PCF50732H/F1 데이터 시트보기 (PDF) - Philips Electronics

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PCF50732H/F1

Baseband and audio interface for GSM

Philips Electronics 

Philips Semiconductors

Baseband and audio interface for GSM

Objective specification

PCF50732

9.1.1 GMSK MODULATOR

The input signal of the GMSK modulator is a bitstream

coming from the baseband serial interface, with a

sampling frequency of 270.833 kHz. Typically 148 bits are

modulated during a normal burst, and 88 bits during an

access burst. Using this bitstream, the GMSK modulator

generates digital I and Q components as described in

“GSM recommendation 05.04”.

This is done in three steps:

1. First the incoming bitstream is differentially encoded

by an EXOR operation on the actual bit and the

previous bit

2. The instantaneous phase (ϕ) is calculated using a

gaussian filter with an impulse response of 4 taps

3. A look-up table provides the cosine (I component) and

the sine values (Q component) of the phase (ϕ).

The look-up table also interpolates the signal to a

16 times higher frequency (4.333 MHz).

9.1.2 10-BIT DACS

The two 10-bit DACs are working at a sampling rate of

4.3333 MHz. They convert the digital I and Q components

of the GMSK modulator to differential analog

I and Q signals.

9.1.3 LOW-PASS FILTER

The analog output signals of the DACs are filtered by

analog reconstruction low-pass filters.

These filters remove high frequency components of the

DAC output signals and attenuate components around the

4.3333 MHz sampling frequency. The low-pass filters

have a cut-off frequency of approximately 300 kHz, with

very linear phase behaviour in the pass band.

The baseband receive section can be switched between

two modes of operation:

• ZIF (zero IF) mode for radio sections, which convert the

receive signal down to baseband. In this mode the ADC

is sampled at 6.5 MHz, the decimation filter samples

down by a factor of 24 with a pass band as specified in

Fig.3. The serial interface output BDIO delivers

2 × 12-bit values for I and Q components at

270.833 kHz.

• NZIF (near zero IF) mode for radio sections, which

converts the receive signal down to a centre frequency

of 100 kHz. In this mode the ADC is sampled at 13 MHz,

the decimation filter samples down by a factor of 24 with

a pass band as specified in Fig.3. The serial interface

output BDIO delivers 2 × 12-bit values for I and Q

components at 541.667 kHz.

9.2.1 RECEIVE ADC

The receive ADCs are Σ∆ analog-to-digital converters that

convert differential input signals into1-bit data streams with

a sampling frequency of 6.5 or 13 MHz.

9.2.2 DIGITAL DECIMATION FILTER

Digital filtering is required for:

• Suppression of out-of-band noise produced by the

Σ∆ ADC

• Decimation of the sampling rate (6.5 or 13 MHz) by 24

• System level filtering.

The digital filtering is performed by a digital FIR filter with a

group delay for this running average filter of approximately

23 or 11.5 µs respectively. The filter uses twos

complement arithmetic.

9.2 Baseband receive path

The baseband receive path consists of two parts:

• Receive ADC: Σ∆ analog-to-digital converters

• Decimation filter: digital decimation filters for I and Q.

1999 May 03

10

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