A Microcontroller AM Transmitter (and a Lesson in EMI)

It turns out that my post on the PLL of the LPC1768 was a bit too harsh. It is true that it can’t generate too many high frequencies when the reference frequency is in the kHz range. But it can generate quite a bit of frequencies with a 1MHz reference. Around 1MHz, it can synthesize frequencies spaces 2kHz apart. Around 10MHz, the frequencies are 20kHz apart, and so on. You get the idea.

When I realized that I can do this, I decided to test this by generating a signal that an AM radio can pick up. I succeeded. But let’s talk about the PLL first.

The idea is simple. We will divide the 12MHz clock signal of the mbed board by 12, to generate a 1MHz reference. Now we multiply it by an integer between 275 and 550 to generate the PLL output frequency. To generate a clock signal that is slow enough to drive the microcontroller (100MHz or less), we need to divide by at least 6. I will assume that we divide by 5 and that we don’t multiply by 500, to keep the math simple. We have a CPU Clock frequency between 55MHz and 100MHz; the possible frequencies are 200kHz apart (the 1MHz spacing of the output of the PLL divided by 5). This clock signal will drive a timer or PWM module (I used PWM1), which will divide it again to obtain the output frequency and drive one of the pins.

I tried this first with a very slow output frequency of 1Hz to make sure the PLL works as I think it should and to test everything. The PWM module drove pin PWM1.2, which is connected to one of the mbed on-board LEDs.

Next, I upped the frequency to 1MHz and tested it on a scope; the signal was a nice 1MHz square wave. By connecting and disconnecting this signal from a pin using an interrupt-service routine driven by a another timer (I used the systick timer), I modulated the 1MHz signal. I used a modulation frequency of 1kHz. Connecting the pin of the mbed to a 30cm piece of wire generated a signal strong enough to be easily picked up by an AM radio at a distance of up to a meter or so. Turn on the mbed and you hear the 1kHz signal on the AM radio, turn it off and you don’t.

There are obviously better ways to modulate the RF signal. One is to vary the width of the pulse, like Michael Kleinigger did with an AVR microcontroller. This works reasonably at low RF frequencies, when the divisor you use to generate the RF signal from the CPU frequency is high (a large divisor gives you many pulse widths). It does not work well at higher frequencies. On the LPC1768 another option is to use the digital-to-analog converter to drive an analog modulator.

Also, the signal emitted by my 30cm wire is not a 1MHz modulated signal; it is a modulated square wave, which also contain large amount of energy in harmonics of 1MHz. To eliminate them, you need a filter; Michael’s article shows a very simple one. For regular use (where legal), a more sophisticated filter is necessary, and a small power amplifier also won’t hurt, at least to isolate the microcontroller from the antenna.

This little experiment is also an interesting lesson in how easy it is to generate electromagnetic interference (EMI). All I did was to connect a 1MHz digital signal to a 30cm piece of wire. This generated enough electromagnetic energy to be easily received by a radio. I did this intentionally, but it’s very easy to do that unintentionally, with long wires on a PCB or in a box. If it’s not shielded (or driven differentially to cancel out the field), it will emit.

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