Toshiba has developed a high-temperature superconductor (HTS) tunable bandpass filter for wireless basestations that will eliminate interference between wireless systems

The future is broadband. That's the promise behind a new generation of content-rich services over wireless broadcasts and telecommunications. But it could also be a future of frustration, interference and lost signals, as a growing number of different systems and services crowd finite radio frequencies.

This article was originally published on Electronicstalk on 23 February 2001 at 8.00am (UK)
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Optimised use of the radio spectrum is a social and economic must, and Toshiba is poised to help realise it, and has developed a high-temperature superconductor (HTS) tunable bandpass filter for wireless basestations that will eliminate interference between wireless telecommunications systems.

The new filter appears at an opportune time, as more and more countries make the shift to digital communications - and the challenges that come with it.

Take Japan, where a new broadband service, IMT-2000, will be launched in May this year.

Although carriers have been allocated 20MHz of bandwidth, they will only be allowed to use 15MHz of it when the service starts.

The remaining 5MHz will be used to provide a buffer against cross-interference with the Personal Handyphone System (PHS), which uses a very close frequency.
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Giving up a quarter of the bandwidth in this way is a less than perfect solution, particularly when measured against projections of vast increases in traffic and the need to maximise speed and signal quality.

Clearly full use of the frequency band is a must - and that is where Toshiba's HTS can help.

The company's new HTS tunable bandpass filter operates at around 2GHz.

It is a microstrip-line type, with a YBCO superconductor thin film grown on a dielectric substrate.

Until now, filter design methodology has simply supported an increase in the number of resonators, a key component of the filter, as the means to achieve steeper signal attenuation.

However, this approach also increases the number of adjustment points and makes bandwidth tuning very difficult.

The Toshiba solution gets round this tradeoff by adopting two bandpass filters, one with steep attenuation at a lower band-edge, the second with steep attenuation at a higher band-edge.

These are fitted monolithically on a substrate and each is covered with an additional dielectric plate.

The frequency characteristic of the whole bandpass filter can be obtained by superposing that of each filter by mechanical control of the dielectric plates, with only two adjustment parameters.

The new filter achieves steep attenuation, 30dB/1MHz at both higher and lower frequency band edges.

It also has a 5MHz bandwidth variation and 10MHz centre frequency shift, both of which are tuned independently.

The tunability of the new bandpass filter means that, once installed in a system, it would not have to be replaced in response to any future specification changes in bandwidth or centre frequency.

It would also end the need for trimming, the complex fine tuning usually needed to adjust the frequency of the filter.

Toshiba has successfully verified the operation of the new filter in the lab.

The next stage is development of a compact prototype for installation in wireless basestations.

Once that is done, cellular subscribers everywhere can look forward to faster, better services.