# An Ultra-Wideband Microwave Photonic Channelized Receiver with Zero-IF Architecture
## 3 Results and Discussion
### Balanced Detection and IQ Demodulation  
The output optical signal of the narrowband OBPF passed through a 1:2 optical splitter, and the upper and lower parts were connected to two BPDs through PCs and PBSs. By adjusting the two PCs, the phase difference between the optical carrier and the RF signal was 0\({}^{\circ}\), and the in-phase channel information was then obtained after passing through the PBS and BPD. The phase difference between the other optical carrier and the RF signal was 90\({}^{\circ}\), so that the quadrature channel information was obtained after passing through the PBS and BPD.  
Balanced detection can effectively suppress the common mode interference (mainly including second-order intermodulation distortions (IMD2) and DC terms). In order to demonstrate the distortion suppression and linearization ability, two-tone signals with frequencies of 21 and 21.01 GHz and the powers of 0 dBm were used as the input RF signal.  
Figure 6a shows the results without balanced detection, where a conversion gain of -41.2 dB, a noise figure (NF) of 53.1 dB, and a third-order input intercept point (IIP3) were obtained. Although the third-order spurious free dynamic range (SFDR3) reached 106.7-dB-Hz2/3, the second-order intermodulation distortions (IMD2) were always the dominant distortion and dragged the second-order SFDR (SFDR2) to only 76.5 dB-Hz1/2, which determined the overall SFDR of the system. Figure 6b shows the results with balanced detection, the conversion gain was -35.2 dB, where the 6 dB improvement benefits from the balanced detection. The NF with balanced detection was measured to 50.1 dB and the SFDR was 107.5 dB-Hz2/3, which also improved compared to those without balanced detection. What is more important, since the simulation is performed under ideal conditions, the comparison between the two shows that the balance detection can theoretically completely cancel the IMD2. Figure 7 shows the spectra of the I and Q electrical signals obtained by balanced detection. The bandwidth of the I and Q baseband signals was 300 MHz, which is half of the bandwidth of the RF signal. Thanks to the balanced detection, no obvious distortion and DC terms were observed.  
Figure 6: _Cont._Channel crosstalk is mainly caused by the residual optical carrier and sidebands. In order to get simulation results that agree with practical applications, the extinction ratio of the MZMs in the simulation was set to 30 dB. The channel crosstalk was tested and is shown in Table 2. We can see the channel isolations are all over 25 dB.  
Figure 8 shows the simulated error vector magnitude (EVM) curves and constellation diagrams when the power of the RF signal is tuned from \(-20\) to 20 dBm. We can see clearly that when the RF power increases, With the increase of RF power, EVM will decline steadily and eventually stabilize at 1,7%. Especially when the RF input power is greater than -5 dB, the EVM will drop below 10%, indicating that the system has a considerable dynamic range under these conditions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sub-Channel** & **1** & **2** & **3** & **4** & **5** \\ \hline
1 & 0 & 25.5 dB & 26.5 dB & 27.5 dB & 28.5 dB \\
2 & 25.5 dB & 0 & 25.5 dB & 26.5 dB & 27.5 dB \\
3 & 26.5 dB & 25.5 dB & 0 & 25.5 dB & 26.5 dB \\
4 & 27.5 dB & 26.5 dB & 25.5 dB & 0 & 25.5 dB \\
5 & 28.5 dB & 27.5 dB & 26.5 dB & 25.5 dB & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Measured channel crosstalk between all sub-channels.  
Figure 6: Measured powers of fundamental term, second-order intermodulation distortions (IMD2), third-order intermodulation distortions (IMD3), and noise floor in output IF signals as a function of RF input power. (**a**) Without balanced detection; (**b**) with balanced detection. Two-tone RF frequencies: 21 and 21.01 GHz. LO frequency: 20.9 GHz. RF power: 0 dBm.  
Figure 7: Measured I and Q spectrum with balanced detection.