Question

a) AM transmitters more commonly use low-level modulation, while FM transmitters more commonly use high-level modulation. True or False

b) Explain the basic elements of a Single Side Band (SSB). What element distinguishes it from a DSB transmitter?

c) As regulated by FCC law, what component is used to generate the carrier frequency of broadcast transmitters?

d) A …. uses a ROM look-up table to generate sinewaves or other signal types for carrier signal generation.

e) Explain the difference between a Class C amplifier and Class A-B amplifier.

6. What would you use to interface a balanced output to an unbalanced input?

Answer 1

B)

The concept of single side-band (SSB) is very simple: if you don't need two side-bands, get rid of one! To make that happen, you merely add a component to your system that removes the extra side-band. That component is called a band pass filter.

Here's what the SSB transmitter looks like:

Note that the band pass filter has removed the lower side-band (LSB) and the carrier from the spectrum. The remainder is transmitted.

The receiver cannot output the signal as is, it must first restore the signal to what is should be before demodulation. The receiver in a SSB system has its own carrier signal (from a local oscillator) that is puts back in. The receiver looks like:

By having its own carrier signal, the receiver makes the signal back into what would be sent by a conventional AM system. The remaining signal is processed normally.

So, there are two modifications to make it work

• The transmitter adds a band pass filter before amplification for transmission and
• The receiver adds a local carrier signal back into the signal prior to processing

C) In modern transmitters, there is a crystal oscillator in which the frequency is precisely controlled by the vibrations of a quartz crystal.

E)

Class AB amplifier falls between Class A and Class B. It seeks to overcome the cross-over distortion by slightly turning on the transistors so that they conduct for slightly more than half the cycle and the two devices overlap by a small amount during the switch-on / switch-off phase, thereby overcoming the crossover distortion.

This approach means that the amplifier sacrifices a certain amount of potential efficiency for better linearity - there is a much smoother transition at the crossover point of the output signal. In this way, Class AB amplifiers sacrifice some of the efficiency for lower distortion. Accordingly class AB is a much better option where a compromise between efficiency and linearity is needed.

A Class C amplifier is biassed so that it conducts over much less than half a cycle. This gives rise to very high levels of distortion, but also it enables very high efficiency levels to be achieved. This type of amplifier can be used for RF amplifiers that carry a signal with no amplitude modulation - it can be used for frequency modulation with no issues. The harmonics created by the amplifier effectively running in saturation can be removed by filters on the output. These amplifiers are not used for audio applications in view of the level of distortion.

Class C amplifiers typically use a single active device that is biased well into its off region. As the signal is applied, the top peaks of the signal cause the device to run into conduction, but obviously for only a small portion of each input-waveform cycle.

At the output the circuit uses a high-Q, L-C resonant circuit. This circuit effective rings after it is hit by each pulse so that the output contains an approximation to a sine wave. Filtering is required on the output to ensure that the level of harmonic is sufficiently low.

Typically, the conduction angle for the transistor is significantly less than 180° - often around the 90° region. Efficiency levels can be as high as 80%, but values of 66% are more normal when circuit losses, etc are taken into account.