BK4819

Proprietary and Confidential

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channel filtering, a variable gain amplifier (VGA) and a high precision analog-to-

digital converter(ADC). The block diagram of the FM receiver is shown in Figure

2.

At the RF front-end part, the LNA is a differential low-noise amplifier with single-

ended input. The LNA is followed by a quadrature mixer that down-converts the

RF signal directly to IF signal. Low-IF image rejection architecture is

implemented in order to eliminate the external SAW filters.

At the IF part, the down-converted in-phase IF signal (IF/I) and quadrature-phase

IF signal(IF/Q)are first filtered by the BPF, and then amplified by the VGA.The

Sigma-Delta ADC sample analog IF signal from VGA, and convert it to digital IF

signal. Then the digital signal will be send to DSP for second down-conversion

and audio processing.

To avoid serious distortion with high-level input power, AGC function is added to

automatically adjust the gain of LNA and the gain of VGA.

LF

DIV

CP

PFD

Frac-N

-

Mod

Reference Clock

from OSC

Ramping

IF/I

IF/Q

Rfin

Rfout

LNA

Mixer

Filter + VGA + ADC

Buffer + PA Driver

VCO

FM Receiver

FM Transmitter

RF Frequency Synthesizer

Do/I

Do/Q

Figure 2. Radio Block Diagram

1.2.2 FM Transmitter

The transmitter is a single-ended amplifier including a buffer and a PA driver. The

block diagram of the transmitter is shown in Figure 2. Due to FM modulation is of

constant envelope, the amplifier works in saturated mode to save current

consumption.

The output power of FM transmitter can be programmed from -24dBm to +7dBm

through 3-wire SPI interface.

滤波器