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Quadrature (I/Q) Modulator Sideband Suppression

Quadrature modulators are used to conserve bandwidth for a given data rate. This is accomplished by modulating two orthogonal data streams onto a common carrier. If the phases and amplitudes of both data stream (in-phase "I" and quadrature "Q"), then one of the sidebands is completely cancelled out. If there is no DC bias feed-through, then the carrier itself is completely cancelled out. In practice, complete cancellation is never accomplished, but without too much work, achieving 40 dB of sideband cancellation is not hard to do. Even 60 dB is relatively easy; however, preventing drift due to thermal and mechanical effects is not so easy, and the result is that a "textbook" quadrature alignment during alignment can look pretty bad over time.


Click to see chartWithout showing all the trigonometry and algebra steps in-between, here are the basics of quadrature modulation. See the chart below.


In-Phase Data  (I) Quadrature Data  (Q) Carrier  (C)
RF Cafe - Quadrature modulator I RF Cafe - Quadrature modulator Q RF Cafe - Quadrature modulator Carrier
After Multiplying I & Q by the Carrier and Adding

Note that the D cos(Ωct) term is the carrier and disappears if D (the DC component) equals zero.

Lower Sideband Envelope  (LSB) Upper Sideband Envelope  (USB)
RF Cafe - Quadrature modulator LSB RF Cafe - Quadrature modulator USB

where G is the amplitude imbalance (ratio) and f is the phase imbalance, and RF Cafe - Quadrature modulator Gain

Sideband Suppression Equation
Quadrature modulator Sideband Suppression - RF Cafe
Phase Error  (f)
Quadrature modulator Phase Error - RF Cafe
Picture of Sideband & Carrier Suppression
Carrier suppression & sideband suppression - RF Cafe
Chart of Sideband Suppression as a Function of Phase and Amplitude Imbalance
This chart was created with Excel, using the above equations.

Sideband suppression vs. phase & amplitude error - RF Cafe

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