10-09-2013, 04:28 PM
ASK - AMPLITUDE SHIFT KEYING
AMPLITUDE SHIFT.pdf (Size: 327.78 KB / Downloads: 533)
generation
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 1. These are related to the number of levels adopted by the digital 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.
bandwidth modification
As already indicated, the sharp discontinuities 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.
Both these options are illustrated in Figure 3, which shows one of the generators you
will be modelling in this experiment
demodulation methods
It is apparent from Figures 1 and 4 that the ASK signal has a well defined envelope.
Thus it is amenable to demodulation by an envelope detector.
A synchronous demodulator would also be appropriate.
Note that:
• envelope detection circuitry is simple.
• synchronous demodulation requires a phase-locked local carrier and therefore
carrier acquisition circuitry.
With bandlimiting of the transmitted ASK neither of these demodulation methods
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.
bandwidth estimation
It is easy to estimate the bandwidth of an ASK signal. Refer to the block diagram of
Figure 3. This is a DSB transmitter. It is an example of linear modulation. If we
know the message bandwidth, then the ASK bandwidth is twice this, centred on the
‘carrier’ frequency.
Using the analogy of the DSB generator, the binary sequence is the message (bit rate
‘μ’), and the sinewave being switched is the carrier (‘ω’).
Even though you may not have an analytical expression for the bandwidth of a
pseudo random binary sequence, you can estimate that it will be of the same order as
that of a square, or perhaps a rectangular, wave.
modelling with a DUAL ANALOG SWITCH
It is possible to model the rather basic generator shown in Figure 2.
The switch can be modelled by one half of a DUAL ANALOG SWITCH module.
Being an analog switch, the carrier frequency would need to be in the audio range.
For example, 15 kHz from a VCO.
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 OSCILLATOR as 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.