March 2003

7

MICRF002/RF022

MICRF002/RF022

Micrel

Applications Information and Functional

Description

Refer to figure 1 “MICRF002 Block Diagram”. Identified in the

block diagram are the four sections of the IC: UHF

Downconverter, OOK Demodulator, Reference and Control,

and Wakeup. Also shown in the figure are two capacitors

(C

ceramic resonator. With the exception of a supply decoupling

capacitor, and antenna impedance matching network, these

are the only external components needed by the MICRF002

to assemble a complete UHF receiver.

For optimal performance is highly recommended that the

MICRF002 is impedance matched to the antenna, the match-

ing network will add an additional two or three components.

Four control inputs are shown in the block diagram: SEL0,

SEL1, SWEN, and SHUT. Using these logic inputs, the user

can control the operating mode and selectable features of the

IC. These inputs are CMOS compatible, and are internally

pulled-up. IF Bandpass Filter Roll-off response of the IF Filter

is 5th order, while the demodulator data filter exhibits a 2nd

order response.

Design Steps

The following steps are the basic design steps for using the

MICRF002 receiver:

1). Select the operating mode (sweep or fixed)

2). Select the reference oscillator

3). Select the C

4). Select the C

5). Select the demodulator filter bandwidth

Functional Diagram

Peak

Detector

AGC

Control

2nd Order

Programmable

Low-Pass Filter

5th Order

Band-Pass Filter

Programmable

Synthesizer

Control

Logic

Resettable

Counter

Reference

Oscillator

Cystal

or

Ceramic

Resonator

CAGC

ANT

SEL0

VDD

VSS

SEL1

SWEN

REFOSC

430kHz

Switched-

Capacitor

Resistor

WAKEB

CTH

DO

MICRF002

RF

Amp

IF

Amp

IF

Amp

Compa-

rator

Wakeup

Reference and Control

UHF Downconverter

OOK Demodulator

SHUT

Figure 1. MICRF002 Block Diagram

Step 1: Selecting The Operating Mode

Fixed-Mode Operation

For applications where the transmit frequency is accurately

set (that is, applications where a SAW or crystal-based

transmitter is used) the MICRF002 may be configured as a

standard superheterodyne receiver (fixed mode). In fixed-

mode operation the RF bandwidth is narrower making the

receiver less susceptible to interfering signals. Fixed mode is

selected by connecting SWEN to ground.

Sweep-Mode Operation

When used in conjunction with low-cost L-C transmitters the

MICRF002 should be configured in sweep-mode. In sweep-

mode, while the topology is still superheterodyne, the LO

(local oscillator) is swept over a range of frequencies at rates

greater than the data rate. This technique effectively in-

creases the RF bandwidth of the MICRF002, allowing the

device to operate in applications where significant transmit-

ter-receiver frequency misalignment may exist. The transmit

frequency may vary up to

±0.5% over initial tolerance, aging,

and temperature. In sweep-mode a band approximately

1.5% around the nominal transmit frequency is captured. The

transmitter may drift up to

±0.5% without the need to retune

the receiver and without impacting system performance.

The swept-LO technique does not affect the IF bandwidth,

therefore noise performance is not degraded relative to fixed

mode. The IF bandwidth is 430kHz whether the device is

operating in fixed or sweep-mode.

Due to limitations imposed by the LO sweeping process, the

upper limit on data rate in sweep mode is approximately

5.0kbps.

Similar performance is not currently available with crystal-

based superheterodyne receivers which can operate only

with SAW- or crystal-based transmitters.