FM Receivers

Abstract

This paper will discuss the design of an FM receiver. It will begin with a brief historical backdrop of FM broadcasting and its use in society. It will continue by providing the necessary mathematical background of the modulation process. Furthermore, it will enumerate some of the advantages of FM over other forms of modulation, namely AM.

Finally, the paper will discuss the design of a basic FM receiver as well as introduce some circuits and circuit components which the reader may not be familiar with.

Introduction

Frequency modulation (FM) was invented in 1936 by an American electrical engineer/ inventor named Edwin H. Armstrong. Possessing numerous advantages over the existing

AM broadcasting system, as will be discussed later, in combination with relatively low cost of implementation, resulted in its rapid growth. In the years following World War

Two, there were 600 licensed stations broadcasting in the U.S. By 1980, the number grew to 4000. On another historical note, in 1961 stations began broadcasting in stereo.

The basic receiver design consists of the following components. An antenna is used to convert electro-magnetic waves into electrical oscillations. Amplifiers are used throughout the receiver to boost signal power at radio, baseband and intermediate frequencies. The core of the FM receiver, the discriminator, comes in various circuit forms and is used in detection and demodulation. Basically, its role is to extract the intelligence or message from the carrier wave. Another component, essential in most electronic circuits, is the power supply (DC or AC converted to DC). Finally, a transducer (speaker in the case of Radio) is needed to convert the message signal into its final form (audio, mechanical, etc¡­). Other components more specific to FM receivers are mixers combined with local oscillators used for frequency manipulation, limiters to control amplitude, de-emphasis and other filter circuits.

2

Mathematics of FM

Unlike amplitude modulation (AM) where the message or modulating signal, call it m(t), is used to modulate the amplitude of the carrier signal, frequency modulation, as the name implies, uses m(t) to transform the frequency of the carrier. The amplitude of an

FM signal should remain constant during the modulating process; an important property of FM. A general FM signal can be described by the following:1

¦µFM(t) = Acos(¦È...

... middle of paper ...

...n its output proportional to s(t).

Over a short time interval, this variation ¡Ö C(wc-wo)t. Thus, the system continues to loop until the frequency of the VCO output matches or ¡°locks¡± onto the incoming frequency.

The time it takes for the system to ¡°lock¡± is called the acquisition time. Once the frequencies match, s(t) becomes s(t) = AB/s sin(¦Èc-¦Èo)

For an incoming FM signal s(t) = AB/2 sin(¦Èc(t)-¦Èo)

= AB/2 sin (kf ¡Òm(¦Ó)d¦Ó ¨C¦Èo)

Running s(t) through a differentiator results in an AM signal which can be easily demodulated using envelope detection.

Once the signal has been demodulated, it is then passed through a de-emphasis circuit, as mentioned earlier. Typically, it is then amplified one last time before heading to the output transducer.

Conclusion

In conclusion, the modulation/demodulation process for FM signals has proven to be much less straight forward than simple AM modulation. However, FM has considerable advantages and its use in radio, satellite and radar applications make it indispensable.

Many methods of signal and system analysis along with filter and feedback design are employed in the building of an FM receiver, whether it be analog, digital or otherwise.