AD20msp415

–3–

REV. 0

AUXILIARY SYSTEM FUNCTIONS

The GSMP and the VBC perform a number of auxiliary func-

tions that are essential to build a complete mobile radio.

A general radio section constitutes the three functions of trans-

mitter, receiver and synthesizer. Figure 2 shows how the baseband

chipset interfaces to a typical radio architecture. The transmitter

is fed with baseband analog I and Q signals from the VBC and

upconverted to 900 MHz for GSM applications and 1800 MHz

for PCN applications.

A dedicated power amplifier increases the RF signal to the

required level. The receiver amplifies the antenna signal, down-

converts it to an intermediate frequency (IF) and amplifies it

there again. After second conversion to baseband the I and Q

components of the signal are fed into the VBC.

The three auxiliary functions, AGC, AFC and Power Ramping

are included to interface to the radio section.

Power Ramp Envelope

To meet the spectral and time-domain specifications of the

transmitted output signal, the burst has to follow a specified

power envelope. The envelope for the power profile originates in

the GSMP as a set of coefficients, down-loaded and stored in

the VBC. This envelope profile is fed to the RAMP DAC on the

VBC with each burst. The analog output is fed into the RF

power amplifier, controlling the power profile and absolute level

of the transmitted data. The power control loop of the power

amplifier can also feedback an error control signal that indicates

whether the output functions are out of specification and the

radio can be switched off accordingly.

Automatic Gain Control (AGC)

The mobile radio has to cope with a wide range of input signal

levels. The major part of the overall gain is provided in the IF

amplifier. The incoming signal level is analyzed in the GSMP

and a digital gain control signal is sent to the VBC. The AGC

DAC generates the appropriate analog control signal for the IF

amplifier.

Automatic Frequency Control (AFC)

The mobile radio tracks the master clock provided by the base

station to compensate for temperature/frequency drifts in the

crystal oscillator. Drift of the crystal oscillator over time and

temperature has to be compensated as well as frequency shifts

due to the Doppler effect in the case of a moving mobile radio.

The received signal is analyzed in the GSMP and a digital con-

trol signal is generated. This signal is sent to the AFC DAC in

the VBC to control the voltage controlled, temperature compen-

sated crystal oscillator (VCTCXO).

Synthesizer Control

The GSMP and the respective parts of the Layer 1 software

control the overall timing and frequency generation of the radio

subsystem. This includes control signals for two synthesizers,

power-down control signals and power amplifier monitor sig-

nals. Detailed information can be found in the AD6422 data

sheet.

SYNTHESIZERS

RADIO

SUBSYSTEM

CONTROL

AGC

AFC

CLOCK

PAERROR

AGC DAC

I

Q

Q

GSMP

BASEBAND/AUXILIARY SECTION OF VBC

DIGITAL FIR FILTER

GSMSK

MODULATOR

BURST

STORE

PA

IF

Tx DAC

Tx DAC

RAMP DAC

AFC DAC

SUB DAC

DIGITAL FIR FILTER

DIGITAL FIR FILTER

DIGITAL FIR FILTER

DIGITAL FIR FILTER

SERIAL

BASEBAND

PORT

SERIAL

AUXILIARY

PORT

RAMP RAM

RAMP CONTROL

Rx ADC

Rx ADC

13MHz

VCTCXO

Figure 2. Control of Radio Subsystem