AD20MSP415 View Datasheet(PDF) - Analog Devices
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AD20MSP415 Datasheet PDF : 8 Pages
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AD20msp415
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.
BASEBAND/AUXILIARY SECTION OF VBC
SERIAL
BASEBAND
PORT
BURST
STORE
GSMSK
MODULATOR
DIDGDIIGGTIAITTLAALFLIFRFIIRFRIFLFITILLETTREERR
Tx DAC
Tx DAC
Rx ADC
DDIIGGIITTAALL FFIIRR FFIILLTTEERR
Rx ADC
SERIAL
AUXILIARY
PORT
RAMP RAM
SUB DAC
AGC DAC
RAMP DAC
AFC DAC
GSMP
RADIO
SUBSYSTEM
CONTROL
PA
Q
I
IF
Q
AGC
RAMP CONTROL
AFC
CLOCK
13MHz
VCTCXO
PAERROR
SYNTHESIZERS
Figure 2. Control of Radio Subsystem
REV. 0
–3–
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