A Novel Magnetic Coupling for Miniaturized Bandpass Filters in Embedded Coaxial SIW
Round 1
Reviewer 1 Report
In this paper, a BPF using a shunt connected LC parallel resonant circuit is realized by short-circuited coaxial SIW and capacitive circular plate by using LTCC techniques. Comparison with the prior arts and literatures is concrete and detailed. Compact and spurious-free wide stop-band characteristic is realized.
1. In line 117, there is a description "enables also to shift up the first spurious mode of the SIW cavity, thus widening the stop-band bandwidth." Can you comment on why the first spurious mode can be suppressed by the proposed method?
2. In section 2.3, it is examined changes in Q and k in detail using design parameters height h and width w of the coupling probe. However, in section 2.4, both he and hk are exactly half of the total height hi = 2.24 mm. Can you comment on the relevance of the results of section 2.3 and the actual parameters used in section 2.4?
3. In lines 196-197, there is a description that the coupling between the circular plates is the cause of the decrease in Q. Can you comment on how the coupling between the three coaxial SIW is prevented?
4. In Figure 12, Can you comment on the reason why the difference between the S21 simulation and the measured value in stop-band is as large as 15-20 dB?
Minor comments
1. In line 92, it is written as ”visible in Figure 2-(b)”. However “(a)” and “(b)” is not found in Figure 2.
2. Although the value of MF in Table 2 is "-96.8%", according to the definition of equation (8), it seems a mistake of "+96.8%".
3. In line 106, there is a description of susceptance slope parameter is b = 86.5 mS. So “Coaxial b = 0.085” in Table 2 seems to be a mistake of 0.0865.
4. In line 79 and 88, the "green tape" is probably a trademark, so it seems better to use a general “dielectric”.
Author Response
Review Response File
Article Title: “A Novel Magnetic Coupling for Miniaturized Bandpass Filters in Embedded Coaxial SIW”
The authors would like to thank the reviewers for their valuable comments and suggestions, which have contributed towards the improvement of the quality of the new manuscript version. The edited text and the answers to the reviewer’s raised questions are marked in red in this letter. In the revised manuscript, the corresponding revisions are also marked in red for easy reference.
ASSISTANT EDITOR
Comments to the Authors:
It has been reviewed by experts in the field and we request that you make major revisions before it is processed further. … .Please revise the manuscript according to the reviewers' comments and upload the revised file within 10 days. … .Please provide a cover letter to explain point-by-point the details of the revisions in the manuscript and your responses to the reviewers' comments. Please include in your rebuttal if you found it impossible to address certain comments. The revised version will be inspected by the editors and reviewers.
Reply: Thank you for handling the publication of this manuscript. The paper has been revised and edited based on the comments made by the reviewers. Every numbered comment has been addressed in detail in the resubmitted revised version, and this cover letter explains all changes performed.
REVIEWERS' COMMENTS:
Reviewers' Comments to Author:
Reviewer: 1
In this paper, a BPF using a shunt connected LC parallel resonant circuit is realized by short-circuited coaxial SIW and capacitive circular plate by using LTCC techniques. Comparison with the prior arts and literatures is concrete and detailed. Compact and spurious-free wide stop-band characteristic is realized.
Reply: Thank you for appreciating the values of the submitted manuscript.
1. In line 117, there is a description "enables also to shift up the first spurious mode of the SIW cavity, thus widening the stop-band bandwidth." Can you comment on why the first spurious mode can be suppressed by the proposed method?
Reply: Thank you for the comment. It is important to mention that the first spurious mode of the cavity resonator is not suppressed by using the proposed approach. By using a coaxial SIW topology, it is possible to reduce strongly the resonant frequency of the fundamental mode of the cavity resonator, since a new mode is excited inside the cavity. In addition, the EM field of the first spurious mode is altered due to the inner via holes and capacitive patch that provoke an increase of its resonant frequency, thus widening the stop-band bandwidth of the device.
Specifically, in the proposed coaxial SIW cavity resonator, the fundamental mode is the TEM mode, while the first spurious mode is the TE101 mode. In the CSIW resonator presented in Section 2.2, their resonant frequencies are:
- Resonant frequency of the TEM mode is 1.55 GHz;
- Resonant frequency of the TE101 mode is 11.6 GHz.
Figure 1 shows the TE101 mode EM field inside such cavity resonator (i.e. top: EM field vectors; bottom: EM field magnitude).
Figure 1 EM field of the first spurious mode (TE101) in the coaxial SIW resonator.
In this context, if we consider a standard SIW cavity resonator having the same cavity size, the TE101 fundamental mode resonates at 8.79 GHz, while the first spurious mode, which are the degenerate mode TE102/TE201, are resonating at 13.94 GHz.
Figure 2 shows the comparison of the S21-S11 responses between the proposed CSIW resonator (dotted lines) and a standard TE101-based SIW resonator (solid lines).
Figure 2 Simulated S21-S11 responses of a (dotted lines) coaxial SIW resonator compared to the responses of (solid lines) a standard SIW resonators having the same cavity sizes.
