# Filtering harmonics

#### Art

Joined Sep 10, 2007
806
Hi Guys,

Just a question.
If you want to pass a fundamental such as a carrier through various parts of a transmitter,
and aim to filter harmonics with either pass or stop filters, I understand that attenuated aftifacts
from the original oscillator could get re amplified, but essentially,
can parts of a circuit other than amplification also reintroduce harmonics?
... such as passing a known intermediate freq through a resonant transformer (IF filter stage)?
Cheers, Art.

#### alfacliff

Joined Dec 13, 2013
2,458
usually done with low pass filters. harmonics are weakier than the origional frequency signal.

#### BR-549

Joined Sep 22, 2013
4,938
"can parts of a circuit other than amplification also reintroduce harmonics?"

Absolutely. Modulation, demodulation, rectifying can cause harmonics. Even filtering can cause harmonics.

The art of design can minimize it, but the potential is always there.

This is because the charge carriers, are in oscillation at frequencies much higher than the signal frequencies we use.

#### crutschow

Joined Mar 14, 2008
27,171
Any non-linearity in the signal chain can cause harmonics.

#### SLK001

Joined Nov 29, 2011
1,548
If you want to pass a fundamental such as a carrier through various parts of a transmitter,
and aim to filter harmonics with either pass or stop filters...
You generally don't worry too much about harmonics going into a transmitter, since the transmitter is the biggest non-linear element in the chain. As long as the fundamental is ~10dB or better above the harmonics, then the harmonics will be swamped by the harmonics generated by the transmitter. It is after the xmitter that strong harmonic filters are placed to attenuate the harmonics. A good UHF class C transmitter will usually output harmonics in the ~40 dBc range, since it is tuned to the fundamental, not the harmonics. Adding a harmonic filter (usually a low pass filter) with an additional 40dB of harmonic rejection will give you an output with harmonics -80dBc or better, depending on your particular regulatory requirements.