Modulacion

Amplitude Shift Keying & Frequency Shift Keying
Aim: To generate and demodulate an amplitude shift keyed (ASK) signal and a binary FSK signal.

Intro to Generation of ASK Amplitude shift keying - ASK - in the context of digital communications is a modulation process, which imparts to a sinusoid two or more discrete amplitude levels. These are related to the number of levels adopted by thedigital message. For a binary message sequence there are two levels, one of which is typically zero. Thus the modulated waveform consists of bursts of a sinusoid. Figure 1 illustrates a binary ASK signal (lower), together with the binary sequence which initiated it (upper). Neither signal has been bandlimited.

Figure 1: an ASK signal (below) and the message (above) There are sharp discontinuitiesshown at the transition points. These result in the signal having an unnecessarily wide bandwidth. Bandlimiting is generally introduced before transmission, in which case these discontinuities would be ‘rounded off’. The bandlimiting may be applied to the digital message, or the modulated signal itself. The data rate is often made a sub-multiple of the carrier frequency. This has been done in thewaveform of Figure 1. One of the disadvantages of ASK, compared with FSK and PSK, for example, is that it has not got a constant envelope. This makes its processing (eg, power amplification) more difficult, since linearity becomes an important factor. However, it does make for ease of demodulation with an envelope detector. Intro to Bandwidth Modification

As already indicated, the sharpdiscontinuities in the ASK waveform of Figure 1 imply a wide bandwidth. A significant reduction can be accepted before errors at the receiver increase unacceptably. This can be brought about by bandlimiting (pulse shaping) the message before modulation, or bandlimiting the ASK signal itself after generation.

Figure 2: ASK generation method Figure 3 shows the signals present in a model of Figure 2,where the message has been bandlimited. The shape, after bandlimiting, depends naturally enough upon the amplitude and phase characteristics of the bandlimiting filter.

Figure 3: original TTL message (lower), bandlimited message (center), and ASK (above) Intro to Demodulation It is apparent from Figures 1 and 4 that the ASK signal has a well defined envelope. Thus it is amenable to demodulation byan envelope detector. With bandlimiting of the transmitted ASK neither of these demodulation methods (envelope detection or synchronous demodulation) would recover the original binary

sequence; instead, their outputs would be a bandlimited version. Thus further processing - by some sort of decision-making circuitry for example - would be necessary. Thus demodulation is a two-stage process: 1.recovery of the bandlimited bit stream 2. regeneration of the binary bit stream Figure 4 illustrates.

Figure 4: the two stages of the demodulation process Modeling an ASK Generator It is possible to model the rather basic generator shown in Figure 2. The switch can be modeled by one half of a DUAL ANALOG SWITCH module. Being an analog switch, the carrier frequency would need to be in the audiorange. The TTL output from the SEQUENCE GENERATOR is connected directly to the CONTROL input of the DUAL ANALOG SWITCH. For a synchronous carrier and message use the 8.333 kHz TTL sample clock (filtered by a TUNEABLE LPF) and the 2.083 kHz sinusoidal message from the MASTER SIGNALS module. If you need the TUNEABLE LPF for bandlimiting of the ASK, use the sinusoidal output from an AUDIO OSCILLATORas the carrier. For a synchronized message as above, tune the oscillator close to 8.333 kHz, and lock it there with the sample clock connected to its SYNCH input. This arrangement is shown modeled in Figure 5.

Figure 5: modeling ASK with the arrangement of Figure 2 Demodulation of an ASK signal Having a very definite envelope, an envelope detector can be used as the first step in recovering...