Direct-conversion I/Q modulation is an attractive solution for advanced RF transmitters, enabling complex modulation at lower cost. A direct I/Q modulator can generate an RF waveform of arbitrary phase, frequency and amplitude. In a typical application, the I/Q modulator produces a single-sideband output signal. For an ideal modulator with perfect 90° phase shift between the I-mixer and Q-mixer local oscillators, and with no other undesired phase or gain impairments, the modulator output will contain only the desired sideband. In practice, non-idealities arise from other sources of phase error such as baseband I/Q delay mismatch and DAC skew.

The error terms of interest in this analysis are the I/Q skew error due to the baseband signal source (φ_DGEN), the I/Q baseband skew error due to the modulator (φ_MOD), and the LO quadrature error (φ_LO) (Figure 1). Note that time delay errors, such as φ_DGEN and φ_MOD, result in a frequency-dependent phase error which can not be compensated by a frequency-independent I/Q phase adjustment. Consequently, they may limit the effectiveness of calibration procedures and the extent of image suppression.

In order to achieve the best image rejection for a broadband communications channel (e.g., W-CDMA), it is important to understand these error sources. This article provides a measurement method to determine the sources of RF and baseband phase error. The method requires a baseband I/Q signal generator with a user-adjustable phase (φ_GEN) between its I and Q channel outputs. Using this signal source, a sequence of three measurements are made. The individual error terms, φ_DGEN, φ_MOD, φ_LO, are derived from these measurements.

As shown and defined in Figure 1, there can be phase differences between the I and the Q path for the baseband generator (φ_DGEN) and within the I/Q modulator (φ_MOD). There may also be a phase error φ_LO between the quadrature LO signals, LOI and LOQ, in the modulator. Consequently, an unwanted upper sideband signal at (ω_LO + ω_BB) is present at the RF output along with the desired lower sideband signal at (ω_LO - ω_BB). Using trigonometric identities and small angle approximations, it can be shown that the upper sideband suppression is given by:

R_SB=20·log(φ_LO+φ_GEN1+φ_DGEN+φ_MOD)-6.02 [dB] (1)

Note that the phases φ are in radians. The image term can be minimized by adjusting the signal generator phase setting to

φ_GEN1= -φ_LO-φ_DGEN-φ_MOD. (2)

This configuration differs from that of Figure 1 in that the differential baseband connections to the modulator's I inputs are reversed (Figure 2). The controllable signal generator phase, φ_GEN2 is adjusted to null the image. The desired signal in this case is the upper sideband signal (ω_LO + ω_BB), and the image signal is at (ω_LO - φ_BB).

The lower sideband suppression is given by:

R_SB=20·log(φ_LO-φ_GEN2-φ_DGEN-φ_MOD)-6.02 [dB] (3)

The image is minimum for:

φ_GEN2 = φ_LO-φ_DGEN-φ_MOD. (4)

In this configuration (Figure 3), the I and Q differential inputs are exchanged. The controllable signal generator phase, φ_GEN3 is adjusted to null the image. The desired signal is now the upper sideband frequency component (ω_LO + ω_BB), and the image is a lower sideband signal at (ω_LO - ω_BB). The lower sideband suppression is given by:

R_SB=20·log(φ_GEN3+φ_DGEN+φ_LO-φ_MOD)-6.02 [dB] (5)

The image is minimum for:

φ_GEN3 = -φ_LO-φ_DGEN+φ_MOD (6)

We can solve equations, (2), (4) and (6) to give:

φ_LO = (φ_GEN2 - φ_GEN1)/2 (7)

φ_DGEN = -(φ_GEN2 + φ_GEN3)/2. (8)

φ_MOD = (φ_GEN3 - φ_GEN1)/2 (9)

As an example, the techniques described here were used to determine the error terms for an LT5528 direct I/Q modulator. A Rohde and Schwarz AMIQ I/Q modulation generator was used as the baseband signal source. For five different LT5528 examples, φ_LO, φ_DGEN and φ_MOD were measured using 5 MHz and 10 MHz baseband frequencies. From the measured phase errors, equivalent delay errors can be calculated. The results in Table 1 show that the baseband signal generator phase error φ_DGEN is dominant in this setup and is equivalent to a delay error of about 300 ps, compared to the LT5528's baseband phase error φ_MOD, which is equivalent to a delay of only about 25 ps - 30 ps. In this example, the baseband signal source phase error φ_DGEN is the major error source. In an actual application, it should be carefully characterized and compensated.

Peter Stroet is a design engineer with Linear Technology Corp., Milpitas, Calif.

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