## 3 Results and Analysis  
After the final simulation, a high-performance 0.34 THz sub-harmonic mixer was fabricated, assembled, and tested, as shown in Figure 9. The photographs of the 0.34 THz mixers are shown in Figure 9b,c. The mixer's circuit was integrated on a 50 \(\upmu\)m thick quartz substrate and was fixed on the copper block with silver glue. In the last part of the mixer, a 50-ohm microstrip line was used to connect the quartz substrate and connector. The material for the black substrate was Rogers RT/Duroid 5880. The IF signal was output by SMA connectors.  
The performance of the mixer was measured by a down-conversion method as shown in Figure 9a. In the measurement of SSB conversion loss, the mixers' RF signal was provided by a signal generator (1465 L from Ceyear) with a multiplier module (82401 TA from Ceyear) and was measured by a THz power meter (Erickson's PM5). To drive the mixers, a \(\times\) 12 multiplier chain was used to provide sufficient LO power. In terms of the IF signal generated by the diode's mixing, it was measured by a calibrated spectrum analyzer (4051 P from Ceyear). The power of the RF signal was set at a low level to make sure that the mixer works in their linear range. The return loss of RF signal and the loss of IF output  
Figure 8: The simulated results of the improved active circuit model and the overall structure model. (a) SSB conversion loss; (b) RF return loss; (c) LO return loss; (d) RF insert loss.  
Figure 9: The measurement of the 0.34 THz sub-harmonic mixers and its photographs under a microscope. (a) Test scenarios for sub-harmonic mixers; (b) The photograph of the 0.34 THz mixer; (c) The photograph of the mixer block.  
lines were measured by a vector network analyzer. All the measurements were carried out at room temperature.  
In respect of the 0.34 THz sub-harmonic mixer, the power of the RF input signal was set at \(-13\) dBm to \(-15\) dBm and the LO driving power was maintained in the range of 5.5-6.5 mW. In the RF frequency range of 320-360 GHz, the measured SSB conversion loss of this mixer was better than 12 dB. When the IF frequency was fixed at 1 GHz and 2 GHz, the measured SSB conversion loss was better than 10 dB in the RF frequency range of 325 to 340 GHz, which is illustrated in Figure 10a,b. In the LO frequency range of 163-175 GHz, the measured SSB conversion loss of this mixer was 8.95-12 dB with large bandwidth. The measured results of this mixer are almost consistent with the simulated results. This mixer also achieves a good return loss at the RF port, as Figure 10e shows. In the frequency range of 310-355 GHz, the measured RF return loss was less than \(-7.5\) dB. In contrast with our previous work, combining single-stage reduced matching technology with GDM, the measured results are much better, as shown in Figure 10f.  
The comparison between the performances of different terahertz sub-harmonic mixers is shown in Table 1. The proposed 0.34 THz sub-harmonic mixer is close to the international and domestic levels in terms of conversion loss and optimal LO power. The proposed 0.34 THz mixer combining two-stage reduced technology with an improved active circuit model have the characteristics of simple structure, low conversion loss, high stability, and large working bandwidth.  
Figure 10: _Cont._