AN-1039

Application Note

Rev. 0 | Page 6 of 8

CALIBRATION PROCEDURE

Correcting all of the modulator’s imperfections is a multistep

process. Start by looking at the procedure for LO leakage cor-

rection which results in a constellation that is offset from the

origin. A single sideband spectrum is applied to the transmitter

and is monitored on the spectrum analyzer. Next, small diffe-

rential offset voltages are applied to the I and Q inputs. Applying

differential offset voltages to the I and Q inputs should not be

confused with changing the dc bias levels (also referred to as the

common-mode level) on these pins, which has no effect. This is

done as an I offset sweep followed by a Q offset sweep (or vice

versa). Returning briefly to Figure 1, note that the AD9788 (a

16-bit, 800 MSPS dual DAC) conveniently includes two aux-

iliary DACs that can be used to couple differential dc offset

voltages on I and Q lines. This coupling is performed externally

using resistor dividers.

Figure 5 shows how sweeping the I and Q offset voltages alters

the LO leakage. Start by sweeping the I offset voltage around 0 V

while holding the Q offset voltage at 0 V. With modern IQ

modulators exhibiting unadjusted LO leakage in the −40 dBm

range and having voltage gains in the −5 dB to +5 dB range, an

offset voltage sweep range of ±5 mV is more than adequate to

identify the location of the null (in this example, ±2 mV is

adequate to identify a nulling voltage somewhere between

100 μV and 200 μV). Note, however, that the first pass (black

trace) only manages to reduce the LO leakage to just under

−40 dBm. This clearly indicates that the Q offset needs

correction. The second pass (blue trace) involves sweeping

the Q offset around 0 V with the I offset held at the value that

yielded the first I null. Note that a Q offset of 400 μV reduces

the LO leakage a further 10 dB to around −50 dBm. However,

a third pass is required. The trough from the first pass is quite

shallow because the Q channel had not yet been adjusted. This

makes it difficult to identify the ideal I nulling voltage. A third

pass (red trace) that involves again sweeping the I offset while

holding the Q offset at 400 μV, identifies the optimum I nulling

voltage to be 150 μV.

–80

–70

–60

–50

–40

–30

–20

–10

0

–2

–1

0

1

2

I AND Q DIFFERENTIAL OFFSET VOLTAGES (mV)

FIRST PASS – I OFFSET ADJUST

SECOND PASS – Q OFFSET ADJUST

THIRD PASS – I OFFSET ADJUST

Figure 5. Multipass LO Leakage Compensation Sweeps

CORRECTING FOR QUADRATURE AND I/Q GAIN

ERRORS

A similar procedure can be used to correct quadrature and I/Q

gain mismatch. IQ modulator family data sheets typically specify

the quadrature phase mismatch and I/Q gain imbalance in

degrees and decibels, respectively, along with the sideband

suppression (also in decibels). Using this information, it is

advisable to perform the first optimization pass on the weaker

of the two specifications, that is, the specification which most

contributes to the sideband suppression. For example, assume

that the device data sheet specifies a sideband suppression of

−40 dBc, comprising of 1 degree of phase imbalance and 0.1 dB

of gain imbalance amplitude. In this case, it is advisable to first

try to adjust phase because making a gain adjustment has

almost no effect as long as the 1 degree of phase error is present

(see Figure 4).

Figure 6 shows the results of a gain sweep followed by a

phase sweep. In the first pass, the gain delta between I and Q

is adjusted over a range of approximately ±2 dB. The TxDAC®

in Figure 1 facilitates this adjustment by providing internal

gain adjust auxiliary DACs. The sweep yields a null of around

−57 dBc for a gain difference of approximately −0.1 dB (gain is

scaled on the top axis). Next, adjust the skew between I and Q.

This drives the null down further to −60 dBc for a phase adjust

of −0.05°.

–70

–60

–50

–40

–30

–20

–10

–0.4

–0.3

–0.2

–0.1

0

0.1

0.2

0.3

0.4

PHASE ADJUST (Degrees)

–2

–1

0

1

2

GAIN ADJUST (dB)

SECOND PASS (PHASE ADJUST)

FIRST PASS (GAIN ADJUST)

Figure 6. Multipass Sideband Suppression Compensation Sweeps

In this case, a third pass is not necessary and does not yield

further improvement. This stems from the fact that the

unadjusted phase error is very close to the optimized value

(~0.05°). Thus, the first-pass gain adjust yields a deep trough

that is only slightly improved during the phase sweep. This

contrasts to the LO leakage nulling where a third pass yielded

further improvement.