Table 2 (absolute maximum ratings) on page 4 of the HMC8191 datasheet states that the maximum source/sink current is 3mA. That is, the total current sinking or sourcing on the IF1/IF2 pins should not exceed this current. When nulling the LO, the IF1 or IF2 voltages do not need to match and some current will flow. The voltage required to null the LO is typically around -0.1 to 0.1V, with fine voltage steps of around 0.001V depending on the nulling depth.

When adjusting the DC voltage on each IFX port:

1. Adjust IF1 from 0V to 0.1V in steps of 0.01V or greater and stop adjusting when the LO leakage starts to deteriorate from the previous voltage.
2. If the above adjustment does not improve, adjust IF1 from 0 V to -0.1 V in steps of 0.01 V or more, and stop adjusting when the LO leakage starts to get worse than before.
3. Adjust IF2 from 0V to 0.1V in 0.01V steps and stop adjusting when the LO leakage starts to get worse than before.
4. If the above adjustment does not improve, adjust IF1 from 0V to -0.1V in steps of 0.01V, and stop adjustment if the LO leakage worsens than before.

At this point, if you have not yet reached your desired LO leak amount, go back to step 1 and repeat the steps.
However, the DC voltage does not return to 0V, and starts adjusting +/- in 0.1V steps from the voltage found in the first adjustment, fine-tuning the offset.

(Published in October 2022)

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