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  • Amplitude-Shift-Keying Receiver

  • Created: Sep 03, 2014

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Amplitude Shift Keying (ASK) is a data transmission by varying the amplitude of the transmitted signal. The ASK is composed of three blocks namely the transmitter, channel, and receiver. This application note describes a low cost, high performance UHF short-range radio ASK receiver design using the Microchip Technology rfRXD0420. It features low cost single conversion superheterodyne receiver architecture.

The frequency tolerance of the crystal should be within the communication system's tolerances (transmitter and receiver) and in accordance with local radio regulations. Components C10, C11, and R3 comprise a second-order low-pass loop filter for the PLL synthesizer. The components selected have a wide loop bandwidth to suppress noise over a wide frequency range. The rfRXD0420 is a single conversion superheterodyne architecture with only one IF frequency (flo = 423.22MHz). Care should be taken to filter the image frequency (frf-image = 412.52MHz). A SAW filter can effectively filter the image frequency with a minimum of 40dB attenuation. The SAW filter has the added benefit of filtering wide-band noise and improving the signal-to-noise ratio (SNR) of the receiver. Components C15, L3, and C17 provide collector current via a pull-up, impedance matching between the LNA and 1IF stages, and decoupling (C17). Pins 1IF+ (Pin 6) and 1IF- (Pin 7) are open-collector outputs that are connected to external pull-up resistors (R5, R4 respectively). A ceramic IF filter (F2) is placed between 1IFOUT (Pin 9) and 2IFIN (Pin 11) to filter the 10.7MHz IF signal. Selection of the ceramic filter bandwidth depends on the signal rate of the incoming digital data signal. The Received Signal Strength Indicator, RSSI (pin 21), is the final signal in the receiver chain. This baseband signal is proportional to the log of the RF input signal at 2IFIN (pin 11). The RSSI signal is first low-passed filtered and then compared to a dynamic reference voltage (created by RC low-pass filter R1 and C2) to determine if the received signal represents a binary one or zero. The internal operational amplifier (OPA+, OPA-, and OPA) is configured as a comparator. The comparator circuitry is also known as a data slicer.

The reference design is suitable for: wireless remote command and control, Remote Keyless Entry (RKE), security systems, and low power telemetry applications. The specifics of this receiver reference design are: single channel, fixed frequency at 433.92MHz ASK modulation and signal rate of 4800 baud.



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