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Introduction
How does anyone, with limited knowledge and resources, develop an interest in radio and communications?
Project 60 is a modular radio project intended to show one path that could be followed that starts with a very basic 'crystal set' and developments into an amateur radio type communications receiver covering 100kHz to 30MHz.
Experimenting forms an essential part of this project with opportunities to develop and improve upon the ideas and module designs given.
Project 60 is designed to operate from a supply voltage of 9 - 14V, with the 'normal' supply being 12V.
An important design feature of this work is that the components used must be readily available and inexpensive, with many of the same type and value components being used in the different sections of the system as it develops. This will enable some of the components to be bought in larger quantities, so reducing the cost. There is also the opportunity to use components that have been 'liberated' from old equipment.
Basic radio theory
Radio spectrum to 30MHz
Basic tools
Basic equipment
A baseboard is useful to mount the various subsystems needed as the system develops.
Subsystem Construction. Whenever possible, the subsystems are all constructed on strip board following this standard design.
Basic Radio Subsystems
The first subsystem to construct is an audio amplifier to drive the speaker on the baseboard. The amplifier will take the weak signals received and increase their strength so that they can be heard. There are several ICs which can be used, two are described below.
The LM386 amplifier uses very few components, but will only provide a small output power and has a maximum supply voltage of 12V.
The TBA820M amplifier is more complex to build and uses more components but provides a reasonable output power and will operate over the full supply voltage range of 9 - 14V.
With the audio amplifier built it can be fitted to the baseboard and the speaker.
A Volume control panel is now needs a so that the loudness can be controlled.
The Radio Subsystems can now be constructed.
The first is the tuning control panel which holds a variable capacitor and is used for selecting the frequency for the radio station.
This panel should be fixed to the right hand side of the front of the baseboard, BUT a 7cm wide space should be left at the end to allow for another control panel (the waveband panel) to be fixed in place at the very end.
The next subsystem to construct is the demodulator, which will extract the information (music/speech) from the radio signal.
This should be mounted onto the baseboard just to the left of the tuning control panel.
The final component needed for the basic radio is an inductor (coil).
Adding More Frequencies
With the basic radio working and receiving stations on the medium wave band, the waveband panel gives opportunity to extends the frequency range of the radio from less than 100kHz to around 30MHz.
Improving selectivity and sensitivity.
While the basic radio system is capable of receiving strong radio signals, its ability to separate out the different stations (selectivity) and its ability to receive weak stations (sensitivity) are both very poor.
This is mainly due to the efficiency (Q) of the tuned circuits (inductors and tuning capacitors) being degraded by the aerial and demodulator circuit.
A radio frequency (rf) amplifier significantly improves both the selectivity and sensitivity of the simple radio.
With two subsystems now needing to be supplied with power, it is worth adding a power supply panel to the baseboard. This will make it easier to add further subsystems as the project expands as well as connecting a stable 12V power supply.
A stable 12V power supply can most easily be provided by a commercial mains operated, plug mounted unit, or a small 12V rechargeable battery supply.
Some local radio station signals can be very strong and can overload the rf amplifier. This can be prevented by using an attenuator to reduce the input voltage to the rf amplifier from the aerial.
Attaching the aerial and earth to the radio with crocodile clips, is fine initially, but as the radio system progresses a more reliable method is desireable.
The aerial panel incorporates an attenuator as well as input sockets for the aerial and earth systems.
The radio stations received on the lower frequency bands can usually be separated fairly well by the radio system constructed so far. However, those on the higher frequency bands will often overlap. A technique employed in the early days of radio to improve the selectivity of a tuned circuit even further is to include it into a circuit that is just at the point of oscillation. This is known as 'Regeneration' and can be achieved by feeding a small amount of the output signal from the rf amplifier back to the input of the rf amplifier. The amount of signal fed back to the input has to be carefully controlled to ensure that the rf amplifier does not completely oscillate.
When properly adjusted, the selectivity and sensitivity of the radio system will significantly improve and it will also be possible to hear morse code radio stations.
Regeneration control panel.
It is important to ensure that the rf amplifier does not oscillate as it will then start to transmit radio signals causing interference to others.
Crystal Calibrator.
The Crystal calibrator is not a direct subsystem of the Project 60 receiver but can be used to identify the frequency that the radio system is tuned to.
It forms a useful piece of test equipment in its own right.
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