From a9102a637f8adc862371ee7b9c4353aed37689c9 Mon Sep 17 00:00:00 2001 From: Tyler Perkins Date: Sat, 15 Jan 2022 21:30:01 -0500 Subject: [PATCH] Update for 15-01-22 21:30 --- tech/QAM.wiki | 28 +++++++++++++++++++++++++++- 1 file changed, 27 insertions(+), 1 deletion(-) diff --git a/tech/QAM.wiki b/tech/QAM.wiki index dcc3155..80d0111 100644 --- a/tech/QAM.wiki +++ b/tech/QAM.wiki @@ -1,3 +1,29 @@ = Quadrature Amplitude Modulation = -Quadrature Amplitude Modulation +Quadrature Amplitude Modulation or QAM is a type of digital modulation. We can +modulate two sin waves that are pi / 2 out of phase and add them together. +When you do this, the resulting signal's phase and amplitude relies on the +input signals. Because it is known that they are out of phase by pi / 2, we +know we can seperate them again. + +The condition of being out of phase by pi / 2 is known as quadrature. + +QAM is used in serveral digital modulation schemes including 802.11 and GOES. + +== Constellation diagram == + +QAM is often shown on a constellation diagram. This is often a circle with points +around a center. The angle of the point, mesured counterclockwise from the horizontal +axis, represents the phase shift of the carrier wave with respect to the reference. +The distance from the center idicates the amplitude of the signal. + +For a set baud rate, an incoming signal will hold a phase and amplitude, +occuping a sample. This combination maps to a finite number of sybmols on the +constallation diagram representing some combination of bits. For a QAM signal +the amplitude may vary, however for QFM or QPM signal, however, this is not +true. + +These are also often called "IQ" diagrams because they consist of a horizontal +I and veritcal Q axis. The I axis reprsents a cosin "in phase (with the +carrier)" signal. The Q represents a sine wav shifted by pi/2 from the carrier +(aka the _quadrature_ carrier).