EC-22.4 Amplifiers, Mixers and Oscillators
You can design a matched amplifier for gain or for noise using scattering parameters, explain what a mixer does and why the image is a problem, and state what phase noise is and why an oscillator's purity limits a whole system.
Three blocks make a radio: something that makes a signal larger, something that moves it to another frequency, and something that produces the reference the moving is done against. Each has one central difficulty. The amplifier cannot have its best gain and its best noise at the same time, the mixer produces an unwanted response as well as the wanted one, and the oscillator is never quite at one frequency.
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Amplifier design from scattering parameters, and the stability circles
A transistor characterised as a two-port can be matched for maximum gain, and the same data says which source and load impedances would make it oscillate instead. Checking stability before matching is not optional.
NPTEL: Design Principles of RF and Microwave Filters and Amplifiers (IIT Kharagpur) · CourseMatching for gain against matching for noise, and why they differ
The source impedance that extracts the most power from a transistor is not the one that gives it the lowest noise figure, so a first stage is deliberately mismatched. This is the most consequential trade in receiver design.
NPTEL: RF Transceiver Design (IIT Roorkee) · CoursePower amplifiers: efficiency, back-off and the classes
A transmitter spends most of the system's energy, and running an amplifier near compression is efficient and distorting. The operating classes are different answers to that same conflict.
NPTEL: RF Integrated Circuits (IIT Delhi) · CourseThe mixer as a multiplier, and the sum and difference it produces
Multiplying two sinusoids produces components at the sum and difference frequencies, which is how a signal is moved to a frequency where it is easier to filter and digitise. Everything else about mixers follows from that.
NPTEL: RF Transceiver Design (IIT Roorkee) · CourseThe image frequency, and the three standard ways of dealing with it
Two different input frequencies map onto the same output, so one of them must be removed before the mixer or cancelled inside it. Filtering, a high intermediate frequency and an image-reject architecture are the usual answers.
Oscillators, phase noise, and why purity limits the whole system
A real oscillator has energy spread around its nominal frequency, and that skirt mixes strong neighbouring signals onto the wanted channel. It is often the limit on how closely two channels can be spaced.
NPTEL: RF Integrated Circuits (IIT Delhi) · Course
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