stream with a pulse density that is proportional to the amplitude of the
analog input signal.
There are three gain control bits for the receive path. The RXGAIN bit in control register one results in a
+20dB gain of the receive signal when set to a “1”. This 20dB of gain compensates for the loss through
the DAA while on hook. It is used for Caller ID reception. This gain is realized in the front end of ASDM.
The other gain bits in control register 1, RXG1:0, compensate for differences in loss through the receive
path.
RXG1
RXG0
Receive Gain Setting
0
6dB
0
1
9dB
1
0
12dB
1
0dB
Table 10: Receive Gain
The output of ASDM is a serial bit stream that feeds three digital sinc
3 filters. Each filter has a [sin(x)/x]3
frequency response and provides a 16 bit sample every 288 clock cycles. The filters are synchronized so
that there is one sample available after every 96 analog samples or at a rate of 16kHz for Fs=8kHz. The
output of the sinc
3 filter is a 17 bit, two’s compliment number representing the amplitude of the input
signal. The sinc
3 filter, by virtue of holding action (for 96 sample period), introduces a droop in the
passband that is later corrected for by a 48 tap FIR filter that follows. The maximum digital word that can
be output from the filter is 0d800h. The minimum word is 12800h.
The output of the sinc
3 filter is input to another 48 tap digital FIR filter that provides an amplitude
correction in the passband to the output of the sinc
3 filter as well as rejecting noise above Fs/2 or 4kHz for
Fs=8kHZ. The output of the this filter is then decimated by a factor of 2; so, the final output is 16 bit, two’s
compliment samples at a rate of 8 kHz.
Figure 6 and Figure 7depict the sinc
3 filter’s frequency response of ASDM along with the 48 tap digital
FIR response that compensates for it and the resulting overall response of the receiver.
相关PDF资料
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