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ADRF6720-27 DC bias does not lead to datasheet results


I'm working with the ADRF6720-27 eval board. In the datasheet a DC bias of 2.68 Vdc for the baseband inputs (I+, I-, Q+, Q-) with 1 Vpp is recommended. At the moment, I'm only using both the I inputs. The signal source is a function generator where I apply the settings of 1 Vpp and 2.68 Vdc for a 1 MHz sine wave. The signals were checked with an oscilloscope and look fine (min ~2.18 V, max ~3.18 V). When applying these signals to the eval board I get the results below. The current consumption of the board is at 385 mA. I'm using an external reference at 153.6 MHz (DIV4 for PFD). My LO frequency is 2300 MHz.

However, when dividing the offset in half to 1.34 V, the RF output spectrum looks good and the output power peak Pout is more or less at 3.5 dBm. The values for second and third harmonic are not matching at all. Current is now at 350 mA. The new spectrum is shown in the next figure.

Is there something I misunderstood? I'd really like to get the same results as shown in the datasheet on p. 4 for 2300 MHz.

Thank you in advance for your help! :)