1. Field of the Invention
The present invention relates to a bandpass filter with wide bandpass characteristics and steep attenuation characteristics which is used suitably for UWB (Ultra Wide Band) in the wireless communications field, and communications equipment using the bandpass filter. UWB is expected to be utilized as a data transmitting medium for PC peripheral equipment such as external storage devices, printers and scanners, or as a data communications medium for digital TVs, projectors, digital steel cameras, digital video cameras, etc.
2. Description of the Related Art
In recent years, attention is being given to UWB as a means of communications. This UWB is different from wireless local area network (herein after referred to as “W-LAN”) in communication length and data transmission rate.
According to IEEE802.11.b, one of the standards for W-LAN, the communication length is 30-100 m, transmission power is 500 mW and transmission rate is about 11 Mbps. On the other hand, in UWB, while communication length is as short as 10 m for a passband of 3.1-4.9 GHz, the transmission power is as low as 100 mW and the transmission rate is 100 Mbps for a communication length of around 10 m, and 480 Mbps for a communication length of less than 2 m, and therefore, data transmission at higher rate is possible as compared with W-LAN.
As discussed above, one of the features of UWB is achieving a high transmission rate by use of a wide frequency band. Its fractional bandwidth (bandwidth/center frequency) is greater than 40%, or 110% or more depending on the case.
Another feature of UWB is that its average transmission power density is defined to be as low as −41.25 dBm/MHz or less. The value of −41.25 dBm/MHz corresponds to a radiant power generating an electric field strength of 54 dBμV=500 μV/m at a distance of 3m from the wave source.
To take an example of a spectral mask under an outdoor circumstance, with the range of 3.16 GHz to 4.75 GHz being set as the base(0 dB) for bandwidth of wireless equipment, it is defined so that it is less than −20 dB at 3.1 GHz and less than −30 dB at 1.61 GHz. Meanwhile, since it is necessary to prevent interference with W-LAN (IEEE802.11.a/b/g) in substantial conditions of use, there are respective attenuation characteristics required for 2.48 GHz and 5.15 GHz.
As described above, a bandpass filter inserted in transmission and receiving signal flow paths in wireless communications equipment for UWB is required to have a wide frequency band (fractional bandwidth of 40% or more) and low loss and high attenuation.
SAW filters and BAW filters using crystalline quartz or piezoelectric ceramics that have high Q factor as the base material have been conventionally used as bandpass filters with low loss and high attenuation in narrow frequency bands. The fractional bandwidths of these are 3-4% or less at a center frequency of 2 GHz, and passbands thereof are 0.06-0.08 GHz, which are two orders of magnitude narrower than those of UWB. Since bandwidths in these materials are determined depending on the electromechanical coupling coefficients of crystalline quartz and piezoelectric substrates, extending the bandwidths to be used as bandpass filters for wider frequency bands has been difficult when the material is taken into consideration.
For this reason, using a dielectric filter including a plurality of dielectric resonators with high Q combined together has been known as one approach for obtaining a bandpass filter with steep attenuation characteristics in a frequency band of 2-5 GHz. However, in order to produce a dielectric filter so that it has a center frequency of 3.98 GHz, a bandwidth of 1.6 GHz, and attenuation of less than −30 dB at 2.48 GHz and 5.15 GHz where W-LAN operates, the size thereof is bound to be as large as about 10×3×1.5 mm, which is disadvantageous. Dielectric filters thus fail to have a wide passband and small size at the same time.
A primary object of the present invention is to provide a small size bandpass filter with a wide passband in UWB and steep attenuation characteristics in a narrow frequency band, and wireless communications equipment using the same.