AN-1039

Application Note

Rev. 0 | Page 8 of 8

COMPLEX MODULATION

CONCLUSIONS

While a detailed discussion is beyond the scope of this article,

it is worth mentioning that all of the issues associated with

modulator imperfections can be avoided with a slightly differ-

ent transmit architecture. Many modern DACs incorporate

complex modulators, that is, digital engines that convert

baseband I and Q data up to a low intermediate frequency (IF).

These signals, which are still in Cartesian I and Q format, drive

the IQ modulator. Because modern IQ modulators, such as

the ADL5375, have baseband input bandwidths of as high as

750 MHz, low IFs in the 100 MHz to 250 MHz range can be

easily accommodated. When an IQ modulator is driven by

such a signal, the output spectrum is essentially a single

sideband spectrum similar to what is shown in Figure 3.

While modern IQ modulators offer excellent out-of-the-box

quadrature accuracy, IQ gain imbalance, and LO leakage, their

performance can be improved further using calibration. If the

transmitter incorporates a loop-back receiver as part of a digital

predistortion scheme, the receiver can also be used to conti-

nuously monitor and correct the imperfections of the IQ

modulator. The post-calibration performance is only limited

by the available compensation step sizes and the ability of the

receiver to precisely measure the constellation degradation.

In transmitters that do not contain a loop-back receiver, factory

calibration is a reasonable alternative. A single calibration in the

middle of a operating band most likely causes degradation at

the band edges. As a result, calibration at multiple frequencies

within a band is more effective. When temperature drift is

factored in, factory calibration at the ambient temperature

typically improves LO leakage and sideband suppression by

around 10 dB to 15 dB.

The lower sideband becomes the modulated carrier and is

displaced from the LO by a frequency offset equal to the

intermediate frequency. The imperfections of the IQ modulator

now manifest themselves as out-of-band effects, which can be

filtered away, resulting in in-band EVM, which is not affected

by the IQ modulator’s imperfections.

However, this approach comes at some cost. Care must be taken

to filter out the LO leakage along with the undesired upper

sideband. In contrast, a Nyquist filtered zero IF spectrum is

completely free of spurious components apart from harmonics

of the LO. In addition, as the frequency of the low IF increases,

the distortion of the DAC and IQ modulator increases slightly.

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