MICRF500

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 MICRF500 consists of a PLL frequency synthesizer and a power amplifier. The frequency synthesizer MICRF500 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 of MICRF500 has two different N, M and A frequency dividers.

FSK modulation of MICRF500 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 MICRF500 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 of MICRF500 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 of MICRF500  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.

A two pin serial interface of MICRF500 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. MICRF500 gives maximum input sensitivity and transmit output power.