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A new testbed was developed with an LSNA to perform multi-harmonic broadband measurements for periodically modulated signals with bandwidth exceeding the IF receiver bandwidth of the LSNA. This testbed is intended for characterizing PAs using periodically modulated (multitone) signals often referred as multisines. Multisine signals can be synthesized to approximate the CCDF and PAPR of digitally modulated signals such as WiMAX and LTE (Long Term Evolution). In the experiment conducted a multisine with 13 tones distributed over 15 MHz was used which provided a PAPR of 6 dB. Using this excitation, the nonlinear response of a 80 W Doherty GaN PA at 2.16 GHz was characterized over a 80 MHz bandwidth. Inband signal distortion and spectral regrowth were recorded in the 63 tones measurement over the 80 MHz measurement bandwidth targeted at both the fundamental and second harmonic. This increased measurement bandwidth enables one to characterize the nonlinear vector response of the PA in its 3rd and 5th order inter-modulation bands at the fundamental frequency. The broadband vector results at the second harmonic obtained in the same measurements provides additional invaluable information for the multi-harmonic optimization of the efficiency of the Doherty PA under test. It is believed that the multi-harmonic broadband vector characterization capabilities provided by the reported testbed should assist with the development and verification of multi-harmonic behavioral models for wideband modulated signals.