Conventional bandstop (notch) filters using distributed elements, such as 90-degrees long stubs or coupled-lines, have a reentrant response at approximately three times the first notch resonance. The ratio of the center frequencies of the 2nd harmonic (spurious) to the 1st fundamental center frequency is limited to 3:1, for conventional designs. This is due to the spectral properties of the tangent function in the Richards transformation of the distributed elements. As a result, occurrence of this reentrant stopband limits the achievable passband widths. This limit is significant above the stopband region, when it is desired to simply reject a large-signal interfering with small, wideband signals, when the large signal is also limited to a certain discrete band of frequencies.
A typical occurrence of this scenario is when the military spread spectrum signals generated by MIDS or JTIDS are located in physical proximity to a sensitive receiver covering a bandwidth much wider than that of the interfering large signal. This desired wide passband contrasts to the other, more common, application for notch filters, in which it is desired to “roof” the bandstop characteristic with a bandpass or lowpass filter, to reduce generated harmonics. The wide passband scenario made possible by this new design is most useful for receiver protection, while the alternative is most often used in systems generating high power, i.e. the transmitter.
Using a new design approach, the upper passband width can be extended significantly. The design simply employs loading capacitors at the end of the open ended parallel-coupled lines, thus reducing the physical length (and it is the physical length that encounters the periodicity of the tangent function in the network Richards transformation). This has the result of pushing the 2nd stopband much higher in frequency. Because the filter now includes both distributed lines and lumped elements, the initial synthesis must be “tuned” on the computer, using a special optimization technique, called co-simulation.
At RS Microwave, this is accomplished using electromagnetic parameter extraction within Ansoft HFSS, followed by circuit level optimization within Ansoft Designer. The use of lumped and distributed elements within bandstop filters should make possible application of this technique down to perhaps a few hundred megahertz, thus facilitating the design of bandstop filters providing the same responses as are achievable with “pure” lumped element designs, but with much lower losses.
Fig.1 shows a quasi-elliptic bandstop filter using parallel-coupled lines, loaded with parallel-plate lumped capacitors. The 1st notch is centered at 1.08 GHz with 22% bandwidth, at the 50 dB rejection level. The 2nd harmonic passband occurs at 6 GHz. The ratio of the 2nd to 1st harmonic bandstop resonance is extended from 3:1 to 5.56:1 for 22 % rejection bandwidth. This quasi-elliptic rejection (achieved by asymmetric placement of stopband transmission zeros) shape factor has a steeper stopband skirt and quite wide rejection bandwidth than Chebychev case.
The Specifications for this filter are as follows:
P/N 50151B-2Fig.2 displays the outline drawing and photo of this filter. The design was implemented as a machined, air-slab structure, and is intended for rejection of the military MIDS/JTIDS passband, at high power levels (80 W peak, 20 W average), to alleviate certain communication cosite interference issues for the receiver.
JTIDS/MIDS rejection filter
Minimum passband: DC-900, 1286-5000 MHz with 1 dB maximum loss
Minimum passband: 900-921, 1266-1286 MHz with 2 dB maximum loss
Minimum –50 dB rejection: 969-1206 MHz
Minimum power handling: 80 W peak, 20 W average
Fig.1 Measured performance of wide passband JTIDS/MIDS
band rejection filter
Fig.2 Outline of wide passband JTIDS/MIDS band rejection filter