диафрагмированные волноводные фильтры / 7ac4e4af-063a-4ed2-b8d5-43cf78ede2ce

Frequency (MHz)

Frequency (MHz)

(b)

Figure 18. Four-channel manifold multiplexer after first optimization of filter parameters:

(a) superimposed channel transfer characteristics and (b) CPRL.

Frequency (MHz)

Figure 19. Four-channel manifold multiplexer: final performance of CPRL.

Demonstration

of the Piecewise Optimization Process

The piecewise optimization process will be demonstrated through the optimization of a four-contigous-channel Ku-band waveguide manifold multiplexer. The channel electrical specifications for this particular case were satisfied with 5-2 quasielliptic filters with 30-dB rejection lobe levels and DBWs of 38-MHz and 40MHz center frequency spacings, which falls within the definition of contiguity.

Designing the filters as singly terminated prototypes and attaching them to the manifold with the initial manifold spacings and stub lengths gives a poor performance, as shown in Figure 16. The CPRL is poor and one of the channels is unrecognizable.

Figure 17 shows the dramatic improvement that results from the optimization of the along-manifold lengths. Now the channel rejection characteristics are close to design, and the average CPRL is on the order of 10 dB.

Further improvement in CPRL is obtained after first optimization of the stub lengths and the first four parameters (first coupling iris MS1 , first cavity tuning state M11 , second coupling M12 , second cavity tuning state M22) of each filter. The computed response following this procedure is described in Figure 18. The inner rejection lobe levels are close to the design of 30 dB, but the outer lobe levels—because there are no neighbors on the outer side of the group—have dropped to about 24 dB. The effect of not having a

Further Optimization

Using EM Techniques

Optimization techniques for multiplexer design have been developed to a high degree of sophistication now, and worthy of note here is the space mapping technique [23], which involves optimizing a coarse model [circuit or hybrid circuit model with embedded electromagnetic (EM)-modeled components] and a fine model (e.g., full-wave electromagnetic modeling). Because of the large number of optimization variables, the fine-model EM simulator takes a formidable amount of computer CPU time and has to be used sparingly during the optimization process. On the other hand, the coarse simulator can analyze the circuit very rapidly, especially if the S-parameters of fixed

A direction filter is a four-port device in which one port is terminated in

a load.

and noninteracting elements (e.g., the manifold waveguide junctions) are modeled in advance by EM techniques and are available to the main program either as look-up tables or with the call of a rapid specialist routine. It, therefore, provides a means to simulate the manifold multiplexer with a good degree of accuracy and efficiency.

To illustrate the complete multiplexer design procedure including EM optimization, we consider a 10channel manifold-coupled output multiplexer in the frequency band of 3.5-4.25 GHz. Eight channels have a bandwidth of 1.5%, and the remaining two have a bandwidth of 0.8%. Every channel is a fifth-degree dielectric resonator (DR) filter as shown in Figure 21.

Ansoft HFSS is used as a fine model of every channel and the network model is used as a coarse model.

Ideal channel coupling values are obtained in the first step of the design procedure in the preceding sections. Space-mapping optimization [23] is then applied to each channel to get the optimal channel dimensions. For example, the results of applying space-mapping optimization to the first channel are shown in Figure 22 (seven iterations are required). An accurate multiplexer model is obtained by replacing each channel with the corresponding S-parameter sweep obtained by HFSS at the optimal channel dimensions. As a result, the new multiplexer model includes channel dispersion and spurious modes. Finally, the manifold parameters are reoptimized to meet the required specifications. Figure 23 compares the multiplexer ideal response and the EM response, where every channel is replaced by its simulated S-parameters (by Ansoft HFSS). The measured response of the multiplexer is shown in Figure 24. The spurious modes predicted by

(a)

Frequency (GHz)

(b)

Figure 22. Responses of the first channel of the 10-channel DR output multiplexer (solid line is the ideal response and dotted line is the EM response at the optimal design parameters); |S21| and |S11| in dB.

Frequency (GHz)

Figure 23. The ideal response of the 10-channel DR multiplexer prototype (solid line) versus the EM response (dotted line).

Frequency (GHz)

Figure 24. Measured response of the 10-channel DR multiplexer.

EM analysis in Figure 23 correlate very well with the measurements in Figure 24. Finally, a fully integrated multiplexer assembly is shown in Figure 25 together with input and output circuitry.

Conclusions

Following a brief review of different types of multiplexer configurations, a systematic design approach has been outlined for the design of manifold-coupled multiplexers. The piecewise approach, optimizing parts of the multiplexer separately in repeated cycles while converging upon an optimal solution, has proved to be very effective for most practical applications. The technique is readily applicable to manifold multiplexers incorporating an arbitrary number of channels, regardless of their bandwidths and channel separations. There are no restrictions on the design and implementation of channel filters onto the manifold; they may be asymmetric, and may incorporate transmission zeros, group delay equalization zeros, or both. The manifold itself is a transmission line, be it a coaxial line or a rectangular waveguide or some other low-loss structure. The costly EM simulation is used economically on manifold junctions and channel filters through the use of space-mapping optimization techniques, where EMbased simulators are used to fine-model each multiplexer channel and coupling matrix representation is used to coarse-model the performance. Fine details such as tuning screws may be included in the design process. This design procedure takes into account the effects of dispersion and spurious modes and, as a result, the overall design and final tuning time can be significantly reduced.

Acknowledgement

The authors would like to thank Prof. Raafat Mansour for providing part of the material in the “Multiplexer Configurations” section.

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