AD20msp410

–3–

REV. 0

decision confidence bits supplied by the equalizer. Once these

decoding functions are complete, digitized voice data is trans-

ferred to the ASP through a parallel port. Error control mecha-

nisms are used to ensure adequate bad frame indication.

Speech Decoding (ASP)

Encoded speech data is transferred at 20 ms intervals from the

PLP to the ASP in blocks of 260 bits plus the Bad Frame

Indicator (BFI). The speech decoder supports a Comfort Noise

Insertion (CNI) function that inserts a predefined silence

descriptor into the decoding process. The ASP also implements

control of talker side-tone and short term echo cancellation.

The resulting data, at 104 kb/s, is transferred to the BBC

through a dedicated serial path.

Voice Digital-to-Analog Conversion

The Voice DAC function of the BBC uses a sigma-delta con-

verter to convert and noise shape the signal. The 13-bit linear

values are converted to the analog domain and filtered to avoid

any images. The resulting differential signals can be controlled

in volume and drive directly a small earpiece as well as a

separate auxiliary output.

AUXILIARY SYSTEM FUNCTIONS

The ASP, the PLP and the BBC perform a number of auxiliary

functions which are essential to build a complete mobile radio.

A general radio section constitutes the three functions of

transmitter, 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

BBC 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 BBC.

The BBC, ASP and PLP provide three auxiliary functions for

interfacing to the radio subsystem. These auxiliary functions

include AGC, AFC and Power Ramping.

Power Ramp Envelope (BBC)

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

BBC. This envelope profile is sent to one of the auxiliary DACs

on the BBC with each burst. The analog output is fed into the

RF power amplifier, controlling the power profile and absolute

level of the transmitted data.

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 ASP and

the PLP and a digital gain control signal is sent to the BBC. A

10-bit auxiliary DAC generates the appropriate analog control

signal for the IF amplifier. Additionally gain control can be

implemented by using two output flags of the ASP.

FLAGS

13 MHz VCTCXO

13 MHz VCTCXO

SYNTHESIZER

CONTROL SIGNALS

AGC

AFC

RAMP CONTROL

AGC

PAERROR

LOCK

10-BIT DAC

I

Q

I

Q

PLP

ASP

BASEBAND/AUXILIARY SECTION OF AD7015

DIGITAL FIR FILTER

RAMPING RAM

GSMSK

MODULATOR

BURST

STORE

PA

IF

10-BIT DAC

10-BIT DAC

8-BIT DAC

DIGITAL FIR FILTER

DIGITAL FIR FILTER

DIGITAL FIR FILTER

DIGITAL FIR FILTER

BASEBAND

SERIAL

INTERFACE

AUXILIARY

SERIAL

INTERFACE

Figure 2. Control of RF Section