ADMV1455BBCZ-R7 datasheet(80/141 Pages) AD

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Data Sheet

ADMV1455

THEORY OF OPERATION

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Rev. 0 | 80 of 141

IMAGE REJECTION OPTIMIZATION

The image rejection capability of a mixer is defined as the ability

to have no gain, or a rejected output signal, for a given RF input

frequency that would otherwise provide the same IF output frequen-

cy. For example, if the intended RF input frequency is 30GHz,

the 2×LO_I and 2×LO_Q mixer LO frequencies are 28GHz, and

the desired IF output frequency is 2GHz, then the RF input image

frequency is 26GHz.

Imperfections in the quadrature phase imbalance of the mixer LO

signals (ideal 90°), imperfections in the mixer output amplitude im-

balance (ideal 0dB), as well as the imbalance characteristics of the

IF hybrid output may cause an IF output signal to be present when

applying this RF image frequency. To overcome these imperfections

and help optimize the image rejection capability of the ADMV1455,

the phase of the mixer LO signals and amplitudes of the mixer

outputs can be adjusted.

The adjustable phase shifters, PH_I and PH_Q, in LO signal chain

allow correcting of the quadrature phase imbalance between the

2×LO_I and 2×LO_Q signals. Each phase shifter has a typical 0.6°

step size, 20° of range, across 32 states. Use Register 0x80A,

Bits[4:0] to adjust the phase of 2×LO_I, and use Register 0x809,

Bits[4:0] to adjust the phase of 2×LO_Q.

The IF_I and IF_Q signals from the mixer feed into fine adjust

attenuators, DSAI and DSAQ. These attenuators allow for small

0.1dB steps and 1.5dB of total amplitude control for these signals

to optimize the image rejection capability of the ADMV1455. Use

Bits[7:4] set DSAQ.

When applying an RF image frequency and observing the IF output

power level, there are various options to perform image rejection

optimization. See the Sensitivity Guided Optimization section and

the Coarse and Fine Sweep Optimization section for two possible

options.

Sensitivity Guided Optimization

The first option for performing image rejection optimization uses

sensitivity analysis to assist in making decisions to narrow the

scope of possible optimization values. To perform this optimization,

apply an RF image frequency and take the following steps while

measuring the IF output power:

1. Set both PH_I and PH_Q to 15.

2. Set both DSAI and DSAQ to 0.

3. Step through the DSAI values starting from 0. Stop sweeping

when a local minima for the IF output power has been achieved

and retain the DSAI value that provides the local minima.

4. Repeat Step 3 for DSAQ.

5. Set PH_I to 14 and then 16 to determine the optimum direction

to step through. If PH_I = 14 provides a better result than PH_I

= 16, then step through the values from 13 to 0; otherwise, step

through values 17 to 31. Stop sweeping when a local minima

for the IF output power has been achieved and retain the PH_I

value that provides the local minima.

6. Repeat Step 5 for PH_Q.

7. With the values determined in Step 3 to Step 6 for DSAI,

DSAQ, PH_I, and PH_Q, further optimization may be possible

by adjusting each ±1 value.

Coarse and Fine Sweep Optimization

The second option for performing image rejection optimization uses

a coarse sweep followed by a fine sweep to determine the optimi-

zation values. To perform this optimization, apply an RF image

frequency and take the following steps while measuring the IF

output power:

1. Set both PH_I and PH_Q to 0.

2. Set DSAQ to 0. Step through the DSAI values starting from 0 to

15. Record the IF output power for each value.

3. Set DSAI to 0. Step through the DSAQ values starting from 0 to

15. Record the IF output power for each value.

4. Pick the combination of values from Step 2 and Step 3 that

provide the lowest IF output power. Note, that one fine step

DSA value will be zero.

5. Perform a nested coarse sweep of PH_I and PH_Q for values

0, 4, 8, 12, 16, 20, 24, and 28. There are 64 combinations of

those values for both phase shifters that are tested. Record the

combination of values that provides the lowest IF output power.

6. Use the combination of values determined in Step 5 to then

determine the fine sweep range by adding and subtracting 4

from each value. Perform a nested fine sweep of PH_I and

PH_Q for these values. For example, if Step 5 yielded PH_I =

16 and PH_Q = 12, then the fine sweep range for PH_I is 12

to 20 and the fine sweep range for PH_Q is 8 to 16. Excluding

the coarse values from the fine sweep, there are an additional

80 combinations of fine sweep values. Pick the combination of

values that provides the lowest IF output power.

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