RF109

2400 MHz Digital Spread Spectrum Transceiver

2

Conexant

100646A

Conexant Proprietary

1/19/00

LNAATTN

Synthesizer

LNAIN

Modulator

Gain

Control

RFO1

RFO2

LNA

FREF

GMCRES

RXQ

AGC

RXI

STROBE

CLK

DATA

TXD

External

CSERVO

MODSET

PS1

PS2

LPF

VCO

External

Serial

Interface

External

CSERVO

Power

Mgmt.

RXEN

SYNTHEN

TXEN

Figure 2. RF109 Block Diagram

Technical Description

Receive Path_______________________________________

The LNA provides two gain levels for coarse Automatic Gain

Control (AGC), which are selected via the LNAATTN control.

The signal is down-converted to In-phase and Quadrature-

phase (I/Q) baseband signals using a matched pair of mixers

and the LO.

The receive baseband bandwidth has a bandpass characteristic.

The I/Q baseband signals are internally low-pass and high-pass

filtered to attenuate out-of-channel signals and to remove DC

components. The low-pass cutoff is determined by the GmC

connected between pins 32–33, and pins 34–35.

The baseband high-pass cutoff frequency should be set much

lower than the low-pass cutoff frequency or else the servo loop

will become unstable.

The optimum receive bandwidth values are:

A matched pair of VGAs provide fine AGC. The differential I/Q

baseband signals are DC-coupled to the RXI+, RXI-, RXQ+, and

RXQ- outputs, respectively.

Transmit Path_______________________________________

The transmit path consists of an amplifier and a mixer. The

mixer modulates the LO with baseband data supplied to pin 8.

The transmit RF outputs from the RF109 are differential and

matched for a 100

Ω differential load. If a single-ended

connection is required, then the unused output must be suitably

terminated by a 50

Ω resistor (as shown in Figure 5).

The transmit output power is determined by the output power

control inputs, PS1 (pin 21) and PS2 (pin 22), and by the value

modulator which is then multiplied by a factor set by the state of

PS1 and PS2. PS1 and PS2 input programming is given in the

Transmitter Section of Table 3.

LO Generation ______________________________________

The LO is generated by a programmable PLL frequency

synthesizer and a 2.4 GHz external VCO. Synthesizer

performance parameters are determined by the loop filter, the

external reference oscillator, the sensitivity and phase noise of

the VCO, and the frequency synthesizer programming.

The RF109 requires differential inputs for VCO1 (pin 38) and

VCO2 (pin 39). The typical differential input level is 200 mVp-p.

A BALUN transformer, shown in Figure 5, is used to generate

differential signals from a single-ended VCO output.