Introduction and Motivation
The purpose of this Instructable is to demonstrate how to design and build a resonator circuit that will let us tune to a given AM radio frequency. We will also briefly discuss how radio frequencies are transmitted and how amplitude modulation (AM) works.
Equipment and Materials
Listed below are the materials necessary for building and testing this circuit.
- Function Generator
- Power Supply
- Variable Capacitor
- Inductor - a rod and coil; can be made by hand
- AM radio antenna (optional)
- Steel rod
- Cable, magnetic wires
- Commercial radio - for testing
A tip before starting:
Use a commercial radio (one you know already works) to test your workspace to make sure you will be able to pick up AM radio signal. Some buildings are AM-insulated and will block AM signals even for commercial radios. You may also want to take note of which radio stations seem strongest at your location if you want the best chance of picking something up.
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Step 1: Designing the Resonator
Designing the Resonator
The resonator is a bandpass filter that filters out frequencies outside of some range centered on its characteristic resonant frequency. A bandpass filter consists of an inductor and a capacitor in parallel. The resonant frequency (in Hertz) of the filter will be
where L is the inductance of the inductor in Henries and C is the capacitance of the capacitor in Farads. (For more information on filter circuits, take a look at this).
The frequencies of AM radio signals range from about 500 kHz to about 1600 kHz. Ideally, to be able to access this wide range of AM frequencies, you will need a variable capacitor with a wide enough range to give a resonant frequency of about 500 kHz at greatest capacitance and a frequency of about 1600 kHz at its least capacitance for an inductance L. Variable capacitors are more difficult to come by than inductors, so it will probably be most practical to find a decent variable capacitor first and then make an inductor of an appropriate inductance.
Important: Keep in mind that any element in your circuit will probably contribute some inductance or capacitance, so your calculations will not be perfect. It’s advisable to try to cover a wider range of frequencies than what seems necessary, to allow for some margin of error.
If you choose to use an external AM antenna, it will contribute a significant inductance to your resonator circuit. Consider placing your antenna in parallel with your dedicated inductor if you’d rather err on the side of having a smaller inductance and in series with your inductor if you’d rather have a larger inductance. Note that using an external antenna is not necessary, because a rod and coil inductor will act as an antenna. If you do not use an external antenna, you will need to make sure to have a rod and coil inductor.
If you can measure the inductance of your antenna, note that inductors in series add as
and inductors in parallel add as
Step 2: Making an Inductor
Making an Inductor
Making your own inductor can be as simple as wrapping wire around a rod. However, it is important to calculate the number of turns of wire required given the widths of your wire and rod, the material of your rod, and your desired inductance. This website is useful for calculating the inductance of a coil of wire given the wire and rod radii, the relative permeability of the material of the rod, and the number of turns of wire in the coil. The relative permeability of the material of your rod is fairly simple to look up. Common values of relative permeability can range from about 1 for air, wood, and aluminum to 100 for steel and up to about 640 for ferrite.
While making a coil with fewer turns seems tempting to save you some work, realize that having more turns on your coil allows for a much greater margin of error.
Step 3: Simulating a Radio Station
Simulating a Radio Station
Because it’s so difficult to know exactly the inductance and capacitance of your resonator circuit, it can be extremely helpful to tune your resonator to a very clear, strong signal of known frequency. Luckily, this is not difficult to do with a function generator, and this will allow you to control the frequency of your very own simulated radio station!
First, you will need to wrap a few turns of wire around a steel rod. The number of turns of wire or the sizes of the rod and wire should not matter for this, since the purpose of this coil is simply to produce a magnetic field to be picked up by your antenna (either the rod and coil inductor made in the previous step or an external antenna, if you’re using one). Connect the ends of the coil to the output of your function generator, and set the output of the function generator to be the frequency of the radio station you will want to try to pick up. The time-varying voltage of the function generator will cause a time-varying current to run through the wire coil, which in turn will produce a time-varying magnetic field. This magnetic field will be picked up by your antenna and interpreted like a radio frequency.
Step 4: Tuning to a Simulated Radio Station
Tuning to a Simulated Radio Station
Now that you’ve created your own radio station, place the coil close to your antenna to ensure that your signal will be strong and clear. Look at the output of your resonator on an oscilloscope. The signal from your coil should be fairly obvious, but try adjusting the capacitance of your variable capacitor to see if you can notice a difference. Once you’re sure that what you’re seeing is the signal from your simulated radio station, adjust the variable capacitor until the signal you see appears to be at a maximum. That configuration should be the ideal capacitance for picking up a radio signal of the frequency set by the function generator.
Step 5: Tuning to a Real AM Signal
Tuning to a Real AM Signal
Once you’ve found the configuration you want to use to pick up your desired frequency, turn off the function generator. Your resonator should be set to pick up radio signals of that frequency, but you may still want to do a little fine-tuning with your capacitor to make sure you’re picking up the radio frequency as best as you can.
Note that the signal you are picking up is not the audio signal, that is, you cannot take this output to a speaker and expect to hear the radio broadcast. AM radio signals is amplitude-modulated, so that the audio signal is modulated over a carrier wave (see illustration below).
On an oscilloscope display, an amplitude-modulated signal may look something like this:
In order to obtain the audio signal, it is necessary to build a demodulator to demodulate the AM signal, but that is a topic for another day.