RF

Circuits incorporating high-temperature superconducting thin films offer unique properties for a variety of high-frequency device applications, in many areas related to the strongly progressing telecommunications market.

Superconducting RF components offer two fundamental benefits: lower loss and a lower thermal noise environment. These two benefits can be enjoyed by all passive and active RF components typically encountered in a telecommunications system: power dividers, couplers, filters, amplifiers, etc.

In the case of superconducting filters, the benefit of lower loss can also be interpreted as a benefit in improved filter selectivity. Namely, if individual resonators have almost no insertion loss, it means that more of them can be cascaded to create filters with sharper skirts – delivering greater ability to reject unwanted signals. Thus a superconducting filter will always have larger rejection than a conventional filter for a given level of insertion loss.



The benefit of lower insertion loss extends to active devices, such as low-noise amplifiers, as well: the smaller loss experienced by the matching components in a cryocooled environment translates into lower noise figure and higher amplifier gain.

Noise figure is a degradation of the signal-to-noise ratio, caused by active and passive components in an RF chain. It effectively represents the ratio of the output noise, to the noise which would remain if the device itself did not introduce noise.



The benefit of lower noise environment is enjoyed by all components, superconducting or not. All components will exhibit lower noise figures at lower temperatures, due to (i) lowering of the noise floor, and (ii) lowering of any resistive losses. Generally, noise figure improvement for passive and active components scales linearly with temperature: so the noise figure of a passive device at cryogenic temperature is always much smaller than its insertion loss.

For example, the noise figure of a filter with 1 dB insertion loss is only 0.3 dB at 77 K. The noise figure of a low-noise amplifier at 77 K is 0.4 dB – about a third of its noise figure of 1.2 dB at room temperature.