73M1903C-IM/F 查看數據表（PDF） - Teridian Semiconductor Corporation

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73M1903C-IM/F

Modem Analog Front End

Teridian Semiconductor Corporation 

73M1903C

Modem Analog Front End

DATA SHEET

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=8kHz rate. It up-samples (interpolates) the data to

16kHz rate rejecting the images at multiples of 8kHz 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 (–3.56dB) 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

1.536MHz. It possesses nulls at multiples of 384kHz 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 shows the frequency response of the transmit path from TXD to TXAP/TXAN.

The transmit bandwidth is about 3.65kHz when Fs=8kHz. The bandwidth scales with Fs, the sampling

rate. In case of Fs=9.6kHz , then the bandwidth is 3.65kHz x 9.6/8 = 4.38kHz and Fs=10.28kHz, the

bandwidth is 3.65kHz x 10.28/8 = 4.69kHz. This is applicable for both transmit and receive path filters.

Figure 8: Overall TX path frequency response at 8kHz sample rate

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© 2005-2008 TERIDIAN Semiconductor Corporation

Rev 4.3

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