8MHz AM radio transmitter

This is an 8MHz amplitude modulated (AM) radio transmitter, which I built mainly for work, and also as an exercise in electronics (it is the first RF transmitter I’ve ever built). We wanted to have a simple radio transceiver design at our disposal for some possible projects in the future that might require a primitive means of short range wireless communications. Also considered was the possibility of getting kids to build this as part of the courses we teach in secondary schools.

The AM transmitter, fabricated in a poorly executed 'Manhattan style'.

The AM transmitter, fabricated in a poorly executed ‘Manhattan style’.

The circuit consists of 3 parts: oscillator, buffer, and mixer. It runs on a power supply voltage of 3.3V.

The oscillator produces the 8MHz carrier signal. It is a minimalist single-transistor (Q1) Pierce oscillator, and the design comes from National Semiconductor Application Note 400 (AN400 – A study of the crystal oscillator for CMOS-COPS, Fig 6). The resonating element is an 8MHz ceramic resonator (X1) with built-in capacitances (we have quite a few lying around in the office). An additional capacitance C1 is necessary to get the circuit to oscillate properly.

The buffer isolates the oscillator from loading by the mixer. It is a standard emitter follower circuit using Q2. The oscillator will not work properly if connected directly to the mixer. I did not explore the possibility of eliminating the buffer by increasing the input impedance of the mixer.

Circuit diagram of the transmitter.

Circuit diagram of the transmitter.

The buffered carrier signal goes to the mixer circuit, where its amplitude gets modulated by the analog signal input from IN1. This is just a common-emitter ‘amplifier’ with the transistor (Q3) biased close to / in the saturation region which is necessary for the whole thing to work as a mixer. The output of the mixer connects directly to the antenna, which is nothing more than a rather long (>30cm) length of wire.

Besides transmitting analog signals, the circuit also accepts digital signals at IN2. As the logic state of the signal from IN2 changes, it influences the bias point of the transistor Q2, and in turn, the gain of the emitter follower. This causes a variation in the amplitude of the carrier wave at the follower’s output – i.e. a cheat implementation of amplitude shift keying (ASK). R10, relative to R3 and R4, determines the size of the amplitude shift. The tank circuit composed of L1 and C5 is tuned to 8MHz, and its purpose is to block the carrier signal from entering whatever circuit is connected to IN2 (and wreaking havoc over there).

I connected the  output of a microphone amplifier to IN1 of the transmitter, and tuned into the transmission with a radio. The effective range is about 2 metres, which is quite suitable for our purposes. Beyond that distance, an audio signal becomes much less intelligible.

The circuit design can certainly be made simpler, and I think there is much room for improvements to be made. One of these will concern the spectral purity of the output, since I have no doubt that the circuit generates lots of spurs due to the somewhat haphazardly biased transistors; many of the component values in the circuit were a result of trial and error! I have so much more to learn about electronics! Finally, the antenna is also much too long, so some work on it is required before the transmitter can be useful.

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