73M1903C-EVM Maxim Integrated Products, 73M1903C-EVM Datasheet - Page 23

• Manufacturer Part Number: 73M1903C-EVM
• Description: BOARD DEMO 73M1903C WORLDWIDE
• Manufacturer: Maxim Integrated Products

Specifications of 73M1903C-EVM

• Main Purpose: Interface, Analog Front End (AFE)
• Utilized Ic / Part: *
• Primary Attributes: *
• Secondary Attributes: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Embedded: -

DS_1903C_033
73M1903C Data Sheet
6.3
Modem Transmitter
The modem transmitter begins with a 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 0.664 (–3.56 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 8 and Figure 9 show the frequency response of the transmit path from TXD to TXAP/TXAN.
The transmit bandwidth is about 3.65 kHz when Fs=8 kHz. The bandwidth scales with Fs, the sampling
rate. In case of Fs=9.6 kHz , then the bandwidth is 3.65 kHz x 9.6/8 = 4.38 kHz and Fs=10.28 kHz, the
bandwidth is 3.65 kHz x 10.28/8 = 4.69 kHz. This is applicable for both transmit and receive path filters.
Figure 8: Overall TX Path Frequency Response at 8 kHz Sample Rate
Rev. 5.0
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