Radio Receivers

Overview

RX Power Reception Selection Demodulation Reproduction

Rx

Types CW AM (SSB) FM

Crystal Tuned Radio Receiver Superhet

RX

Functions Reception Selection Demodulation Reproduction

Reception - Power

Inverse square law

Reception - Power

Reception

Aerial / antenna Polarisation Direction

Yagi array

Reception

Sensitivity Weak signal noise

Selectivity Band width filter 1% of RX Frequency

Stability

Reception

Crystal radio AM (not CW, SSB or FM) Poor sensitivity and selectivity

Reception

Tuned radio Improves selectivity and sensitivity Amplifiers (difficult to stabilise)

Reception

Superheterodyne Best selectivity, sensitivity and stability

(tuned oscillator)▪ CW, AM, SSB, FM

Reception

1st stage Buffer Amplifier Filter

Selection

Mixer Intermediate Frequency Smaller band width (5 kHz) – receiver bandwidth Local oscillator – controls reception frequency

fs – fo = fif

fs + fo = fif

Selection

Image frequencies two frequencies can produce the same

IF

Demodulation

CW and SSB AM FM

Demodulation

Beat Frequency Oscillator Mixes with IF to produce an audio tone |fif – fbf |= Beat Frequency

fif + fbf remove using a filter Used in SSB and CW (no carrier wave) Change crystal to change sideband

Demodulation

AM RX Detector

Envelope detector Product detector

Demodulation

Envelope detector

Envelope detector circuit

Demodulation

Product detector Works for AM and SSB fif . flo

Filter result

Demodulation

FM RX Quadrature detector▪ Multiply with signal phase shifted 90o

Phase Lock Loop (PLL)▪ Error signal

Digital Signal Processor

Demodulation

FM RX Foster-Seeley Discriminator▪ Frequency control using a centre tapped

transformer

▪ Ratio detector

Reproduction

Low Frequency amplifier

Questions?