Q.
What is the low frequency operating limit for ADL5385 and ADL5386?
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A.
The ADL5385 and ADL5386 are specified to work down to output frequencies of 50 MHz. In practice you can operate them at slightly lower frequencies. There are however two constraints. Firstly, the output of the IQ Modulator is ac-coupled. This results in a 3 dB high-pass corner frequency of approximately 10 MHz (see attached plot). In practice, you should not operate close to this frequency as it will result in phase and amplitude distortion of the output signal. Operation at a minimum frequency of around 30 MHz is more realistic.
In addition to this constraint, the gain of the LO input path decreases with decreasing frequency. Below, the level shown in the attached plot, the IQ modulator core will not operate (you will see a massive rise in the noise floor). So for good operation down to 30 MHz, an LO power level that is well above this is recommended (an LO power level of 0 dBm was found to work well).
Author's Note (May 3rd, 2013): Since this original posting was made, the datasheet for ADL5385 has been updated and now includes specs at 30 MHz and a minimum specified operating frequency that has been reduced from 50 MHz to 30 MHz with the proviso that a mimumum LO power level of 0 dBm should be used.
