Fig. 3. Simulated (dashed curves) and experimental (solid curves) results of tunable lowpass Hilbert transformer with 90-degree phase shift.( a. purple 5.6 GHz ,b. green 7.1Ghz, c. blue 1.2Ghz, d. yellow 4.1Ghz and e. red 2.6Ghz)
The wavelength channels for both positive (solid red line) and negative (solid grey line) taps were separately measured by an optical spectrum analyser (OSA), achieving good performance that agreed well with the theory (blue dot). Finally, the weighted and delayed replicas were combined and converted back into the RF domain via a balanced photodetector (Finisar BPDV2150R).
The system RF frequency response was characterized with a calibrated vector network analyser (VNA, Agilent MS4644B) to measure the RF transmission and phase response. Fig. 3 (a-e) presents the simulated (dashed curves) and measured (solid curves) RF frequency response for both the magnitude and phase of the standard Hilbert transformer, yielding variable bandwidths ranging from 3.4 to 15.3 GHz. The centre frequency of the Hilbert transformer was set into half of the FSRRF, which was 19/2 = 9.5 GHz in our case.