Superconductor Filters and LNAs

Superconductivity filters are based on the principle that resistance to current flow
reduces with temperature. Superconductivity filters are made from either thin or thick
film deposition processing that promises very good conductivity when cooled to temperatures
of 90 K or below. Because there is less resistance, the mechanical form factor
of the filter can be made smaller for a given selectivity; though you do then need to add
a cooler to make the device work.
An example from SuperConductor Technology Inc (www.suptech.com) is a hightemperature
superconducting filter and LNA system (dimensions are W: 8.4 inches, L:
23 inches, H: 7 inches) deployed in cellular base stations between the antenna and
receiver. Using only 50 W of power, the Stirling cryogenic cooler maintains the filter
and LNA at a steady 77 K. Together with the sharpest filters available, the product has
a noise figure of < 0.5 dB, as compared to conventional solutions with 2.5 dB.
When used to achieve selectivity, it is claimed that a 16-pole filter can be implemented
for 3G applications giving an additional 2.5 MHz of selectivity (-60 dBc) over
and above a conventional 11-pole filter, with a 1-dB reduction in the noise floor.
These filters are used mainly in receive applications, since high powers cause heating
and hence loss of performance. Although given the low insertion loss, they can
handle several Watts. Even on the receive side, a base station can easily be looking at
-10 dB/0 dB of received signal energy.
Thick film superconductors are better at handling this incident energy and can
deliver a Q of 50,000, giving a close-in, deep rejection figure of better than 100 dB
within 3 MHz. However, they are more expensive than thin film superconductors.
Superconductor filters and superconductor LNAs have not as yet been widely
deployed in either 2G or 3G wireless networks. Partly this is due to network operators’
concerns about maintenance (windshield time), and partly it is because conventional
filter technologies continue to improve.
Figures 13.18 and 13.19 show a conventional ceramic filter and preamplifier module.
It is a good illustration of how intermod performance can be traded against noise performance.
This is a tower-mounted conventional LNA product.
For tower-mounted amplifiers (receive-side amplifiers), network operators generally
prefer to use relatively simple devices. The idea of having to climb a mast to service
a supercooled LNA is not particularly attractive.