Q.
Is there a Performance Benefit Associated with Driving the ADL5375 Local Oscillator Input Differentially?
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A.
The LO inputs of most IQ Modulators can be driven single-ended(the unused LO input is ac-coupled to ground) or differentially. While most of ADI's IQ Modulators are specified for single-end LO drive, differential LO drive can improve performance. Differential LO drive results in some common mode rejection, cancellation of even-order LO harmonics and can improve modulator quadrature accuracy. While differential LO drive has traditionally required the use of an external balun to convert the signal from a single-ended VCO, to differential mode, modern PLL/VCOs such as the ADF4350 provide a differential output which can be directly interfaced to the IQ Modulator or Demodulator.
The attached plots compare Sideband Suppression, OIP2 and 2nd harmonic distortion when the ADL5375-05's LO input is driven single-ended and differentially. The most notable performance difference is sideband suppression in the 1500 MHz range. The measurements showed no difference in output power, LO leakage, 3rd order distortion, and OIP3. The TC1-1-43+(600MHz ~ 4GHz) transformer was used to generate the differential LO.
Based on these results, we would recommend that if a differential LO is available (e.g. from a device like the ADF4350), then it is worth using this to drive the ADL5375 differentially. However, if only a single-ended LO signal is available, in this case it is probably not worthwhile to spend the money on a balun to achieve a differential LO drive.
