Radio transmitter design

An important legal requirement is that the circuit does not radiate significant radio wave power outside its assigned frequency band, called spurious emission.

[1] High-power transmitters may have additional constraints with respect to radiation safety, generation of X-rays, and protection from high voltages.

[2] Typically a transmitter design includes generation of a carrier signal, which is normally[3] sinusoidal, optionally one or more frequency multiplication stages, a modulator, a power amplifier, and a filter and matching network to connect to an antenna.

More elaborate transmitters allow better control over the modulation of the emitted signal and improve the stability of the transmitted frequency.

This was accommodated by allocating the short wave amateur and marine bands in harmonically related frequencies such as 3.5, 7, 14 and 28 MHz.

The RF generator in a microwave oven, electrosurgery, and induction heating are similar in design to transmitters, but usually not considered as such in that they do not intentionally produce a signal that will travel to a distant point.

Such RF devices are required by law to operate in an ISM band where interference to radio communications will not occur.

This will produce 100% modulation and can be done by inserting a transformer in series with the high voltage supply to the anode so that the vector sum of the two sources, (DC and audio) will be applied.

A disadvantage is the size, weight and cost of the transformer as well as its limited audio frequency response, especially for very powerful transmitters.

The audio input operates the regulator in such a way as to produce the instantaneous anode voltage needed to reproduce the modulation envelope.

Using a balanced mixer a double side band signal is generated, this is then passed through a very narrow bandpass filter to leave only one side-band.

[5] By convention it is normal to use the upper sideband (USB) in communication systems, except for amateur radio when the carrier frequency is below 10 MHz.

Connecting the 90° delayed signal from either the audio or the carrier (but not both) to the other mixer will reverse the sideband, so either USB or LSB is available with a simple DPDT switch.

Shaping circuits are used to turn the transmitter on and off smoothly instead of instantly in order to limit the bandwidth of these sidebands and reduce interference to adjacent channels.

Indirect FM employs a varicap diode to impose a phase shift (which is voltage-controlled) in a tuned circuit that is fed with a plain carrier.

Early text messaging such as RTTY allowed the use of class C amplifiers, but modern digital modes require linear amplification.

However, they require a heated cathode which consumes power and will fail in time due to loss of emission or heater burn out.

Solid state devices, either discrete transistors or integrated circuits, are universally used for new transmitter designs up to a few hundred watts.

The majority of modern transmitting equipment is designed to operate with a resistive load fed via coaxial cable of a particular characteristic impedance, often 50 ohms.

To connect the power stage of the transmitter to this coaxial cable transmission line a matching network is required.

For solid state transmitters this is typically a broadband transformer which steps up the low impedance of the output devices to 50 ohms.

Commonly an SWR meter and/or directional wattmeter are used to check the extent of the match between the aerial system and the transmitter via the transmission line (feeder).

High power short wave transmission systems typically use 300 ohm balanced lines between the transmitter and antenna.

If the transmitter continues to cause interference during this test then a path exists by which RF power is leaking out of the equipment and this can be due to bad shielding.

RF leakage from microwave ovens, while rare, may occur due to defective door seals, and may be a health hazard.

In modern equipment there are three main types of spurious emissions: harmonics, out of band mixer products which are not fully suppressed and leakage from the local oscillator and other systems within the transmitter.

Protection features must also prevent the human operator and the public from encountering the high voltages and power which exist inside the transmitter.

In addition, either resistors which bleed off the high voltages or shorting relays are employed to insure that capacitors do not retain a dangerous charge after turn off.

With large high power transmitters, the protective circuits can comprise a significant fraction of the total design complexity and cost.

Either AC transformers or DC power supplies are required to provide the values of voltage and current needed to operate the various circuits.