I'm OOK - You're OOK?

In today’s connected world, multiple modes of communication between circuits are needed - ASK and OOK are two such protocols. This article describes possible application solutions and their application simplicity when compared to other protocols available in the market.

Amplitude Shift-Key (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 Key (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 small-distance wireless applications – examples of which can span 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 – e.g. ~2 MHz in some low-frequency wired communications in base-stations to ~433 MHz in short-range wireless communications that make use of the ISM (Industrial, Scientific, and Medical) band.

Various wireless technologies have made headway in today’s consumer world – Bluetooth, ZigBee, Wi-Fi. These protocols offer means of secure communication between devices, and typically operate in the 2.4 GHz ISM band using a combination of FSK – Frequency-Shift-Keying, PSK – Phase-Shift-Keying and ASK – Amplitude-Shift-Keying (Amplitude Modulation). 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 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 and 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.

Parameter	Bluetooth	Zig-Bee	ASK / OOK - general ISM
Frequency	2.4 GHz	2.4 GHz	315 MHz - 2.4 GHz
Battery Life	Low	High	High
Speed	800 kbps	200 kbps	2 Mbps
Relative Cost	Medium	Medium	Low
Industry Standards	Yes	Yes	No

Simple ASK/OOK hardware implementations 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-to-point wired infrastructure and wireless infrared type of link is possible. Depending on the application, implementation costs can be 2x to 5x higher for alternative technologies. Security can still be over-laid on this link by incorporating bidirectional 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 band-pass 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 2 MHz to 1.6 GHz. In particular, it delivers a logarithmic voltage proportional to signal level between -58 dBV to -13 dBV (i.e., 1.25 mVrms to 223 mVrms). Figure 1 shows its use in an ASK receiver signal chain.

The RF signal fed into the RFIN pin is externally AC coupled. Since the MAX9933 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 extremely sensitive to very small signals – thus allowing the ASK receiver to discriminate between small input 1 and 0 signal levels easily. 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 MAX9933 is tested as an envelope detector, and a MAX9030 comparator is used with an adaptive reference to generate digital output bits. The test waveform has a 10 MHz carrier frequency, and a 40 kbps data rate. The CCLPF filter capacitor is 150 pF and R-C filter is comprised of a 100 kΩ resistor and a 0.22 µF capacitor.

Figure 2: Response of MAX9933 RF Detector to RF input signal with modulation frequency of 10 MHz, 40 kbps data rate. The two waveforms show output response (yellow) to input signal (blue) of (a) -10 dBm ASK signal, and (b) -40 dBm OOK signal. The two waveforms at the MAX9930 comparator inputs are shown in pink and green at the bottom.

OOK: 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 automatic gain control (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 as shown in Figure 3. 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.

Figure 3: Circuit showing use of MAX9930 in OOK applications.

OOK transmitter

An OOK transmitter’s simplicity is without equal. It 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 is a very good example of a part that uses the input digital data stream to modulate output of a crystal-based PLL oscillator feeding a power amplifier. 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).