3

PLUTO

PIN DESCRIPTION (continued)

No

Pin Name

Type

A/D

Description

55

RXQP

Input

Analog

Receive Q Channel Input (+ve)

56

RXQM

Input

Analog

Receive Q Channel Input (-ve)

57

SUB

Ground

Substrate - Ground

58

RXIP

Input

Analog

Receive Q channel input (+ve)

59

RXIM

Input

Analog

Receive Q channel Input (-ve)

60

VDD

Power

Power Supply

61

GND

Ground

Ground

62

VREF<0>

Input/Output

Analog

CDMA Receive Circuit Voltage Reference De-Coupling

63

AD<0>

Input

Analog

AUX ADC Input

64

AD<1>

Input

Analog

AUX ADC Input

65

AD<2>

Input

Analog

AUX ADC Input

66

AD<3>

Input

Analog

AUX ADC Input

67

Vtest

Output

Digital

RX Filter tuning tone output - pulled low when inactive

68

EnTest

Output

Digital

RX Filter tuning mode control output - pulled low when inactive

69

FC_Q

Output

Analog

RX Filter Q channel FC control

70

FC_I

Output

Analog

RX Filter channel FC control

71

BAL

Output

Analog

RX Filter Gain Balance Control

72

VDD

Power

Power Supply

73

GND

Ground

Ground

74

TXIP

Output

Analog

Transmit Circuit channel Complementary Output (+ve)

75

TXIM

Output

Analog

Transmit Circuit I channel Complementary Output (-ve)

76

SUB

Ground

Substrate - Ground

77

TXQP

Output

Analog

Transmit Circuit Q channel Complementary Output (+ve)

78

TXQM

Output

Analog

Transmit Circuit Q channel Complementary Output (-ve)

79

FMTX

Output

Analog

Transmit Circuit FM output

80

VREF<1>

Input/Output

Analog

Transmit Circuit Voltage Reference De-coupling

FUNCTIONAL DESCRIPTION

Baseband TX interface circuit

The Pluto baseband transmit circuit acts as an interface

between the baseband signal processor and the RF/IF sections

in a CDMA/AMPS compatible mobile telephone.

The TX circuit has two modes of operation :

CDMA mode, transmit data that has previously been encoded

by the baseband digital signal processor is converted to

equivalent analog signals by matched digital-to-analog

converters, these signals are then filtered to remove the image

of the sample clock that would otherwise be present at the

output before being output to the I and Q modulator as

differential signals.

FM mode, transmit data is treated in much the same way as

in CDMA mode except that only one DAC is used and (because

of the much lower bandwidth of AMPS signals) a different

reconstruction filter is used before the analog fm signal is output

to the mixer as a single ended signal.

CDMA Transmit Signal Path

CDMA TX DACs

In CDMA mode two matched 8-bit DACs are used to

generate the in-phase and quadrature signals, the input

data for the DACs is obtained by multiplexing over an 8-bit

parallel input port (TXD<7:0>). The transmit data rate is

twice as fast as the differential transmit clock (TXCLK).

Incoming data that is valid during the rising edge of the

transmit clock is loaded into in In-Phase DAC & incoming

data that is valid on the falling edge of the transmit clock is

loaded into the Quadrature DAC - I and Q values must be

modified in the digital baseband chip to account for the half-

cycle delay between them.

CDMA Analog Reconstruction Filters

The frequency spectrum at the output of the transmit

DACs contains unwanted frequency components.

Reconstruction filters are used to smooth the DAC output

signals, providing continuous time output signals at the I and

Q output pins thereby removing these undesirable signals.

The low pass filters used are 5th order Butterworth,

continuous time filters with a nominal cut-off frequency of 1.2

MHz. These filters are designed to have a linear phase

response in the pass band. On-chip reconstruction filters

minimise the phase and amplitude mismatch between I and

Q channels.