## 3 Concept of Dual-Comb Operation  
A frequency comb is a radiation source that can generate many peaked frequency components simultaneously, making it suitable as a probe source for simultaneous multipoint measurements with a Doppler reflectometer. The most important issue in multi-frequency simultaneous measurement is the receiving technique, which requires multiple filter banks and detectors to differentiate and measure multiple frequencies, which increases the cost. Another technique, other than filter banks, is to collect all components of the frequency comb with a wideband digitizer and apply digital signal processing. In this case, however, it is difficult to collect data for a long-time duration due to memory issues. To reduce these problems, the dual-comb system was devised. To illustrate the concept, the following is an example of a probing range in the Ka-band (26.5-40 GHz). As shown in Figure 4, the Probe signal generated by a frequency comb has multiple frequency peaks with a bandwidth of about 14 GHz. When the Local frequency is set outside the Ka-band, the down-converted intermediate frequency (IF) bandwidth is also about 14 GHz, as shown in Figure 4a. When the Local frequency is exquisitely shifted from the peak near the center value of the Probe frequency range, the IF frequency bandwidth can be reduced by about half, as shown in Figure 4b. By setting the Local frequency in this way, the previously developed measurement system [6,8] could be applied to plasma measurements. Is there any way to lower the IF bandwidth? We have devised a new method for this project. That is the dual-comb method. Thereby, a frequency comb is used not only for the Probe signal, but also for the Local signal, as shown in Figure 4c. It is important to note that the clock (fundamental) frequencies of these two frequency combs are slightly offset. In this way, the IF bandwidth is expected to be significantly reduced to about 0.5 GHz.  
A confirmation test was conducted to see if the IF could actually be lowered. The conceptual test circuit shown in Figure 5 was built and the frequency spectrum was observed. The frequency combs created with a 190 MHz (\(f_{1}\)) clock and a 200 MHz (\(f_{2}\)) clock are input to the mixer as RF and LO, respectively. If m and n are integers greater than or equal to 0, the mixer outputs \(|\)m\(f_{1}-\)n\(f_{2}|\) frequency components. Therefore, the  
Figure 4: Schematic diagram of generating IF frequency. The blue (top), red (middle), and green (bottom) lines indicate the frequency components of probing comb, local, and IF, respectively. Here, frequency band of RF probe depicted as Ka-band. (**a**) Local frequency set outside RF frequency range. (**b**) Local frequency set near center RF frequency range. (**c**) Dual comb cases. Here, frequency difference between probe and local source drawn with 10 MHz shift.  
mixing frequencies of RF and LO comb components that are closest each other would line up at 10 MHz intervals. The frequency spectrum of this test is shown in Figure 5c. Thus, it was confirmed that the dual-comb method can generate discrete IF components at each frequency step, and that IF bandwidth reduction can be achieved.