Building and testing of an FM receiver


Building and testing of an FM receiver

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An FM receiver is just an electronic device that can take wavelength of a radio wave from a different range of wavelengths like that of X-Ray, microwaves, ultraviolet wavelength, and many others. On receiving or taking the radio wave it, in turn, convert the information carried on it into a useful and the required information. To this, it needs a particular frequency wave and to accomplish this task, a component by the name antennae is used. This antenna enables the FM receiver to be able to use the electronic filters to get the required radio frequency signal from other different signals. After the antennae pick the necessary signal, it sends it to an electronic amplifier for amplification to increase its power signal for further processing(Gassmann, 1976). By so doing, it recovers all the desired information by the process of demodulation. From all wide range of radio wave FM is the popular one but there is also AM which a bit common though not popular as FM. In FM radio broadcasting frequency modulation is widely used because of its larger signal-to-noise ratio which enables it to reject radio frequency interferences.

The students were required to build an FM receiver circuit on a stripboard. An implementation of an FM receiver was given in a circuit diagram below.

Diagram of a circuit

The students were required to analyze the circuit and identify the function of each functional block and constituent components of each block. They were then required to build a circuit on stripboard and make it operational to receive a local FM radio broadcast. The students were grouped into groups of two students per group. The students in each group were required to record all activities concerning this lab project assignment with the time and date noted. The activities recorded includes a literature review, datasheet search, details on any session attended on practical electronic skills such as soldering, and building circuit. The students were then required to produce a neat and tidy component which arrange the components in a space-saving configuration.

The students were required to prepare a component list from a circuit diagram and get the list certified by the lab technicians.  Then they can collect the components from the lab and use the project box to save them.


Varying the instantaneous frequency of radio wave to encode the information in the career wave is termed as frequency modulation. This frequency modulation is widely used from broadcasting to communication, and it’s more advantageous than other modes. To do the modulated signal generation, the radio frequency carrier is changed in line with the amplitude of the incoming audio signal. After the conversion of audio signal into radio frequency carrier the new radio frequency oscillates up and down and is termed as a deviation. After that is done, the original information-bearing signal a carrier wave can be recovered by process of frequency demodulation to demodulate frequency modulation the frequency variation is converted into voltage variation by the use of radio receiver(Gassmann, 1976).

The oscillator is adjusted using the C4 capacitor. The potentiometer is adjusted until the best reception is obtained. Oscillator output will have the same frequency deviation as the received signal from FM antenna because of the synchronization. The broadcast audio information causes these deviations in frequency. Low pass filter extracts the audio signal and then is amplified by R2 and then transmitted at the output through the C5 capacitor.

The antenna catches frequency waves in the receiver circuit. Electronic filters are used to separate the radio frequency signal received by the antenna to obtain the desired frequency. The amplifier is used to raise the power signal for processing and recovering the desired audio information using demodulation.

Components of FM receiver circuit block consist of Ammeter, Voltmeter, transistor, resistor R, coil L, capacitances c3,c4 and c5, oscillator.

Components, tools, and equipment

Soldering station, Stripboard, DMM, Oscilloscope, Power supplies, pliers, connection lead, logbook, earphone sockets, coil from hook-up line one, 3 or 4 wire coil, battery snap, earphones and a project box for keeping components and build-in.


Figure 2; Oscilloscope


First, the Voltmeter was connected ammeter, as shown in the figure below.

The resistor was then connected to the circuit, as shown in the figure below.

The coil L1 was the connection to the circuit, as shown below.

Capacitor 2 and capacitor three were then connected, as shown below.

The coil L2 was connected to the circuit, as shown below.

Resistor R2, capacitor 5 and six were then connected to the circuit as shown below.

The speaker was then connected at the end to give a complete FM receiver circuit.

The power supply was then connected to the circuit, and the switch was pressed.

The frequency of the circuit was adjusted by varying the variable capacitor of the circuit; hence, the circuit starts turning to the nearest frequency.

The frequency of the incoming signal of the receiver can be then heard through the speaker when the frequency is matched.

The circuit was then tuned to another frequency using a tank circuit. Another incoming sound at that same frequency can now be heard.

The variable resistor was then tuned to increase or decrease the volume.

The problems encountered include difficulty in soldering where rounded soldiers were formed, which was undesirable. This problem was remedied by making sure that too much soldier was not used.


Audio or Oscilloscope working FM receiver


What is the purpose of the JFET in the circuit?

JFET is a depletion mode device which creates a depletion region in a circuit producing a potential gradient of varying thickness around the PN-junction, and it restricts the current flow through the channel by reducing its sufficient width thus increasing the overall resistance of the circuit itself. Depletion region means a junction formed when an N-type semiconductor channel with P-type region diffuse into N-type channel forming a reverse biased PN-junction around the gate.

