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.