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AD8346_

Correcting Imperfections in IQ Modulators to Improve RF Signal Fidelity

Analog Devices 

Application Note

The second alternative presented previously is to perform

factory calibration. To do this, the popular single sideband

spectrum can be used as a simple but valuable diagnosis

tool during factory calibration. To create a single sideband

spectrum, the I and Q inputs are driven by low frequency

(typically 1 MHz) sine and cosine signals, that is, the base-

band signals are in quadrature. Figure 3 shows the spectrum

that results when these baseband signals are mixed with the LO.

The primary components of the single sideband spectrum are:

• Lower sideband: If the IQ modulator has no imperfections,

this is the only spectral component observed, the result of

multiplication and summing of the baseband sine and

cosine signals with the two in-quadrature LO signals.

• Undesired upper sideband: This undesired component

results from gain and phase imbalances between the I and

Q signal paths along with LO quadrature imbalance.

• Undesired LO leakage: As discussed earlier, the LO leakage

results from I and Q offsets and/or parasitic leakage of the

LO directly to the IQ modulator’s output.

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

CENTER 899.9334MHz 333kHz/

SPAN 3.33MHz

Figure 3. Single Sideband Spectrum

AN-1039

Figure 4 shows a plot that can be used to relate sideband

suppression to I/Q gain mismatch and quadrature mismatch.

From the plot, it can be noted that a quadrature phase error of

1°, coupled with an I/Q gain mismatch of 0.5 dB, results in

−30 dB of sideband suppression. It is notable in this example

that improving the quadrature phase mismatch has no effect

on the sideband suppression unless the gain mismatch is also

improved.

0

–10

2.5dB

–20 1.25dB

–30 0.5dB

0.25dB

–40 0.125dB

–50 0.05dB

0.025dB

–60 0.0125dB

–70

0dB

–80

–90

0.01

0.1

1

10

100

PHASE ERROR (Degrees)

Figure 4. Plot Showing the Relationship Between Modulator Errors and

Sideband Suppression

By using a directional coupler and a power splitter (as shown

in Figure 1), it is quite simple to add an auxiliary output to

the transmitter that can be used during factory calibration. A

spectrum analyzer is connected to this port. Another alternative

would be to connect the spectrum analyzer at the antenna

connector (after the signal has been adequately attenuated).

Rev. 0 | Page 5 of 8

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