What is the turning range for the detector circuit?

between 88 MHz and 108 MHz

Display the spectrum diagram using the oscilloscope


Frequency Modulation Transmission

The transmission of signals in frequency modulation is done by a device known as a frequency modulation transmitter, which is a sub-system taking the information and processing it before transmission. The frequency modulation in telecommunication transfers information by a principle of varying the carrier wave frequency according to the message signal. The information in the carrier signal is first modulated by the transmitter, amplified and then broadcasted over the channel. The amplification which is the increase of the power of the filtered signal is done after the information has reached a receiver and then by the use of electronic filter separated from all other signals picked from the antenna in form of sound, moving images, or data. After amplification, further processing takes place whereafter, the desired information is covered by demodulation. Demodulation is the process by which original information is bearing a signal is removed from the carrier wave. To transmit and receive the frequency modulation signal, the frequency modulation transmitter may use VHF radio frequency of 88.5 to 108.0 MHz The transmitter is commonly used because it accomplishes the most excellent range with less power. These transmitters can vary from those with wires and the wireless one. For the wireless ones, the circuit depends on the induction coil and capacitance.

New development of frequency modulation transmission

The development of frequency modulation started some time back. Due to the ideological conflicts/differences, some engineers saw it suitable for the improvement of frequency modulation. This idea of development was brought by the fact that due to the wireless transmission during old days there was a kind of hissing, clicks and grinding noises which was brought about by the atmospheric electricity during transmission. Great effort was put by radio engineers to get rid of or reduce the so-called static. The engineers achieved this by the invention of an antenna though by this invention, and many considered the problem still unsolved. Edwin H. Armstrong went against their idea but noted that it was a terrible disaster(Wang, et al., 2017). He later succeeded to solve the terrific problem by introducing a system of wideband frequency-modulation radio. This frequency modulation introduces a new and different way of an audio signal impression on a radio frequency carrier wave. On the other side, higher sound fidelity was aided in radios to have relative freedom from static. In progress was improving the frequency modulation, four patents for the frequency modulation technique was obtained, gaining the support of RCA for the new system. Due to financial constraints brought about by the idea of introducing television, some pioneers saw it challenging to introduce and develop another new radio medium. Though the improvement faced some challenges to the extent of break out of the war which led to the death of some engineers, the pioneer engineers struggled with the situation to the level of the frequency modulation we have today which is free from hissing, clicking and grinding noises.

Feature development of frequency modulation transmission

Because many systems operate more effectively on wavelength hitherto not put into use, it is possible to place station much closer geographically to permit the use of the same channel over and over. By doing so in the future, the number of channels may exceed the demand required(Wang, et al., 2017).  Also, some form of the current created by disturbances could be devised to separate the signaling current from the interference by raising the power of transmitting station.

Holding to the fact that the disturbance contains frequency changes, as well as amplitude, could be artificially increased in extent beyond that extent in the disturbing current(Wang, et al., 2017). To add on, the limitation upon network operation imposed by the deficiencies of the wireless connection, because of this limitation of frequency transmission range up to 6000 cycles has to be improved in order not to reduce the quality of transmission to that imposed on the wireless. By so doing, they can also be set up to carry the full frequency range for a long time to come.


This laboratory introduced students to FM, or frequency modulation, signals, building an FM receiver, FM transmissions, new development, and future development FM transmissions, the purpose of the JFET in the circuit, functions, and components of FM receiver circuits block and how to pick up that signal by using a spectrum analyzer. At the beginning of this exercise, the instructor let the students practice soldering on empty pieces of stripboard using recycle parts of stripboard. This practice helps the students to understand the concept of building and testing of an FM receiver and avoid mistakes like using too much soldier, which affect the accuracy of the results obtained. The idea of building the FM receiver demonstrates the principles of FM transmission as well as give a broad overview of how the radio processes the images for transmission.

 References list

Gassmann, G.G., International Standard Electric Corp (ISEC), 1976. FM receiver and demodulation circuit. U.S. Patent 3,961,262.

Ringstad, J.E., TAPE ATHON CORP, 1975. Fm transmission system. U.S. Patent 3,860,873.

Wang, Y., Cass, C.J., Chow, T.P., Wang, F., and Boroyevich, D., 2006, July. SPICE model of SiC JFETs for circuit simulations. In 2006 IEEE Workshops on Computers in Power Electronics (pp. 212-215). IEEE.

Wang, A., Iyer, V., Talla, V., Smith, J.R. and Gollakota, S., 2017. {FM} Backscatter: Enabling Connected Cities and Smart Fabrics. In 14th {USENIX} Symposium on Networked Systems Design and Implementation ({NSDI} 17) (pp. 243-258).


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