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Amplitude shift-keying (ASK) is a popular modulation technique used in digital data communication for a large number of low-frequency RF applications. The source transmits a large amplitude carrier when it wants to send a '1' and it sends a small amplitude carrier when it wants to send a '0' in its simplest form. On-off keying (OOK) modulation is a further simplification of this method, where the source sends NO carrier when it wants to send a '0'.
ASK and OOK communication protocols are commonly used in short-distance wireless applications, such as home automation, industrial networks, wireless base stations, remote keyless entry (RKE), and tire-pressure monitoring systems (TPMS). OOK is especially popular in battery-operated portable applications since such systems can save on transmit power when (not) sending '0's. Carrier frequencies involved can vary greatly depending on the application, such as ≈2MHz in some low-frequency wired communications in basestations to ≈433MHz in short-range wireless communications that make use of the ISM (industrial, scientific and medical) band.
Various wireless technologies, including Bluetooth, ZigBee, Wi-Fi. have made headway in today's consumer world. These protocols offer means of secure communication between devices, and typically operate in the 2.4GHz ISM band using a combination of frequency shift keying (FSK), phase shift keying (PSK), and amplitude shift keying (ASK). The security offered by these approaches includes channel hopping and spread-spectrum modes of communication. Such schemes can be difficult to overhear, offering increased security as well as improved noise immunity. All of these methods spend transmit energy when sending both a '1' and a '0'. Unfortunately, these protocols also have a relatively high complexity and cost of hardware implementation, especially if security and high noise immunity are not hard requirements.
Wi-Fi is specifically aimed at high-data rate, wide-reach applications and is likely an overkill for simple control + monitoring applications. ZigBee is considered ideal for the upcoming field of sensor networks, while Bluetooth has found acceptance in a range of consumer audio devices and personal wireless devices. Table 1 provides a simple comparison of various performance features for Bluetooth, ZigBee, and ASK/OOK approaches.
Simple ASK / OOK hardware implementations though become a relatively straight-forward choice due to their low cost of implementation, in extremely long-life battery operated applications, or if access to point-point wired infrastructure and wireless infra-red type of link is possible. Depending on the application, implementation costs can be 2x to 5x for alternative technologies. Security can still be over-laid on this link by incorporating bi-directional interrogation schemes between transmitter and receiver, such as by exchanging a special code, if necessary. ASK offers better noise immunity compared to OOK, at a lower cost than FSK, but at higher power consumption levels than OOK.
ASK: Amplitude Shift-Keying
ASK receiver front ends typically comprise three blocks: an input bandpass filter to discern the carrier frequency of interest from a broadband input noise spectrum, an envelope detector to extract the information of interest, and a comparator to obtain binary outputs. The comparator trigger threshold is derived from the output of the envelope detector itself; this enables the threshold level to auto-scale with received signal level that can vary depending on length of channel and transmitter strength.
One possible implementation of a front end uses the MAX9933 , an RF power detector that can read input signals with a 45dB dynamic range from 2MHz to 1.6GHz. In particular, it delivers a logarithmic voltage proportional to signal level between -58dBV to -13dBV (i.e. 1.25mVrms to 223mVrms). Figure 1 shows its use in an ASK receiver signal chain:
1. Circuit showing use of MAX9933 in ASK applications.
The RF signal fed into the RFIN pin is externally AC coupled. Since this is a peak-responding RF detector, it essentially functions as a simple envelope detector, even for small mV level signals. Its log transfer function for input RF voltage amplitude vs. output DC voltage gives a proportional-to-dB characteristic that makes it sensitive to very small signals, allowing the ASK receiver to discriminate between small input 1 and 0 signal levels. The value of capacitor CCLPF determines response bandwidth at the chip's output, and thus is determined by data-rate expected. Figure 2 illustrates the output waveform when the power detector is tested as an envelope detector, and a comparator is used with an adaptive reference to generate digital output bits. The test waveform has a 10MHz carrier frequency, and a 40kbps data rate. The CCLPF filter capacitor is 150pF and R-C filter is comprised of a 100kΩ resistor and a 0.22μF capacitor.
2. Response of MAX9933 RF Detector to RF input signal with modulation frequency of 10MHz, 40kbps datarate. The two waveforms show output response (yellow) to input signal (blue) of (top) -10dBm and -20dBm ASK signal, and (bottom) -40dBm OOK signal. The two waveforms at the MAX9930 comparator inputs are shown in pink and green at the bottom.
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