Microwave Journal's sample issue

Design of a Microstrip
open Loop resonator
Bpf with wiDe stopBanD
for wLan

A new microstrip open loop resonator bandpass filter (BPF) for wireless local area networks (WLAN) with a wide stopband response is presented in this article. The BPF consists of a pair of λ/2 open loop resonators and two additional resonators operating at the center frequency of 2.44 GHz. The novel resonator consisting of asymmetrical spurlines and quarter-wavelength open-circuited stubs are used at the input and output (I/O) feed lines to improve the stopband characteristics. The current distribution is demonstrated and analyzed. For the out of band spurious response suppression, its stopband attenuation loss is better than 20 dB from 2.6 to 10 GHz, and its insertion loss is as low as 1.05 dB. Good agreement is achieved between simulated and measured results.

RF and microwave microstrip BPFs with igh selectivity, low insertion loss, sup- pression of spurious sideband and wide
stopband are widely used in recent wireless
communication systems.1 In 2002, the IEEE
extended the 802.11b standard in the 2.4 to
2.4835 GHz frequency range; the fast develop-
ment of WLAN communications has also made
high performance essential for RF BPFs.2
However, many microstrip planar BPFs have
spurious resonant frequencies, which may be
close to the operating frequencies, with severe
influence on the system. 3 Although quarter-
wavelength resonator filters have their first
spurious passband at 3ƒ0, where ƒ0 is the cen-
ter frequency, they require short circuit con-
nections with via holes, which creates a para-
sitic effect difficult to cancel. 4 BPFs using low
temperature co-fired ceramic and stepped
impedance resonators are able to control the
spurious responses, but they can only be imple-
mented in certain filtering configurations with
high insertion loss. 5, 6 The open loop resonator
filter has a pair of attenuation poles at finite
frequencies, making it a viable intermediate
between the Chebyshev and elliptic-function
filters. 7, 8 Defected ground structures (DGS)
have been applied to improve the spurious re-
sponse of the microstrip open loop resonator
BPF; it also results in some problems such as
high insertion loss and electromagnetic com-
patibility interference. 9