# Transmitter and Receiver Circuits for a High-Speed Polymer Fiber-Based PAM-4 Communication Link
## 4 Measurement Results of the PMF Link  
The polymer microwave fiber that was used during these measurements was provided by Lehrstuhl fur Hochrequenztechnik (LHFT), Germany. A photo of the D-band dielectric waveguide can be seen in Figure 13. The length of the fiber is 1 m and it has a rectangular solid cross section (1 mm by 2 mm). Measured reflection loss and transmission loss of the fiber, including transitions from the waveguide to fiber, are shown in Figure 14. The group delay can be seen in Figure 15. Both measurements include the fiber-to-waveguide transitions.  
The entire link was tested in the time domain, transferring data in real time. The output from the PD was then connected using a coaxial cable to a Lecroy LabMaster 10-100 Zi real-time oscilloscope, where the signal could be analyzed. A photo of the fiber connecting the circuits on each probe station can be seen in Figure 16.  
The LO input signal for the RF-DAC was provided by a VDI vector network analyzer (VNA) through an extender WR 6.5. The LO input frequency was set to 130.6 GHz, which together with the probe loss results in an input power of approximately \(-1.5\) dBm delivered at the input of the RF-DAC. The data input was provided by a Keysight M8195A arbitrary waveform generator (AWG).  
Figure 12: Time domain simulated input and corresponding output from the PD. The carrier frequency is 140 GHz.  
Figure 13: The polymer microwave fiber that was used in the test setup.  
First, the PMF link was tested using only one data input (MSB) to create a PAM-2 modulated signal. Bit rates up to 32 Gbps were measured and the data input that was used was a PRBS-9 stream. The eye diagram of the output signal from the PD can be seen in Figure 17a for a 30 Gbps signal and in Figure 17b for a 32 Gbps signal. For a 32 Gbps signal, the bit error rate (BER) was 2.6 \(\times\) 10\({}^{-10}\).  
For the PAM-4 PMF link measurements, a PRBS-9 stream was used at the MSB data port and a PRBS-10 stream was used at the LSB data port. Data rates up to 15 GBaud were tested, corresponding to 30 Gbps. For 30 Gbps PAM-4 transmission, the BER was 4.3 \(\times\) 10\({}^{-10}\). Eye diagrams of the measurements can be seen in Figure 18.  
Figure 16: The setup that was used in the PMF link measurements. The fiber can be seen connecting one circuit on one probe station to the other one on the other probe station.  
Figure 14: Measured reflection loss and transmission loss of the fiber that was used during the measurements.  
Figure 15: Measured group delay of the fiber that was used during the measurements.  
The total DC power consumption for the PMF link was measured to be 123 mW, which results in an energy efficiency of 4.1 pJ/bit for a 30 Gbps data transmission.  
An overview of the link is displayed in Figure 19, and an estimation of the link budget can be seen in Table 1.  
The noise floor of the oscilloscope is approximately \(-\)73 dBm.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Stage** & **Change** & **Power** \\ \hline LO & - & 0 dBm \\ Probe loss & \(-1.5\) & \(-1.5\) dBm \\ RF-DAC & \(-3\) & \(-4.5\) dBm \\ Probe loss & \(-1.5\) & \(-6\) dBm \\ PMF & \(-5\) & \(-11\) dBm \\ Waveguide bends/transitions & \(-2\) & \(-13\) dBm \\ Probe loss & \(-1.5\) & \(-14.5\) dBm \\ PD & \(-1.5\) & \(-16\) dBm \\ Probe loss & \(-0.5\) & \(-16.5\) dBm \\ Cable loss & \(-1\) & \(-17.5\) dBm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Link budget: carrier frequency at 131 GHz.  
Figure 17: Eye diagrams from the demodulated PAM-2 signal at 130.6 GHz. **(a)** 30 Gbps signal. BER \(<\times 10^{-12}\). **(b)** 32 Gbps signal. BER = 2.6 \(\times\)\(10^{-10}\).  
Figure 19: Overview of the link.  
Figure 18: Eye diagrams from the demodulated PAM-4 signal. (a) 11 GBaud signal. BER \(<\) 10\({}^{-12}\). (b) 12 GBaud signal. BER = 2.8 \(\times\) 10\({}^{-12}\). (c) 15 GBaud signal. BER = 4.3 \(\times\) 10\({}^{-10}\).