Digital Signal Modulations with ASK: RF Modulation Schemes Part 3 of 7

We have seen how analog signals can be imprinted in a carrier. Now, we enter the digital world. Modern communication is exclusively made with bits because the most efficient signal processing is digital (microcontroller, DSP, ADC, DAC…). Transmitting and receiving sequences of 1 and 0 is much more convenient, easier to control and robust against noise and interference.
 

Figure 1: Robustness of analog versus digital signals against noise

Figure 1 shows how an analog signal that’s polluted by noise can be difficult to recover, while a digital signal can easily be recovered from noise. An analog signal plus noise just gives us a modified analog signal, destroying the original shape. A digital signal has only two values, 0 and 1, and they can be easily differentiated as “above” or “below” a threshold, making noise easy to distinguish. Of course, that’s as long as noise amplitudes are not in the same range as the useful signal. This is why digital modulations are far more diverse and developed than analog modulations.
 

ASK Modulations Principle

Amplitude shift keying (ASK) is the simplest digital modulation mode. The message has only two values: 1 and 0. The 1 and 0 change the carrier amplitude by high and low levels, respectively. See how the messaging signal in Figure 2 is just a binary flow of 1 and 0:

Figure 2: ASK modulation principle

The ASK modulator circuit in figure 3 can be derived from the ones used for analog signal seen in the first episode of this series. The messaging signal here is just a binary flow of 1 and 0. The mixer multiplies the carrier signal by a flow of a rectangular signal having two values, low and high, modulating the carrier amplitudes. The circuit principle is nearly identical to the one described for analog messages. The only difference between ASK and amplitude modulation is that, in the case of ASK, the input has only two possible levels.

Figure 3: Diagram of a digital message mapped to an ASK modulated carrier
 

Binary Amplitude Shift Keying

ASK has various versions or types. The simplest, described so far, is the binary amplitude shift keying (B-ASK). Because there are only two different amplitude levels for the carrier, only 1 bit of information can be sent per unit of time. This means the symbol rate (symbols per second or SPS) is identical to the bit rate (bits per second or BPS).

Figure 4: Binary amplitude modulation signals (B-ASK)
 

ASK Modulators

An ideal ASK modulator is illustrated below. A carrier frequency is duplicated into two different amplitudes, A and B. The messaging data is connected to a multiplexer, and amplitudes A and B are translated to their corresponding bit value. The ASK output appears then with two different amplitudes.

Figure 5: Diagram of an ideal ASK modulator
 

Since analog mux is not perfect (made by analog switches), such an ASK modulator is not practical to build. Many other ways exist that provide better noise performance, frequency band usage, and circuit integration.

Figure 6: Diagram of a practical, real-world ASK modulator
 

ASK Frequencies

Since the message is a square wave form, the modulation will generate the two main peaks around the carrier frequency. Others will be spaced at the fundamental frequency of the message and all the harmonic multiples of the fundamental. For example, a 1 kHz carrier modulated by a 100 Hz square wave will have the following spectrum:

Figure 7: Binary phase shift keying (PSK) typical frequency spectrum
 

The carrier frequency (1 kHz) is also present in the ASK signal because the data stream has a DC component. Carrier frequency is used at the receiver side to decode the signal.
 

ASK Modulation Index

ASK varies the carrier amplitude with two levels: high (usually the full amplitude of the carrier) and low, which can sometimes have a very wide delta. The modulation index M is a parameter indicating that difference. If A and B are the carrier amplitudes for ASK high and low levels, respectively, then:

M = (A-B)/A (in %)

Figure 8: ASK modulation index
 

At 10% modulation index (labeled as 10%ASK), we would have a weak modulation since the difference between A and B is low. A higher index gives a stronger modulation, and the advantage: the receiver will have more margin to differentiate the data 0 and 1.
 

On-Off Keying (OOK): A Particular Use Case of ASK

When the low level carrier amplitude approaches zero, we have On-Off Keying or OOK modulation where the differentiation between high and low is maximum - i.e., on or off.

Figure 9: OOK modulation principle
 

ASK Modulation Advantages and Disadvantages

Suitable Application Domains for ASK and ADI Solutions

Despite the poor data rate offered and the weak resistance to jammers, ASK modulations are still popular and are used in many applications such as medical telemetry, short distance links (RFID, NFC), and even in digital TV and radio broadcasting. Examples of ASK circuits made by ADI are:

• MAX1472, an automotive antitheft alert system using ASK modulation
• MAX7049, an ISM bands 280MHz-945MHz frequency programmable ASK/FSK transmitter
• MAX7036, an ISM bands 280MHz-945MHz frequency programmable ASK/FSK receiver
• MAX66301, an OOK RFID identification transceiver with SHA3 identification

Further Reading

Previous post

Next post

List of all posts in the series