Amplitude Modulation Receiver Circuit

Figure 1 

The circuit was designed to be able to obtain signals via amplitude modulation where the sensitivity and selectivity is fairly good.

• Amplitude Modulation AM – a method of emphasizing data on an alternating current waveform or radio wave wherein the carried transmitted signal varies with reference to the amplitude; the highest frequency of the modulating data is typically less than 10 percent of the carrier frequency and the overall power signal varies depending on the amplitude of the modulating data

Three BC549 transistor were used to construct this circuit. These are low noise type small signal NPN Silicon epitaxial planar transistors which are subdivide into three groups according to current gain. The circuit will operate under medium wave frequency but can work under higher frequencies if another type of capacitor and tuning coil will be used. The composite pair of transistor Q1 (conducts on emitter follower) and Q2 (conducts on common emitter) results to high gain and high input impedance while the third transistor executes a dual function by demodulating the RF carrier and amplifying the audio signal.

The overall performance of the circuit is influenced by the value and regenerative feedback of the 120K ohm resistor between the Q2 output and tank circuit. İn the existence of excessive feedback, the circuit becomes unstable while insufficient feedback makes the circuit deaf. The selectivity and sensitivity of the circuit varies with the value set on the resistor R1.

Taken from an old radio, a tuning capacitor and a ferrite rod was reused which can be tuned around 550-1600 KHz. The copper wire is wound around the ferrite rod and rotating the rod would void some signals while improving the others. An external antenna, can be of several feet of flexible wire, may be necessary if the location of the receiver is in an area of weak reception. The tuning capacitor’s moving plates should be connected to the base of Q1 while the fixed plates to the junction of R1 and C1. Useless oscillation and instability may occur if the connection of the capacitor is reversed.

Amplitude modulation may be utilized not only in radio communications but also in transmitting human voice electronically, where the voice waves on both sides are modulating the voltage of the direct current loop connected to them by the telephone company. In AM radio, the voltage or amplitude of a carrier with a fixed center frequency by the station is varied by the analog audio signal. Amplitude modulation is also used in digital data, like quadrature amplitude modulation, where both amplitude and phase modulation are used to create different binary states for transmission. AM is also used in modulating the light waves in optical fibers.

Some of the advantages possessed by using amplitude modulation receiver are, simplicity of implementation, can be demodulated using a circuit consisting of very few components and they are cheap as no specialised components are needed. The disadvantages that one may encounter are inefficient in terms of its power usage, inefficient in terms of its use of bandwidth and prone to high levels of noise.

Source:www.zen22142.zen.co.uk/Circuits/rf/amrec.html