In this article, we will talk about amplitude modulation and we will work on some fun circuits to construct.
Introduction to AM Radio Receivers
Many radios have a band switch that might say something like: ”FM, MW, and SW.” This is actually a mistake carried forward by history since the advent of FM, which is transmitted at VHF (88 – 108MHz). It’s wrong because FM (Frequency Modulation) describes a modulation method while MW (Medium Wave) describes a wavelength. But for now, forget about FM (we will cover that in another article), and let’s get clear first about MW, SW, and AM.
Medium Wave (MW) is a band of radio frequencies extending from 530kHz to 1700kHz. On the other hand, short wave or SW extends beyond that and up to about 30MHz. The relationship between wavelength and frequency is described as wavelength = 300/frequency(MHz). So a frequency of 1000kHz has a wavelength of 300m. And the 49m SW broadcast band has a frequency of 300/49 = 6.1MHz.
MW and SW were the main broadcast radio bands until the advent of FM. But they remained popular as they have more channel space than FM stations and offer better distant coverage, especially at night via the ionosphere. These receivers are also easier to build.
Radio stations in the MW and SW bands transmit their material using Amplitude Modulation (AM). This means that the transmitter radio (or carrier) signal is modulated with the music or speech content in a manner that the amplitude of the carrier is varied in relation to the incoming speech or music. AM is also used on all aircraft radios from 108 to 136MHz.
How AM Radios Work
A good start in understanding AM is the venerable crystal set.
Shown above is a simple (and magical if you have ever built one) crystal set. The diode is ideally a germanium type, like the OA81, as it has a lower forward volt drop. But any diode will work, only with less volume. The LS has to be a high-impedance device such as the vintage 2000Ω headphones or a so-called crystal earpiece. C2 is a variable capacitor of about 300 or even 500pF, and L1 is a coil wound on a ferrite rod of about 50 to 60 turns. The tap is at about 5 to 10 turns to be coupled to the antenna. For this marvel to work well, you will need a good earth connection and at least 20m of wire as high as possible outdoors as an antenna. What’s truly amazing here is that this circuit will work without any batteries and provide hours of MW listening fun.
The circuit works as follows:
- When the reactance (that’s like AC resistance) of the capacitor C2 is the same as the reactance of the coil L1, resonance occurs at the frequency f=1/2π√(LC). If they are in parallel (as in our circuit), the combined impedance is very high, and if they are in series, resonance also occurs but the combined impedance is very low. The ratio of this dynamic impedance to any loss resistance present is called Q and the greater the Q, the more selective the circuit becomes. This increase in selectivity enables the circuit to tune into the station you want. If the selectivity is low, you would hear other neighboring stations at the same time. (C1 is a small capacitor as well as the tap to prevent the antenna from damping the Q of the tuned circuit.)
- The diode D1 rectifies and recovers the modulation and the capacitor C3 bypasses the radio (RF) part-leaving the original modulated audio. By changing the capacitance of C2, the resonant point, effectively, the tuning, can be varied across the MW band.
For example, if L was 300uH and C was 100pF, F=1/2π√(300*10-6*100*10-12 ) = 919kHz.
|Unmodulated carrier of 10MHz||Same carrier with modulation, note increase in peak to peak level|
|50% modulated carrier with audio signal show above||Same carrier but with 100% modulation|
The circuit below is basically the same crystal set above, followed by some audio amplification instead of the speaker so that it will work without an antenna or good earth. Also, the LM386 provides enough amplification and drive to power a small speaker instead of the earpiece. C5 and C9 set the overall gain, R3 and C10 prevent unwanted instability of the LM386 by providing a known load at frequencies above audio. The first transistor provides a moderately high impedance to the tuned circuit giving good selectivity. If you find the gain too high, you can remove C5.
|Breadboard of the LM386 radio||Detail of the breadboard|
Before the advent of easy-to-use IC’s such as the LM386, receivers were made from designs using discreet components. A popular choice was the regenerative receiver shown below. Rectification of the AM signal takes place inside Q2, and R3, C3 remove any remaining RF components. Some of the demodulated signals are fed back as positive feedback through R4 into the tap of L1 via the regeneration control R6. This will have the effect of starting to oscillate. The idea is to adjust R6 to a point where oscillation is about to start and back off a bit. This has the effect of greatly increasing the sensitivity and selectivity of the receiver. R5 generates some negative feedback, which improves the audio quality.
C5 and L1 are the tuned circuit, and L1 is 60 turns on a 1cm ferrite rod (about 300uH) with 5 turns added for the tap. In the breadboard below, I am only using one half of the variable cap.
|Regenerative radio breadboard||Detail of the PC|
In conclusion, making your own AM receiver can be easy and gives a lot of rewarding listening pleasure. All of the above circuits were built and tested and worked pretty well.
And here’s a small tip, if you are unable to source a variable capacitor, you can make one by gluing aluminum foil to two sheets of A4 paper and connect them with crocodile clips. Sliding one sheet over the other makes a variable cap provided they don’t make electrical contact.
There are some amazing websites featuring crystal sets, from the sublime to the ridiculous, and a whole community of crystal set fans out there. You may visit them here: http://crystalreceiver.co.uk/radios3.htm or https://en.wikipedia.org/wiki/Crystal_radio. If you would like to build a crystal set, here is a good guide: https://scitoys.com/scitoys/scitoys/radio/ten_minute_radio.html.