Multi-Frequency
AM Transmitter

Circuit : Andy Collinson
Email :

Description

A multi frequency modulated transmitter circuit. The circuit transmits the same input simultaneously across all channels. Although the transmitters are of the FM type, there is enough cross modulation to be heard on AM receivers.

WARNING
It is illegal to transmit without a license in most countries. For this reason, this circuit has very limited power and range and is for educational purposes only.

Notes

Please read the disclaimer on this site before building any transmitter. It is illegal to operate a radio transmitter without a license in most countries. This circuit is deliberately limited in output power for this reason. The tank circuit, L1, C1 (first section), L2,C4 (second section) and L3,C7 (third section) determine output frequency. With a fixed 200uH inductor coil and a range of the tuning capacitor from 50 to 500PF the output frequency range is approximately 500kHz to 1600kHz. The tuning capacitor is shown fixed but may be a variable capacitor in each section. The resonant frequency for different coil and tuning capacitors can be found by using the calculator below.

Tuned Circuit Resonant Frequency Calculator
Capacitance:
Inductance:
Resonant Frequency:


Circuit Notes
The input stage is a single common emitter amplifier, base around Q4 and associated components. The emitter resistor is bypassed by C13 for higher audio gain. The amplified audio signal at Q4 collector is than passed to each of the oscillator stages, via C12, C14 and C15.

Each oscillator stage is a tuned tank circuit, the resonant frequency is varied by the modulating signal at the base of each transistor. This is technically frequency modulation, but there is enough cross distortion in the transistor to produce an AM signal.
Each oscillator stage works as follows. The first stage components, will be used as an example. L1 and C1 form a resonant tank circuit. An oscillator needs two conditions: gain greater than unity and regenerative feedback. Transistor Q1 in stage 1 provides the gain and its emitter resistor R2, is decoupled by C3 for radio frequencies. The signal appearing at the collector is fed back via C2 into the emitter, and passes through the base where it will be amplified again. The oscillation takes a short time about 20us, to build up and can be seen in the simulations. Power in the transmitter is limited by supply voltage and the current through the coil, which is regulated by R2.

Oscillator Waveform

Measuring RF waveforms is always difficult even with an oscilloscope. This is because all oscilloscope probes contains capacitance, and although it may be as little as 10pF, it is often enough to de-tune the oscillator or cause it to fail. The following waveforms are simulated in LTSpice, a freely available circuit simulator. See my LTSpice section link here for more details.

Simulation Circuit

To simulate the multi frequency transmitter, the schematic is modified as below. As there is no "microphone" it is replaced with a signal generator, designation V2 and set to 20mV pk-pk at 1kHz. This value will approximate "close speech" levels, The bypass capacitor across R7 has been left off and each transmitter stage has output notes labelled TR1, TR2 and TR3.

Spectrum Response

Fixed value capacitors have been used in each of the tank circuits and an AC simulation in LTSpice can be run. The AC simulation simulates the frequency domain ( this is the response you would see on a spectrum analyser). In the AC response the range from 500KHz to 2000kHz is simulated using 200 points/decade and the output is shown below:

Three peaks can be seen with the values for each tank circuit at approximately 600kHz, 1.1MHz and 1.4MHz. There are also "image" products in the output spectrum at each of the other frequencies. Looking at waveform V(tr1)you will see an image product at the other two frequencies, 1.1MHz and 1.4Hz but at a much reduced amplitude. This is because the circuit shares the same power supply, and as the same input signal is sent to all three transmitter stages, this is not so important, The bandwidth of each peak is controlled by the Q factor or "quality" factor of the tuned circuit, high Q produces a higher peak and narrow bandwidth where lower Q produces a wider bandwidth. The Q factor of a coil is given by its inductive reactance divided by the resistance of wire used. Using a thicker wire gauze will make a higher Q coil. In practise you can use a medium wave ferrite rod antenna from a salvaged old AM radio.

LTSpice File

Multiband AM The link contains the LTSpice file used to simulate the multi-frequency transmitter. Download and extract the zip file, all components and symbols are native to LTSpice XVII. Only one simulation can be run at once, so you need to right click the simulation command (in red) and prefix it with a period"." to run the simulation.

Adding More Stages

Although drawn with three transmitter stages, it is possible to increase the output stages to transmit on more frequencies. The bandwudth of each transmitter stage is controlled by the Q factor of the tank circuit, as previously stated. Should a wider transmit bandwidth be desired, then the Q factor of the coil can be reduced by placing a small serial resistor of 10 to 47 ohms in series with each coil.
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