# Photonic-Assisted Scanning Receivers for Microwave Frequency Measurement
## 3 Experimental Results
The input RF power derived from the optical sideband to carrier ratio (OSCR), which is defined as the power ratio between the 1 st-order sidebands and the optical carrier, shows a good agreement with the theoretical OSCR value calculated based on the applied DC bias and switching voltages of the modulator employed. Next, we adjusted the input RF power to \(-5\) dBm to further demonstrate this technology. Figure 4c,d show the measured power output of the scanning receiver and the reconstructed optical spectrum of the modulated signal, respectively. When the input RF power was reduced from 0 dBm to \(-5\) dBm, the corresponding OSCR obtained from the reconstructed LSB (USB) spectrum was decreased from 2.7048 to 0.8702 (2.6695 to 0.8585), which indicates a power drop of 4.925 dB (4.927 dB) and agrees well with the theoretical value. Figure 3: (**a**) Top-down SEM image of the fabricated SOI single-ring filter. (**b**) Measured (red) output of the scanning receiver when no RF signal is applied and simulated (green) spectral responses of the optical filter. (**c**) Fourier transform of the measured filter response, \(H(k)\). Inset: Zoomed-in \(H(k)\). Figure 5 shows the measurement error based on the deconvolution method when the RF frequency is varied from 1 GHz to 20 GHz with a step of 1 GHz, during which the upper frequency was limited by the RF signal generator used in the experiment. The power of the input RF signal was fixed at 0 dBm. It can be seen that the proposed method is capable of characterizing a wide range of RF frequencies with an average measurement error of about 19.06 MHz across the whole measurement range. To investigate the resolution of the proposed technique for multiple-tone microwave frequency measurements, a two-tone RF signal was injected to the EOM. Figure 6a,c show the measured outputs of the scanning receiver with the RF signal having two tones with an equal RF power of 0 dBm at 12 GHz and 13 GHz and 12 GHz and 12.4 GHz, respectively. When the two-tone RF signal was spaced 1 GHz apart, Figure 6a shows that the filter shape pulses generated by the 12 GHz and 13 GHz RF frequency components had almost merged, and what should have been two obvious peaks were now barely distinguishable. The measurement resolution without applying the deconvolution technique is thus said to be limited to 1 GHz. Further reducing the spacing between the two-tone RF signals to 0.4 GHz showed complete overlap of the filter peaks as shown in Figure 6c. In contrast, Figure 6b,d illustrate the reconstructed RF spectrum by using the matrix pencil assisted deconvolution method to resolve the RF signals. A clear resolution of the two frequency components corresponding to the two tones in the original microwave signal with almost identical intensity in each sideband was achieved. For a microwave signal with input tones at 12 GHz and 13 GHz, the step differences for the two RF tones were, according to the reconstructed spectrum, 1921 and 2080, which represent measurement errors of 6.25 MHz and 0 MHz, respectively. When the frequency separation was further  
Figure 4: (**a**) Measured output signal and (**b**) reconstructed spectrum with the input RF power at 0 dBm. (**c**) Measured output signal and (**d**) reconstructed spectrum with the input RF power at \(-\)5 dBm. Figure 5: Measurement error as a function of the input RF frequency. reduced to 0.6 GHz (input tones at 12 GHz and 12.4 GHz), the step difference for the two reconstructed RF tones was 1922 and 1977, which translate to small measurement errors of only 12.5 MHz and 43.7 MHz for the two tones, respectively. More importantly, the reconstruction result demonstrates the significance of using the proposed deconvolution method to achieve a much higher resolution of 0.4 GHz, which is only around 30% of the filter bandwidth. This resolution could be further improved by increasing the signal-to-noise ratio of the scanning system, which is primarily attributed to the high fiber-to-chip coupling loss of the employed filter.