March 2010 Archives

Understanding I and Q modulation

This article is borne out of an attempt to explain modern modulation methods to a friend who has only heard of AM and FM.

This article explains and illustrates how a difference between the "I"  and "Q" produces modulation. I can be thought of as In phase. Q is short for Quadrature phase, which is the phase difference when measured from a fixed distance of -90 degrees from the I signal.

Is there is fixed shift in frequency, the resulting modulation is a circle. When viewed along the time axis, the circle becomes a spiral. If the spiral moves to the left (left-hand circular polarization), the frequency is higher than the carrier frequency (I leads Q). The converse is also true; the spiral turns to the right if the frequency is lower than the carrier frequency(Q leads I). Either of these signals, viewed on an oscilloscope, appears as a circle turning one direction or the other depending on the leading component.

If you have the ability to modulate the I and Q phases directly, any known modulation may be synthesized using a mathematical expression. AM is perhaps the most simplistic to represent since resulting occupied frequency is symmetrical. Holding the I phase steady in amplitude and phase while varying the frequency of the Q phase results in frequency modulation. It is almost as if varying the I phase amplitude modulates the signal and the Q phase frequency modulates the signal and the two together make any modulation, but this not accurate since QPSK is a phase modulation. It would be more appropriate to say that phase modulating both axes produces QPSK, and through that ability QAM, FM, and AM may be created. For this example I do not distinguish between AFSK, BPSK or FM, nor MSK or GMSK since they are all frequency-modulated modulations despite individual complexities.

One point of note that I have not seen in the books yet is whether or not the QPSK or PSK signals are subject to pre- and de-emphasis of the signal as a result of the modulation. In two-way radio, phase-modulation came about as a cheap way to frequency modulate transmitters, however it has a built-in pre-emphasis of the transmit signal, amplifying higher frequencies more than lower frequencies as a result of the modulation process. In the data world, this causes the resulting signal to occupy more frequency bandwidth than the symbol rates would otherwise imply.  

Here is another good explanation.

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