by Simon Chambers-Trubatch
Project Overview
A tunable AM radio receiver that isolates the information signal from a broadcast
Tools and Materials
- Variable capacitor (30-130 pF approximately)
- 1000 pF capacitor
- 10 kOhm resistor
- Germanium diode (critical that it be germanium) (precise part number unknown as the germanium diodes were in unlabeled bags, but I tested two variants of germanium diode and both worked, so hopefully this is fairly flexible)
- Ferrous rod OR manufactured inductors of around 100 uH and 30 uH
- Soldering setup: soldering iron, solder
- 4 meters (at least) of wire
- Source of amplitude-modulated signal at around 1MHz (many function generators can do this)
- Oscilloscope
Project Files
Step-by-Step Instructions
Make the inductor. The example pictured measures in at 130 uH, so anything near there should work; slight difference can be accounted for later by tuning the variable capacitor. The photo shows ~50 turns of wire around a ferrous core, which is one recommended approach. Manufactured inductors should also work, though two would be needed, because the antenna needs to be inserted with inductance on BOTH sides, as is discussed in the next step.
The antenna is simply a few meters of wire. One end must be connected to the circuit in the midst of the inductor, such that the inductance is split roughly 1:4 about the point where the antenna connects. The other end of the antenna should extend away from the receiver. In the photo, the inductor is formed from coated wire, and a break in the coating allows the antenna to be soldered on. This kind of small break in the coating may be made in several ways: one way is to melt the coating with a soldering iron and then pick away at what remains with scissors and/or fingernails.
Connect capacitor and inductor in parallel. ideally this is done by soldering a short connections, but a breadboard does work. Using wires and breadboards will introduce small amounts of parasitic capacitance from the breadboard, and will create long loose connections that will tend to shift around, making this parasitic capacitance variable in an uncontrolled way. In the photo, two variable capacitors sit in parallel boosted by a fixed 20 pF capacitor because a variable capacitor with necessary range was not available.
At this point, a test is possible. Turn your antenna towards your source of AM signal. (Even if you are somewhere known to have strong airwaves, a function generator can be a nice, controllable test if it has the capability of providing a modulated signal. Simply hook a few meters of wire to the output and use that as a broadcast antenna, which can be brought near your receiving antenna for the easiest possible test.) Place the oscilloscope's measuring terminal between inductor and capacitor. Note that at this point there is nowhere to put the ground end of the oscilloscope without shorting everything, so it that can be hooked to the oscilloscope's ground pin or even left hanging for this quick-and dirty test. Varying the capacitor across its range, you should be able to receive a modulated signal, which should be strongest at a specific resonant value of capacitance.
Refer to the circuit diagram I made (it is provided on this wiki page) and add the diode and resistor to the in-progress circuit. As with the capacitor and inductor, this is most effectively achieved by soldering, but does work on a breadboard (I tried both). Note here that the diode must be germanium, and the resistance value is important. My suggested and tested value of 10kOhm discharges the capacitor while allowing enough current for proper functioning of the resistor, and the germanium diode is both fast enough and has small enough forward bias voltage to run on this weak megahertz signal.
Another test is possible at this point. Still picking up with the antenna, measure across the resistor with the oscilloscope (notice that now the oscilloscope ground has somewhere to go in the circuit!). The diode should be rectifying the still-modulated signal.
Attach demodulating 1000 pF capacitor, again optimally by soldering. Measuring across the capacitor, we should see a rectified, demodulated signal! In the photo of the oscilloscope screen, this signal is the yellow trace. The blue trace is the signal being broadcast: 3 kHz modulation and 1MHz carrier.
There are a few places you can go from here. The circuit diagram I provide shows a speaker driven by our signal, but one would need to amplify the output signal in order to do this. Another option, depending on one's interest, could be to make a longer antenna for a stronger signal. If the signal is noisy (as in the previous photo) one could refine the circuit implementation, making connections short, tight, and soldered.
