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What Are the Causes of Transmitter Harmonics?

Harmonics, a type of spurious emission, occur when operating signals are outside the transmitter's operating frequency. If these fall into frequency already in use, there can be problems. Possible causes of harmonics can include the transmitter's cabinet or auxiliary RF devices.

Limits on Harmonics
- The FCC puts limits on the harmonics allowed on a transmitter due to the inference on other users. For example, a transmitter with a 100 W output cannot have total harmonic signals more than 10 millwatts. While there will still be some harmonics, they will be small enough to avoid interference with other users.
Detecting Harmonic Transmissions
- An absorption wave-meter or RF spectrum analyzer can be used to test for harmonic transmissions. New equipment has FCC requirements to reduce harmonics. In addition, a low-pass filter can be used to reduce harmonics.
Causes of Harmonics
- There are a number of factors that can cause harmonics. A common cause is intermodulation distortion resulting from mixing signals. Trying to equalize linearity -- the input and output of a transceiver -- can result in harmonics. This can be caused by adding an amplifier to improve linearity.

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