DS_1903_032
73M1903 Data Sheet
5 Modem Transmitter
The modem transmitter begins with an 48 tap Transmit Interpolation Filter (TIF) that takes in the 16-bit,
two’s compliment numbers (TXD) at SDIN pin at Fs=8 kHz rate. It up-samples (interpolates) the data to
16 kHz rate rejecting the images at multiples of 8 kHz that exist in the original TXD data stream and
outputs 16-bit, two’s compliment numbers to a digital sigma-delta modulator. The gain of the interpolation
filter is 0.640625 (–3.8679 dB) at DC.
The digital sigma-delta modulator (DSDM) takes 16-bit, two’s compliment numbers as input and
generates a 1’s bit stream which feeds into a D to A converter (DAC1). The gain through DSDM is 1.0.
DSDM takes 16-bit, two’s compliment numbers as input and generates a 1’s bit stream that feeds into a D
to A converter (DAC1).
DAC1 consists of a 5-tap FIR filter and a first order switched capacitor low pass filter both operating at
1.536 MHz. It possesses nulls at multiples of 384 kHz to allow decimation by the succeeding filter.
DAC1’s differential output is fed to a 3rd-order switched-capacitor low pass filter (TLPF). The output of
TLPF drives a continuous time smoothing filter. The sampling nature of the transmitter leads to an
additional filter response that affects the in-band signals. The response is in the form of sin(x)/x and can
be expressed as 20*log [(sin(PI*f/fs))/(PI*f/fs)] where f = signal frequency and fs = sample frequency = 16
kHz. Figure 10 shows the frequency response of the transmit path from TXD to TXAP/TXAN for a dc to 4
kHz in-band signal including the effect of this sampling process plus those of DAC1, TLPF and SMFLT. It
is important to note that as TXD is sampled at 8 kHz, it be band-limited to 4 kHz.
Figure 10: Frequency Response of TX Path for DC to 4 kHz in Band Signal
Rev. 2.0
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TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
