FM/FSK ICs for a Complete UHF Radio Transceiver

FM/FSK ICs for a Complete UHF Radio Transceiver

MX-COM Inc.
Winston-Salem, NC

The industrial, scientific and medical (ISM) frequencies in the 900 MHz region and the European 868 to 870 MHz bands have been proving ground for a wide array of communications systems, from medical and industrial telemetry to consumer communications. Many of these services have either entered the mainstream of commercial communications at other frequencies or continued to operate in these license-free bands. The model CMX017 transmitter IC and model CMX018 receiver IC are designed to serve the growing number of applications operating in this region (including analog and digital narrowband) as well as spread spectrum radio links, cordless phones, handheld data terminals and wireless local area networks. With the addition of only a few other peripheral components, the two ICs can be integrated to form a small, cost-effective transceiver capable of transmitting and receiving FM or frequency-shift keying (FSK) signals in the 860 to 965 MHz frequency range with input data rates up to 500 kbps.

The CMX017 and CMX018 ICs are single-chip solutions housed in 28-pin SSOP packages, each measuring 0.0054 in³ and operating from 2.7 to 3.3 V DC. Both ICs incorporate the features required in typical battery-powered operating environments, including a power-saving sleep mode. The CMX017 device combines a VCO modulator and RF power amplifier with adjustable maximum output power up to +20 dBm (100 mW). Frequency selection is achieved through a buffered oscillator, which provides sufficient output to drive an external synthesizer or fixed-frequency phase-locked loop (PLL). Two power-saving modes can be utilized: transmit standby and sleep mode. Both modes allow independent power-down control of both the modulator and power amplifier.

The FM/FSK modulator circuit employs an integrated oscillator that can be modulated directly by a DC-coupled input baseband signal to generate an FM or FSK signal. The power amplifier is composed of two fixed-gain stages biased for class B operation with a 50 W impedance. Use of simple external matching networks, shown in Figure 1 , can provide an additional 2 dB of power gain. The external PLL or synthesizer stabilizes the VCO to the desired channel frequency, and the designer has control over both external PLL filtering and VCO tank components (which affect closed-loop bandwidth, settling time and transient response).

Each transmitter stage can be powered down independently, which is useful when the CMX017 device is integrated with the CMX018 IC to form a transceiver. In sleep mode, all sections of the device are powered down and current consumption is reduced to less than 10 mA. In the transmit standby mode, the power amplifier driver and power amplifier stages are powered down while the modulator and buffered oscillator are not. In this situation, the external synthesizer or PLL can be stabilized to the center frequency of the desired channel prior to transmission.

In some systems, only very low output power is required and the CMX017 device can be configured to allow the drive signal to be fed directly from the modulator output to the antenna, bypassing the power amplifier. A simple connection implements this configuration, which greatly reduces current drain while retaining the ability for the modulator output to be enabled and disabled.

A typical application of the CMX017 IC requires the addition of only two ICs: an Analog Devices AD8532-SO8 XXXX and a Cypress WB1315X XXXX. Both the schematic and layout for the circuit are available from the company’s Web site at www.mxcom.com. Other ICs can be employed as well, depending on the end use of the transmitter.

The CMX018 FM/FSK dual-conversion superheterodyne receiver IC is designed to be used either in conjunction with the CMX017 device or as a stand-alone receiver, as shown in Figure 2 . The IC consists of a switched-gain low noise amplifier (LNA), two downconverters and integrated oscillators, a limiting amplifier, a temperature-compensated received signal strength indicator (RSSI) circuit, an FM/FSK demodulator and a zero-power mode control.

As with the CMX017 transmitter IC, the desired frequency is selected via an external PLL or synthesizer driven by the buffered RF oscillator signal from the first downconverter. The switched-gain function of the LNA can be used to increase receiver dynamic range. In the first mode, high gain is selected and the lowest noise figure is achieved, which is useful when maximum sensitivity is required to accommodate low level signals at the antenna. When strong signals that could increase intermodulation distortion are present, the gain of the LNA can be reduced by up to 22 dB (from approximately +16 dBm to -6 dBm). In this case, distortion is reduced at the expense of sensitivity. This capability gives the designer the flexibility to achieve the best balance between sensitivity and intermodulation distortion, depending on the demands of the application.

The first downconverter of the CMX018 IC includes a double-balanced mixer and a low noise preamplifier as well as oscillator components. The high side voltage-controlled LO uses an external varicap diode and tank resonator circuit that produce a first IF of 70 MHz. A buffered oscillator signal drives the synthesizer to establish tuning. The second downconverter (also with a double-balanced mixer and a low noise preamplifier) is a low side configuration that uses an external 60 MHz crystal to produce a second IF of 10.7 MHz.

The limiting amplifier provides IF amplification and limiting prior to the demodulator, and the RSSI circuit results in an indicated voltage of approximately 150 mV with an RF signal level of -100 dBm at the LNA input. The RSSI increases at 20 mV/dB to a maximum of 1.05 V with a -60 dBm input. The demodulator circuit employs a quadrature detector and external discriminator and phase shift network. The power-down mode of the CMX018 IC reduces current drain to less than 10 mA, which is useful in transceiver applications with the CMX017 IC. A typical layout is available from the company’s Web site located at www.mxcom.com.

Typical operating parameters of the CMX017 and CMX018 ICs are listed in Table 1 . The devices are supported by application notes, schematics and circuit layouts, which provide sufficient detail to significantly reduce the time required to produce a working prototype. In addition, the EV0171 evaluation kit for the CMX017 and the EV0181 evaluation kit for the CMX018 are readily available. These kits allow designers to quickly become familiar with the capabilities of the components.

MX-COM Inc.
Winston-Salem, NC
(336) 744-5050