How are RF harmonics affected by the shape of a waveform?

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SUMMARY

The discussion focuses on the impact of waveform shape on RF harmonics, particularly in relation to square waves. It is established that square waves generate an infinite number of odd harmonics, with the amplitude of the n-th harmonic being 1/n of the fundamental frequency. The propagation of high-frequency harmonics through components like MOSFETs is contingent on the frequency response of the circuit, as harmonics are independent of the fundamental once generated. The transfer function of a circuit plays a crucial role in determining how these harmonics are affected as they pass through electronic components.

PREREQUISITES
  • Understanding of Fourier Analysis and its application in waveform analysis
  • Knowledge of RF harmonics and their implications in communication systems
  • Familiarity with MOSFET operation and switching characteristics
  • Basic principles of circuit transfer functions and frequency response
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  • Study the principles of Fourier Analysis in-depth, focusing on harmonic generation
  • Research the effects of RF harmonics on communication systems and mitigation techniques
  • Explore MOSFET frequency response and its impact on signal integrity
  • Learn about circuit transfer functions and how they affect harmonic propagation
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Electrical engineers, RF engineers, and anyone involved in circuit design or analysis, particularly those working with waveform generation and harmonic distortion in electronic systems.

Plat
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How can the power/amplitude of a particular RF harmonic be calculated? I would assume it is some well-defined fraction of the amplitude of the main frequency?

Do RF harmonics from a square-wave drive appear only on the even, odd, or both, multiples of the base frequency?

How do high-frequency harmonics propagate through electronic components? For example, if a MOSFET can switch at 1MHz max, then can it convey harmonics from the source/signal frequency that are above 1MHz? I know I'm greatly simplifying things here.
 
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Fourier Analysis enables us to calculate the amplitude of each harmonic in a repetitive/periodic wave.
 
Plat said:
Do RF harmonics from a square-wave drive appear only on the even, odd, or both, multiples of the base frequency?
A square wave has an infinite number of odd harmonics. The n'th harmonic has an amplitude of 1/n compared with the fundamental.
https://en.wikipedia.org/wiki/Square_wave#Fourier_analysis
 
Plat said:
How can the power/amplitude of a particular RF harmonic be calculated? I would assume it is some well-defined fraction of the amplitude of the main frequency?
In general, RF harmonics are a bad thing, especially in communication systems. Quiz Question -- Why? :smile:

And there is no general way to calculate them. They arise from several different mechanisms, some generating odd harmonics some generating even harmonics, and some generating both.
 
Plat said:
How do high-frequency harmonics propagate through electronic components?
A circuit has a transfer function that shows circuit gain and phase shift across frequency. In linear circuits, the fundamental and each harmonic must be treated separately as each passes through the circuit. The resultant signal will be the sum of all the differently shifted and scaled sinewave harmonics.
The spectrum of the output signal is the spectrum of the input signal multiplied by the transfer function.

Plat said:
For example, if a MOSFET can switch at 1MHz max, then can it convey harmonics from the source/signal frequency that are above 1MHz?
A MOSFET needs to be on or off most of the time to minimise both heating during each transition and the power needed to charge and discharge the gate. For that reason, switching rate is meaningless without qualification. It is unlikely that a switching MOSFET application would be driven by a sinewave alone.
 
Plat said:
How do high-frequency harmonics propagate through electronic components?
It would depend entirely on the 'frequency response' of the system you are dealing with. There is nothing magic about harmonics. They are totally independent of the fundamental, once they have been created. Their frequencies will stay the same - that's all.
To find out more about the relationship between the shape of a waveform in time and its description in 'frequency space', try this wiki article. Frequency and time domain descriptions are just alternative ways of describing the same signal. It is not 'really' one or the other.
 

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