RF transmitter uses AMI encoding

Posted on Jul 20, 2012

The circuit in Figure 2 shows a 16-channel, AMI-encoded RF transmitter for data rates as high as 28.8 kbps. The circuit operates in the unlicensed (FCC Part 15) 902- to 928-MHz industrial, scientific, and medical (ISM) band and is reliable for open-field distances as long as 1000 ft. You can easily modify it to sustain higher or lower data rates. IC1, IC2, R1, and R2 perform the AMI encoding. IC1A, a simple D flip-flop, controls the high-impedance state on Pin 13 of IC2. IC1B alternates the data presented to IC2 whenever the previous data is a logic one. This data has a slight delay to avoid glitches in the output waveform. R1 and R2 set the high-impedance voltage to ½VCC. R2 adjusts the peak deviation of the transmitted RF signal from IC3, nominally set at 50 kHz. C1 and R2 constitute a lowpass filter with corner frequency set to attenuate the data at frequencies above the third harmonic.

RF transmitter uses AMI encoding
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IC3 is a low-power, integrated RF transmitter (Motorola, www.motorola.com) targeting ISM applications. Its voltage-controlled oscillator is a parallel-resonant Colpitts type. The varactor diode, D1, controls the modulation, and D2 sets the center frequency. The modulation, D1, is set to approximately 60 kHz/V, and the frequency adjustment, D2, is set to 7 MHz/V. The mixer of the IC serves as a buffer; you can adjust R3 for desired output power. C2/L1 and C3/L2 conjugately match the source-to-load impedances. These values can vary, depending on the parasitics of your layout. CF1 (TDK Corp, www.tdk.com) provides final filtering of the output before transmission. IC4 synthesizes the desired carrier frequency. The IC internally divides by 512 the frequency that crystal Y1 establishes. With Pin 16 of IC3 tied low, the synthesized frequency is fOUT=(2.048 MHz/512)*65*N, where N represents the digital value present on the N bus of IC. As illustrated, N can vary from 3472 to 3487, yielding 16 discrete output channels from 902.72 to 906.62 MHz in 260-kHz steps. R4 and C4 form a lowpass filter with corner frequency set to a value substantially lower than the internal reference frequency of the synthesizer or the data stream, whichever is lower. As with any RF design, you should give careful consideration to parts placement and shielding. You should also apply generous decoupling. If desired, you can hold Pin 17 of IC3 at logic...

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