Receivers

draw this circuit and explain each partDraw the circuit using a block diagram

AntennaTuned Circuit Detector RF Filter

High Impedance Headphones

Sensitivity and Selectivity This basic crystal radio can be effectively used to receive strong local signals but suffers from poor sensitivity and poor selectivity. Sensitivity is the ability of the receiver to pick up weak signals. The more sensitive the receiver the greater number of stations it can pick up. Selectivity is the ability to distinguish between stations. The LC tuned circuit has a maximum impedance at the resonant frequency f0. The voltage output of the circuit is therefore a maximum at this frequency. If the circuit is loaded, by drawing a current from the circuit the response decreases. This causes the impedance and so the output voltage to fall with greater loading.

The RF amplifierAn RF amplifier is designed to enlarge signals that have

frequencies much higher than audio frequencies. (RF may be generally taken to mean any frequency greater than 20 kHz.) The RF amplifier is placed after the tuned circuit and before the detector.

Engineers experimented with adding more and more tuning circuits and RF amplifiers to improve the ability of a radio to receive even very weak radio signals. These efforts resulted in the development of the tuned radio frequency (TRF) radio. The block diagram for this system is shown on the following page.

AntennaTRF 1 RF

FilterTRF

2Detector

AF Amplifier Loudspeaker

TRF

3

Mechanical link

Antenna  As with all radio stations, the antenna will pick up the electromagnetic radio waves from the atmosphere and convert these into very small electrical currents. Tuned R.F. Amplifier. This block amplifies the very small currents created in the antenna, to improve the sensitivity of the radio receiver, in the same way that it was used in the TRF radio.

Local Oscillator. A new addition to the superhet radio. This is a sine wave generator which is mechanically linked to the tuning capacitor. This ensures that it always produces a frequency at a fixed amount above the resonant frequency of the tuned amplifier. This is typically in the range 450 kHz to 480 kHz.

The mixer The signal from the TRF is amplitude modulated onto the local oscillator frequency. It is in effect re-transmitted to the Intermediate Frequency (IF). The IF is a fixed frequency, usually around 470kHz.

Sketch a frequency spectrum to show all the frequencies present at the mixer. Label the frequencies