What are transmitter spurs?

Transmitter spurs are very important from the perspective of radio amateurs, especially for those who are planning to homebrew radios or use homebrew radios which have not undergone extensive testing. Transmitter spurs are spurious emissions other than the intended output signal. A spur typically comes from a non-linear component in the circuit. These could be in amplifiers, mixers or diodes. Oxide coatings on the mating surfaces of cables and adapters could also give rise to spurs. A spectrum analyzer covering a wide range of frequencies will be needed to check for transmitter spurs. It is similar to looking for harmonics of the fundamental frequency occurring at lower signal strengths. In a broader sense, harmonics are also spurious unwanted emissions. TinySA based on the NanoVNA, is an affordable spectrum analyzer. But I have not been able to get one yet!

Long back, in my homebrewing era, I used to scan for harmonics by keeping my vacuum tube home radio receiver near the transmitter operating at low power. But looking for spurious emissions other than harmonics can be difficult with the receiver alone as you have to check a wide range of frequencies. In case of harmonics, you know exactly where to look as they occur in multiples of the fundamental frequency. There is also a potential risk of damage to the front end of the receiver if the signals are very strong and attenuators will have to be in place. Modern solid state radios will not be as tolerant as my antique vacuum tube radio!

Transmitter spurs other than causing unwanted interference, reduce the efficiency of the transmitter as extra power is used for that. Filter circuits are an important way of eliminating transmitter spurs. Choosing components with good linear response could also mitigate spurious emissions. Proper circuit layout and shielding could also help to reduce generation and radiation of spurious signals from the transmitter.