FAQ: When I use the Auxiliary DACs to compensate for IQ Mod LO Leakage, what resistor tolerance should I use?

Document created by enash Employee on Sep 9, 2009Last modified by AndyR on Jan 31, 2012
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Q.

I want to use the Auxiliary DACs of the AD9779A to compensate away the LO Leakage of an IQ Modulator. I am using the recommended schematic from the data sheet of the ADL5370/1/2/3/4 family of IQ Modulators. What resistor tolerance should I choose for the resistors that couple the signal from the Aux DACs to the main signal path?

Aux DAC Compensation.JPG

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A.

This depends a bit upon your application. If you plan to calibrate the LO Leakage of your equipment one time in the factory (the so-called "set-and-forget" approach), then you should choose 1% resistors. The idea of "set-and-forget" is that you calibrate the LO Leakage in the factory and store the required Aux DAC codes in non-volatile memory (NVM). When the equipment is turned on in the field, these calibration codes are retrieved from NVM and loaded in to the DAC.

 

This suggests that since any errors caused by initial resistor mis-match can be calibrated away. So resistor tolerance should not be that important. However, we have found that the better your pre-calibration LO Leakage is, the better your post-calibration holds up over temperature (assuming that the in-factory set-and-forget routine is only performed at room temperature). So it is worthwhile to use resistors with 1% tolerance to give you a good pre-calibration starting point. This same logic alos applies to the 50 ohm current to voltage setting resistors of the main DACs.

 

In some transmitters, there is a loop-back receiver. The main purpose of this circuit is to monitor the distortion of the High Powered Amplifier (HPA) so that the equipment's digital engine can provide adaptive pre-distortion to reduce overall distortion. The loop-back receiver can also be used to measure the LO Leakge and Quadrature imbalance of the IQ modulator (to do this you need to demodulate the constellation and look for signs of LO Leakage and Quadrature Imbalance). In such a system, the LO Leakage can be continuously monitored and optimized so there is no real value in choosing resistors with very good tolerances.

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