S5-1.1 Amplitude Modulation

Work through these

• Need for modulation

  Why a baseband signal cannot be radiated as it is: the antenna would have to be kilometres long, and every station would sit on top of every other. This is the item that makes the rest of the subject feel necessary rather than arbitrary.

• AM: time and frequency domain description; single tone modulation

  The two pictures of one signal, and the habit the whole course runs on: describe it in time, describe it in frequency, then read the bandwidth straight off the spectrum. Single tone is the case worth being able to sketch from memory.

• Power relations in AM

  Where the transmitted power actually goes. The number to carry away is that a full carrier wastes at least two thirds of its power on something conveying no information, which is the whole reason the suppressed carrier schemes further down exist.

• Generation of AM waves: switching modulator

  How the multiplication gets done with real parts rather than on paper. A switching modulator makes the product from a nonlinearity and a filter, and seeing that is what connects the equation to a circuit you could build.

• Detection of AM waves: envelope detector

  The receiving half, and the cheapest circuit in the subject: a diode, a capacitor and a resistor recovering the message. Choose the time constant badly and you get either diagonal clipping or ripple, and telling those two apart is the practical skill here.

• DSB-SC: time and frequency description; balanced modulators

  Suppressing the carrier saves the power the item above showed was wasted, and costs you the ability to detect with a diode. Balanced modulators are how the carrier is cancelled rather than filtered out.

• Coherent detection of DSB-SC; the COSTAS loop

  Once the carrier is gone the receiver has to regenerate it, in phase. Follow the two arms of the Costas loop once, because the same idea comes back in every coherent digital receiver later in this course.

• SSB modulation: time and frequency description

  Half the bandwidth of the suppressed carrier scheme for the same message, because one sideband already carries everything. The price is a filter with a very sharp edge, which is why this is used where spectrum is expensive rather than everywhere.

• Principle of vestigial sideband modulation

  The compromise between the two schemes above, and the reason analog television used it. Leaving a vestige of the second sideband makes the filter buildable while keeping most of the bandwidth saving.

• AM transmitter; superheterodyne receiver

  The block diagram every radio has used for a century. Fixing the intermediate frequency is what lets one set of sharp filters serve every station, and that single idea is what makes a tunable receiver practical.

• Image frequency rejection and image rejection ratio; receiver characteristics

  The superheterodyne's built-in flaw: a second frequency mixes down to the same intermediate frequency and arrives as interference. Work out why it sits at twice the intermediate frequency away and the rejection ratio stops being a formula to memorise.

• Figure of merit calculation for AM

  The number that lets you compare schemes fairly, by saying how much the modulation itself helps or hurts the signal to noise ratio. It is the basis of the comparison with frequency modulation that the next topic makes.

• Lab: generate and detect an AM signal, and generate DSB-SC with a balanced modulator

  The bench version of the first half of this topic. Seeing the envelope on a scope, then seeing it vanish when the carrier is suppressed, fixes the difference between the two schemes far better than the algebra does.