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RF/Microwave??

Save power with on-off keying

Posted: 01 Dec 2008 ?? ?Print Version ?Bookmark and Share

Keywords:On-off keying modulation? OOK? communication protocols? short-distance wireless applications?

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 and tire-pressure monitoring systems. OOK is especially popular in battery-operated portable applications since such systems can save on transmit power when (not) sending "0." Carrier frequencies involved can vary greatly depending on the application, such as 「2MHz in some low-frequency wired communications in base stations to 「433MHz in short-range wireless communications that make use of the ISM (industrial, scientific and medical) band.

Various wireless technologies!including Bluetooth, Zigbee and 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 FSK, PSK and 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.

Simple ASK/OOK hardware implementations become a relatively straightforward 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 IR 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 with OOK, at a lower cost than FSK, but at higher power consumption levels than OOK.

Shown are circuits using MAX9933 in an ASK application MAX9930 in an OOK application.

ASK implementation
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 the length of the 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).

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.

On-off key
The MAX9930, an RF power detecting controller, was designed for use in a feedback control loop for power amplifiers (PA) in an AGC loop. However, when configured in open-loop (i.e. without a PA to close its feedback loop from OUT to RFIN), it can just as easily be used in OOK applications. A REF voltage representing a threshold well below the lowest "1" signal level that will be received in the application can be used to extract the OOK information.

The RF signal into the RFIN pin is again externally AC-coupled into the chip. The front-end of MAX9930 is a peak-detector, and it internally detects the peak of input RF signal. The voltage fed to the SET pin then acts as a comparator threshold (ideal for OOK detection). Resistors Rfb and Rin provide comparator hysteresis for increased noise immunity. Capacitance CCLPF can be minimized to increase data rate. Rfb, comparator hysteresis resistor is chosen as 300k?, and Rin is set as 10k?.

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." There is really no "ASK transmitter," only a bad OOK transmitter. The receiver system can be either an OOK receiver system (with fixed threshold) or an ASK receiver system (with adaptive threshold). In today's connected world, multiple modes of communication between circuits are needed. ASK and OOK are two such protocols.

- Prashanth Holenarsipur
Lead Product Definer
Maxim Integrated Products Inc.





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