73M1903 Data Sheet
DS_1903_032
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 through the external DAA in normal conditions. (See Appendix A) 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 3 dB point at approximately 40 kHz. The filtered output signal is the input to an analog sigma-delta
modulator (ASDM), clocked at an over sampling frequency of 1.536 MHz for Fs = 8 kHz, 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
+20 dB gain of the receive signal when set to a “1”. This 20 dB 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.
Table 12: Receive Gain
RXG1
RXG0
Receive Gain Setting
0
0
1
1
0
1
0
1
6 dB
9 dB
12 dB
0 dB
The output of ASDM is a serial bit stream that feeds three digital sinc3 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 16 kHz for Fs=8 kHz. The
output of the sinc3 filter is a 17 bit, two’s compliment number representing the amplitude of the input
signal. The sinc3 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 sinc3 filter is input to another 48 tap digital FIR filter that provides an amplitude
correction in the passband to the output of the sinc3 filter as well as rejecting noise above Fs/2 or 4 kHz
for Fs=8 kHz. The output of 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 7 depict the sinc3 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.
18
Rev. 2.0
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
