Jiaxing Song1,
Chuicai Rong1*, Ruilong Zhu2, Wen
Zhang2, Yiping Zhang2,Yubo
Liao1
1School of Physics and Electronic information,
Gannan Normal University. Ganzhou, 341000 China
2Xi ’an
Matrix Wireless Technology Co., Ltd. Xi’an, 710100 China
Email: rongchuicai519@gnnu.edu.cn
Abstract:A novel stripline diplexer design using frequency dependent
couplings to achieve multiple transmission zeros is developed in this
paper. The transmission zeros generated by the frequency dependent
couplings are flexible and controllable, on the basis of the existing
cross-coupled, more transmission zeros are introduced to improve the
frequency selection characteristics. Based on this characteristic, we
designed a 2.6G Hz diplexer, its transmitting channel filter is 5 order
with 4 transmission zeros, and the receiving channel filter is 4 order
with 5 transmission zeros. We fabricated and measured it, the synthesis
results, simulation results, and the tested results are well matched
with each other, which will provide more flexibility in the design of
diplexers for wireless communication system.
Introduction: With the rapid development and wide application of
modern communication technology, the limited spectrum resources are
becoming more precious, in order to make full use of the electromagnetic
spectrum resources, the operating frequency interval between various
communication systems is getting smaller and smaller, and the
anti-interference requirements between adjacent operating bands are also
put forward higher requirements, the demand for highly selective RF
front-end diplexers is also growing[1-2].
The finite transmission zeros are critical for diplexer design to
enhance the rejection ratios, one of the prevalent methods for creating
finite transmission zeros is the basic cross- coupling mechanism, and
another popular mechanism to create finite transmission zeros is based
on the frequency dependent coupling [3-5].
Metal stripline resonators have higher Q values, better harmonic
suppression characteristics and higher reliability than dielectric
waveguide resonators. Here we introduce a inverted E-type metal
resonator, which can be regarded as an evolution of a 1/4 wavelength
step impedance resonator, and they have similar field distribution and
resonant characteristics. The upper space of the stripline resonator is
mainly electric field distribution, and the lower space is mainly
magnetic field distribution. When two stripline resonators are placed
side by side, the electrical coupling \(E_{c}\) mainly exists through
the upper half space coupling window, while the magnetic coupling\(M_{c}\) mainly exists through the lower half space coupling window.
When there are coupling windows in the upper and lower spaces of both
stripline resonators, that is, there are both electrical and magnetic
coupling between the two resonators, the total coupling coefficientk is frequency dependent, and the transmission zero\(f_{\text{tz}}\) produced by the frequency dependent coupling as
follow[6-12]:
\(k=\frac{(M_{c}-E_{c})}{(1-\text{M\ }_{c}E_{c})}\) (1)
\(f_{\text{tz}}=f_{0}\sqrt{\frac{M_{c}}{E_{c}}}\)(2)
As can be seen from (1) and (2), when
the electric coupling in \(E_{c}\)is greater than magnetic coupling\(M_{c}\), the transmission zero \(f_{\text{tz}}\) generated by the
frequency dependent coupling will be at the lower end of the passband.
While the magnetic coupling \(M_{c}\)is greater than the electric
coupling \(E_{c}\), the transmission zero \(f_{\text{tz}}\) will be at
the higher end of the passband.
Diplexer design and measurement: Based on the frequency dependent
coupling of the two stripline resonators described above, the frequency
dependent coupling can generate transmission zero at the high frequency
stopband or the low frequency stopband of filter, respectively, which
can effectively improve the selection characteristics of the filter,
while the asymmetric frequency response can greatly improve the
isolation between channels in the diplexer applications. So here, we
designed and fabricated a 2.6G cross-coupled diplexer with frequency
dependent couplings, to verify the feasibility. The following specs have
been assumed (return loss is 22dB for both channels).
Junction: Resonant node.
RX Filter: Band= [2495, 2575] MHz and \(f_{\text{tz}}\)= [2484,
2605, 2679] MHz, 4th order.
TX Filter: Band= [2615, 2695] MHz and \(f_{\text{tz}}\)= [2561,
2597, 2717] MHz, 5th order.
According to the above technical specifications, we carried out
synthesis, modeling and simulation, the topology with synthesis results
are shown in Fig. 1.