Circuit Ideas: Setting Up a Low-Power Transceiver

March 6, 2008
There are a variety of transceiver chips available that feature low-power consumption.

Edited by Leland Teschler

This makes them candidates for use where power comes from batteries. One chip in this category is the TI CC1000 which works in the UHF range. It is often powered from a 3-V coin cell and uses less than 10 mA in receive mode. Current consumption in transmit mode depends on output power but is less than 30 mA at 10 dBm. Typical applications include home automation, wireless security systems, meter reading, and toys.

The CC1000 is generally intended for use over short ranges. It operates at 315, 433, 868, and 915 MHz but can be programmed in steps of 250 Hz for operation at other frequencies in the UHF range.

The chip operates as a conventional heterodyne receiver. The RF input is converted to an intermediate frequency, then gets filtered and fed to a frequency shift-key demodulator. Demodulated data is available on pin DIO, a synchronized data clock signal at DCLK.

Values for the antenna matching network components C41, L41, and C42 shown here are for operation at 315 MHz using a quarter- wave whip antenna. Of course, antenna VSWR will affect these values, and they must be recalculated when using the chip to transmit and receive at other frequencies.

A voltage-controlled oscillator on the chip uses an external inductor L101 to set its frequency. Its PCB layout should be close to pins L1 and L2 and symmetrical to minimize affects of stray fields and component variations.

The digital bit stream on DIO controls the frequency shift-keyed output. In this case a PIC16F62X processor generates the bit stream and reads the demodulated output through one of its bidirectional I/O ports. Similarly, it handles chip configuration through a three-wire digital serial interface on PCLK, PDATA, and PALE pins.

This particular circuit was battery powered. Circuits powered from an ac supply should incorporate filtering and decoupling to avoid problems from harmonics fed in through the supply.

Douglas Bahniuk
Adelsys Inc.
Cleveland Heights, Ohio.

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