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WE2408

2.4GHz Single Chip FM Transceiver

Unspecified 

WiNEDGE & WiRELESS

Winceiver WE2408

2.4GHz Single Chip FM Transceiver

Receive PLL Synthesizer and VCO

The receive PLL synthesizer with voltage controlled oscillator (VCO), is designed to provide a low

phase noise local oscillator. The Rx VCO operates in a balanced mode at 1.5x the Rx local oscillator

frequency. The VCO frequency is immediately divided by a factor of 2 and then multiplied by 3 to

provide the local oscillator for the quadrature down-conversion mixer. The synthesizer is 17 bits in

total (excluding the proceeding divide by 2). These 17 bits are split into 14 bits to provide the input to

the PLL phase detector to be compared with the reference frequency, with an additional 3 bits

providing fractional-n capability, to allow channel frequency definition in steps of 1/8 of the PLL

reference frequency.

This LO signal also drives the receive synthesizer and demodulator synthesizer.

The Rx VCO center frequency is determined by an external tank circuit comprised of an external,

center-tapped, inductor. The tank circuit is connected to pin 41 and 42.

An external PLL loop filter network, connected to the RPLL pin (pin 39), filters the VCO control

voltage. This control voltage is used to tune the tank frequency of the VCO via an internal, common-

anode, varactor pair. The receive frequency for the WE2408 is programmed via serial interface (Data,

Clock, and Load Enable).

Demodulator synthesizer

This synthesizer consists of programmable and fixed dividers which determine the IF frequency and

the demodulator bandwidth of the digital FM demodulator. The demodulator synthesizer is controlled

by 3 PDR bits of the reference frequency register, programmable via serial interface. The

demodulator bandwidth is programmable between 150kHz and 25kHz.

Variable Gain Amplifiers

The gain of the receiver can be dynamically adjusted via the serial interface to maintain signal linearity

before the demodulator. This enables the achievement of high values of SINAD for an analog FM link.

An Automatic Gain Control (AGC) function is also available on-chip. The gain can be adjusted in 3

locations along the receive path:

x A 10dB RF pad before the down-conversion quadrature mixer

x Four step baseband attenuators in the down-conversion quadrature mixer load circuits

x Three-step baseband Variable Gain Amplifiers after the baseband I and Q low pass filters

In addition, each VGA provides differential to single-ended conversion and amplification of the

baseband signal, prior to the I/Q low-pass filters. These gain stages are referenced to pre-filter

ground (PFGND), an internally generated virtual ground.

LPF1 (I/Q)

This first stage of the I/Q baseband low pass filter (LPF) section consists of active, Sallen-key type

filters. These filters provide a combination of low noise figure and gain along with a wide dynamic

input range. The purpose of these filters is to provide preliminary rejection of the out-of-band

interferences. The reduction of out-of-band interferer levels, reduce the dynamic range requirements

for the following filter stages in LPF2. The –3dB corner frequency of these LPFs are set via external

RC values.

© 2003 WiNEDGE & WiRELESS PTE LTD

Rev Date: 2003 May 23

Page 7

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