Datasheet

73M1903 Data Sheet DS_1903_032

18 Rev. 2.1

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 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 6 dB

0 1 9 dB

1 0 12 dB

1 1 0 dB

The output of ASDM is a serial bit stream that feeds three digital sinc

filters. Each filter has a [sin(x)/x]

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 sinc

filter is a 17 bit, two’s compliment number representing the amplitude of the input

signal. The sinc

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

filter is input to another 48 tap digital FIR filter that provides an amplitude

correction in the passband to the output of the sinc

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 sinc

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.