Figure 3. Circuit showing the MAX9930 RF-detecting controller in an OOK application.
The RF signal fed through the RFIN pin is again externally AC-coupled into the chip. The front-end of the MAX9930 controller is a peak detector, and it internally detects the peak of the input RF signal. The voltage fed to the SET pin then acts as a comparator threshold, which is ideal for OOK detection. Resistors RFB and RIN provide comparator hysteresis for increased noise immunity, with RFB chosen at 300kΩ and RIN at 10kΩ; CCLPF can be minimized to increase the data rate. In Figure 4, the test results are shown with the same input conditions as in the Figure 2 test.
Figure 4. Response of the MAX9930 RF-detecting controller to the RF input signal with a modulation frequency of 10MHz, a 40kbps data rate, and a -40dBm OOK signal. REF is set at 500mV. Output digital bits are shown in yellow, and the input RF signal is blue.
OOK Transmitter
An OOK transmitter's simplicity is without equal. It quite simply involves sending a carrier wave to a PA feeding an antenna/cable to transmit a '1' or to send nothing to transmit a '0'. The MAX1472 VHF/UHF transmitter is a very good example of a part that uses the input digital data stream to modulate the output of a crystal-based PLL oscillator feeding a power amplifier. The receiver system can be either an OOK receiver system (with a fixed threshold) or an ASK receiver system (with an adaptive threshold).
Conclusion
In today's connected world, multiple modes of communication between circuits are needed. ASK and OOK are two such protocols, and this article has described possible application solutions and their simplicity when compared to other protocols available in the market.
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