Editing Amplitude modulation (section)

=={{anchor|AM modulation methods}}Modulation methods==
[[File:ammodstage.png|300px|right|thumb|Anode (plate) modulation. A tetrode's plate and screen grid voltage is modulated via an audio transformer. The resistor R1 sets the grid bias; both the input and output are tuned circuits with inductive coupling.]]

Modulation circuit designs may be classified as low- or high-level (depending on whether they modulate in a low-power domain—followed by amplification for transmission—or in the high-power domain of the transmitted signal).<ref>
{{cite book
 | title = Communication Engineering
 | author1= Atul P. Godse |author2=U. A. Bakshi
 | publisher = Technical Publications
 | year = 2009
 | isbn = 978-81-8431-089-4
 | page = 36
 | url = https://books.google.com/books?id=coQ6ac-fh6QC&pg=PA36
 }}</ref>

===Low-level generation===
In modern radio systems, modulated signals are generated via [[digital signal processing]] (DSP). With DSP many types of AM are possible with software control (including DSB with carrier, SSB suppressed-carrier and independent sideband, or ISB). Calculated digital samples are converted to voltages with a [[digital-to-analog converter]], typically at a frequency less than the desired RF-output frequency. The analog signal must then be shifted in frequency and [[linear amplifier|linearly amplified]] to the desired frequency and power level (linear amplification must be used to prevent modulation distortion).<ref>
{{cite book
|publisher= American Radio Relay League
|title= The ARRL Handbook for Radio Communications
|editor1-last= Silver |editor1-first= Ward
|edition= Eighty-eighth
|year= 2011
|chapter= Ch. 15 DSP and Software Radio Design
|isbn= 978-0-87259-096-0}}</ref>
This low-level method for AM is used in many Amateur Radio transceivers.<ref>{{cite book
|publisher= American Radio Relay League
|title= The ARRL Handbook for Radio Communications
|editor1-last= Silver |editor1-first= Ward
|edition= Eighty-eighth
|year= 2011
|chapter= Ch. 14 Transceivers
|isbn= 978-0-87259-096-0}}</ref>

AM may also be generated at a low level, using analog methods described in the next section.

===High-level generation===
High-power AM [[transmitter]]s (such as those used for [[AM broadcasting]]) are based on high-efficiency [[Class-D amplifier|class-D]] and class-E [[Electronic amplifier|power amplifier]] stages, modulated by varying the supply voltage.<ref>{{cite journal
|author= Frederick H. Raab
|title= RF and Microwave Power Amplifier and Transmitter Technologies – Part 2
|journal= High Frequency Design
|date= May 2003
|pages= 22ff
|url= https://www.scribd.com/doc/8616046/RF-Power-Amplifier-and-Transmitter-Technologies-Part2
|display-authors= etal
|access-date= 8 September 2017
|archive-date= 6 March 2016
|archive-url= https://web.archive.org/web/20160306164053/https://www.scribd.com/doc/8616046/RF-Power-Amplifier-and-Transmitter-Technologies-Part2
|url-status= live
}}</ref>

Older designs (for broadcast and amateur radio) also generate AM by controlling the gain of the transmitter's final amplifier (generally class-C, for efficiency). The following types are for vacuum tube transmitters (but similar options are available with transistors):<ref>
{{cite book
|author= Laurence Gray and Richard Graham
|title= Radio Transmitters
|publisher= McGraw-Hill
|year= 1961
|pages=141ff
}}</ref><ref>
{{cite book
|author=Cavell, Garrison C. Ed.
|title= National Association of Broadcasters Engineering Handbook, 11th Ed.
|publisher= Routledge
|year= 2018
|pages=1099ff
}}</ref>

; Plate modulation: In plate modulation, the plate voltage of the RF amplifier is modulated with the audio signal. The audio power requirement is 50 percent of the RF-carrier power.
; Heising (constant-current) modulation: RF amplifier plate voltage is fed through a [[choke (electronics)|choke]] (high-value inductor). The AM modulation tube plate is fed through the same inductor, so the modulator tube diverts current from the RF amplifier. The choke acts as a constant current source in the audio range. This system has a low power efficiency.
; Control grid modulation: The operating bias and gain of the final RF amplifier can be controlled by varying the voltage of the control grid. This method requires little audio power, but care must be taken to reduce distortion.
; Clamp tube (screen grid) modulation: The screen-grid bias may be controlled through a ''clamp tube'', which reduces voltage according to the modulation signal. It is difficult to approach 100-percent modulation while maintaining low distortion with this system.
; [[Doherty amplifier|Doherty modulation]]: One tube provides the power under carrier conditions and another operates only for positive modulation peaks. Overall efficiency is good, and distortion is low.
; [[Ampliphase|Outphasing modulation]]: Two tubes are operated in parallel, but partially out of phase with each other. As they are differentially phase modulated their combined amplitude is greater or smaller. Efficiency is good and distortion low when properly adjusted.
; [[Pulse-width modulation|Pulse-width modulation (PWM) or pulse-duration modulation (PDM)]]: A highly efficient high voltage power supply is applied to the tube plate. The output voltage of this supply is varied at an audio rate to follow the program. This system was pioneered by [[Hilmer Swanson]] and has a number of variations, all of which achieve high efficiency and sound quality.
; Digital methods: The [[Harris Corporation]] obtained a patent for synthesizing a modulated high-power carrier wave from a set of digitally selected low-power amplifiers, running in phase at the same carrier frequency.<ref>{{Cite patent|title=Amplitude modulation using digitally selected carrier amplifiers|fdate=1981-12-24|country=US|number=4580111|inventor1-first=Hilmer|inventor1-last=Swanson|inventor1-link=Hilmer_Swanson|assign=[[Harris Corp]]|pubdate=1986-04-01}}</ref>{{citation needed|date=May 2021}} The input signal is sampled by a conventional audio analog-to-digital converter (ADC), and fed to a digital exciter, which modulates overall transmitter output power by switching a series of low-power solid-state RF amplifiers on and off. The combined output drives the antenna system.