# Superconducting Sub-Terahertz Oscillator with Continuous Frequency Tuning
## 3 Results and Discussion
### Suppression of Resonant Features. Study of the Modified FFO  
In order to \"smooth\" the IVC of the FFO in the resonant mode, it is necessary to increase the damping parameter \(\alpha\), which would cause the dissipation of the reflected electromagnetic wave inside the junction. This was achieved by introducing an additional resistive layer into the FFO topology. To study the effect of the suppression of the resonances experimentally, several versions of the oscillators were fabricated with a 60-100 nm thick Mo absorber layer, partially covering the FFO and located either along the entire junction or at the FFO ends [66]. Figure 3 shows a schematic representation of the structures with different geometries of absorbing resistors. For a direct comparison, the FFOs without an absorber (see Figure 3a) were fabricated in the same technological run. It should be mentioned that all test circuits contained a small SIS junction to detect the radiation emitted by the FFO [39; 67]; this makes it possible to compare the power produced by FFOs with  
Figure 3: Layout of the FFO of different designs: (**a**) traditional FFO; (**b**) design with resistive layers along the FFO; (**c**) resistive layers along and on both ends of the FFO; (**d**) resistive layers on the non-emitting end of the FFO.  
and without absorbers. Both Nb-AlO\({}_{x}\)-Nb and Nb-AIN-NbN circuits were fabricated and tested.  
The location of the resistive layers along the FFO on its sides (Figure 3b) does not provide complete suppression of geometric resonances. There are still weakly expressed regions with a characteristic stepped structure on the IVC in the resonant mode. The inclusion of additional resistors on both ends of the generator and along the FFO (Figure 3c) allows for the suppression of the resonances completely, but leads to strong losses, since only part of the power generated by the FFO reaches the mixer (especially at high frequencies). The location of the resistors only at the non-radiating end where the fluxons enter the FFO (Figure 3d) results in good suppression of resonances and does not lead to considerable additional power losses.  
The I-V characteristics of the FFO equipped with absorbing layers placed on the non-radiating end are shown in Figure 4a,b. It can be seen that the dependence became smooth, even in the resonant operating mode. Dependence of the differential resistance R\({}_{\text{d}}\) on voltage V for the curve from Figure 4b is presented in Figure 4c. Comparison of the R\({}_{\text{d}}\)-V dependencies for two different designs Figure 3a,d is presented in Figure 5. It is clearly seen that the R\({}_{\text{d}}\)-V dependence has become smoother without sharp jumps, therefore making it possible to smoothly tune the frequency of the FFO at an arbitrary bias current. This was confirmed by direct measurements of the FFO linewidth (see Section 3.4).  
Figure 4: (a) IVCs of the Nb-AlO\({}_{x}\)-Nb FFO with resistive layers on the non-emitting end of the FFO (Figure 3d) measured at different magnetic fields induced by the current in integrated control line (I\({}_{\text{CL}}\) = 20–40 mA); (b) one of the FFO IVC measured at fixed value of I\({}_{\text{CL}}\) = 27.2 mA; (c) differential resistance calculated from Figure 4b.