March 2003

1

MICRF500

MICRF500

Micrel

MICRF500

700MHz to 1.1GHz RadioWire™ RF Transceiver

Final

General Description

The MICRF500 is a single chip UHF transceiver designed for

spread spectrum communication (FHSS) intended for ISM

(Industrial, Scientific and Medical) and SRD (Short Range

Device) frequency bands from 700MHz to 1100MHz with

FSK data rates up to 128k baud.

The transmitter consists of a PLL frequency synthesizer and

a power amplifier. The frequency synthesizer consists of a

voltage-controlled oscillator (VCO), a crystal oscillator, dual-

modulus prescaler, programmable frequency dividers and a

phase-detector. The loop filter is external for flexibility and

can be a simple passive circuit. The VCO is a Colpitts

oscillator which requires an external resonator and varactor.

FSK modulation can be applied externally to the VCO. The

synthesizer has two different N, M and A frequency dividers.

FSK modulation can also be implemented by switching

between these dividers (max. 2400bps). The lengths of the N

and M and A registers are 12, 10 and 6 bits respectively. For

all types of FSK modulation, data is entered at the DATAIXO

pin (see application circuit). The output power of the power

amplifier can be programmed to eight levels. A lock detect

circuit detects when the PLL is in lock.

In receive mode the PLL synthesizer generates the local

oscillator (LO) signal. The N, M and A values that give the LO

frequency are stored in the N0, M0 and A0 registers. The

receiver is a zero intermediate frequency (IF) type in order to

make channel filtering possible with low-power integrated

low-pass filters. The receiver consists of a low-noise amplifier

(LNA) that drives a quadrature mixer pair. The mixer outputs

feed two identical signal channels in phase quadrature. Each

channel includes a preamplifier, a third order Sallen-Key RC

low pass filter that protects the following gyrator filter from

strong adjacent channel signals and finally, a limiter. The

main channel filter is a gyrator capacitor implementation of a

seven-pole elliptic low pass filter. The elliptic filter minimizes

the total capacitance required for a given selectivity and

dynamic range. The cut-off frequency of the Sallen-Key RC

filter can be programmed to four different frequencies: 10kHz,

30kHz, 60kHz and 200kHz. An external resistor adjusts the

cut-off frequency of the gyrator filter. The demodulator de-

modulates the I and Q channel outputs and produces a digital

data output. It detects the relative phase of the I and the Q

channel signal. If the I channel signal lags the Q channel, the

FSK tone frequency lies above the LO frequency (data ‘1’). If

the I channel leads the Q channel, the FSK tone lies below the

LO frequency (data ‘0’). The output of the receiver is available

on the DATAIXO pin. A RSSI (Receive Signal Strength

Indicator) circuit indicates the received signal level.

Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com

A two pin serial interface is used to program the circuit.

External components are necessary for RF input and output

impedance matching and decoupling of power. Other exter-

nal components are the VCO resonator circuit with varactor,

crystal, feedback capacitors and components for FSK modu-

lation with the VCO, loop filter, bias resistors for the power

amplifier and gyrator filters. A T/R switch can be implemented

with 2-pin diodes. This gives maximum input sensitivity and

transmit output power.

Features

• Frequency range: 700MHz to 1100MHz

• Modulation: FSK

• RF output power: 10dBm

• Maximum data rate: 128k bauds

Applications

• Telemetry

• Remote metering

• Wireless controller

• Wireless data repeaters

• Remote control systems

• Wireless modem

• Wireless security system

Ordering Information

Part Number

Ambient Temp. Range

Package

MICRF500BLQ

–40

°C to +85°C

44-Lead LQFP

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