S5-1.2 Angle Modulation

Work through these

• Basic concepts of phase modulation

  The other thing you can vary once amplitude is taken: the angle. Phase and frequency modulation are two views of the same operation, and being able to move between them saves learning the second one twice.

• Frequency modulation: single tone FM

  The standard case worked in full. Modulation index is the quantity everything else here depends on, so get comfortable computing it before moving on.

• Spectrum analysis of sinusoidal FM using Bessel functions

  Where the mathematics gets heavier and the payoff is real: the sidebands come out as Bessel coefficients. The lesson is that this signal has infinite sidebands and you are choosing where to stop, not discovering a natural edge.

• NBFM and WBFM

  Narrowband and wideband are the same mechanism at different modulation indices, not two different schemes. Students routinely file them as separate things and then cannot say where the boundary is.

• Power and transmission bandwidth of FM

  Carson's rule, and why the power stays put while the bandwidth grows. This is the bargain the whole scheme rests on: you buy noise immunity with spectrum.

• Indirect generation of FM: the Armstrong method

  Building a wideband signal from a narrowband one by multiplication, because generating it directly is hard. It is a good example of reaching a specification indirectly when the direct route is impractical.

• Detection of FM: the phase locked loop

  The phase locked loop as a detector, and its first appearance in this subject. It comes back in the digital half and in the timer topic later, so time spent here is not spent once.

• Pre-emphasis and de-emphasis

  Noise in this scheme rises with frequency, so the fix is to lift the high frequencies before transmission and cut them after. The same trick appears in audio recording, which is a good sign it is a real idea rather than an examination one.

• Lab: FM generation and detection; obtain and analyse the pre-emphasis and de-emphasis frequency response

  The bench version of the topic. The pre-emphasis and de-emphasis response is the part worth plotting, because seeing the two curves mirror each other explains why the pair leaves the signal unchanged.