This figure has been included in the manuscript as Figure 3-(b) with the following caption:
‘’Figure 3. Simulated wideband response of (a) the proposed CSIW resonator having b=105.5mS. (b) Comparison between the frequency response of the proposed CSIW resonator (solid lines) and a standard TE101-based SIW resonator (dashed lines) having the same cavity size.’’
In addition, the sentence on line 117 is modified as follows:
‘’ As shown, the use of an embedded CSIW topology enables also to shift down the fundamental mode of the SIW cavity resonator (i.e. the TEM mode in a CSIW resonator) with respect to the first spurious mode, which is the TE101 mode, thus widening the stop-band bandwidth. Indeed, the EM field of the first spurious mode is altered due to the inner via holes and capacitive patch that provoke an increase of its resonant frequency, from 8.79 GHz as fundamental mode of a TE101-based resonator to 11.6 GHz as first spurious mode of the CSIW resonator. Thus, in the proposed simulated CSIW structure, the stop-band bandwidth is higher than 7× f0, as it can be seen in Figure 3-(b).’’
2. In section 2.3, it is examined changes in Q and k in detail using design parameters height h and width w of the coupling probe. However, in section 2.4, both he and hk are exactly half of the total height hi = 2.24 mm. Can you comment on the relevance of the results of section 2.3 and the actual parameters used in section 2.4?
Reply: Thank you for the comment. In Section 2.3, we have proved by means of 3D EM simulations that the proposed magnetic coupling mechanisms enable for a flexible design of CSIW passband filters. In fact, both narrow- and wide-band filtering responses can be easily implemented by exploiting the characteristic of the multi-layer LTCC manufacturing, thus printing conductors that compose the magnetic coupling mechanisms at any available layer of a LTCC stack-up.
Concerning our fabricated devices in LTCC technology, unfortunately, we were not able to choose freely the structure of the LTCC stack-up. Indeed, our CSIW resonator and filter prototypes were included in a multi-project fabrication run that had an already predefined LTCC stack-up. As already mentioned, the latter consisted on 11 green tape layers (10 layers having un-fired thickness of 0.254 mm, and an additional one having un-fired thickness of 0.127 mm that has been added on the top of the tenth layer). The printing of the conductors was allowed only on 5 different layers:
- 2 external layers
o External top layer - on the top of eleventh layer - that has been used for input-output feeding lines and ground-signal-ground (GSG) probe pads to allow for measurements with probe station.
o External bottom layer – on the bottom of the first layer - used for the bottom SIW ground plane.
- 3 inner layers
o At h=1.12 mm - on the top of the fifth layer – used for the coupling mechanisms (i.e. external and inter-resonator), and cover pads for electrical connection among via holes forming the SIW cavity walls.
o At h=1.792 mm – on the top of the eighth layer – used for the inner loading capacitance plate.
o At h=2.24 mm – on the top of the tenth layer – used for the top SIW ground plane.
Since this information is too detailed and not so relevant for the reader, it was not included in the manuscript. However, it is worth mentioning that the proposed 3-pole ECSIW filter, which was designed by using such tight design restrictions, shows remarkable performance in terms of compactness, passband bandwidth and spurious-free band when compared to recent related works, as shown in Table 4, proving the validity of the presented coupling mechanisms.
Finally, the sentence on line 172 is modified as follows:
‘’ It should be noted that external and inter-resonator coupling have been accommodated on the very same layer (i.e. he = hk = 1.12 mm) thus allowing for a stack-up simplification and manufacturing cost reduction. Indeed, the designed CSIW components have been included in a multi-project LTCC fabrication run that had an already predefined LTCC stack-up. In this context, the inner conductor printing was only allowed at h = (1.12, 1.792, 2.24) mm, which limits the filter design flexibility. However, optimized values of the coupling coefficients have been easily obtained by adjusting the probe widths we and wk.’’
3. In lines 196-197, there is a description that the coupling between the circular plates is the cause of the decrease in Q. Can you comment on how the coupling between the three coaxial SIW is prevented?
Reply: Thank you for the comment. Concerning lines 196-197, as it is well known, to precisely estimate the unloaded quality factor (Qu) of a resonator by using the S-parameter transmission response, the resonator should be under-coupled with a S21 response below -20 dB. As presented in Section 2.3, to achieve such coupling condition, the height he of the external coupling mechanism should be lower than 0.5 mm, which means that the coupling mechanism should be printed on a lower green tape layer. As previously mentioned, we were not able to achieve such condition since the LTCC stack-up was already predefined. Therefore, the resonator shows a more coupled configuration that makes the Qu estimation less precise.
On the other hand, the coupling between adjacent CSIW resonators is basically a mixed coupling mechanism with both electric (due to the proximity of the loading capacitive patches) and magnetic (due to the proposed embedded stripline probes) components. Thus, the total coupling corresponds to the difference in magnitude between the two components, where the magnetic component is dominant in our structure. Such coupling condition is extremely similar to the one of our air-gap-based coaxial SIW topology [6,9,14], whose mixed coupling mechanism was deeply studied in [14] in order to propose a new type of electric coupling mechanism between CSIW resonators.
Note that the study presented in Section 2.3 is based on 3D EM simulations of adjacent coupled ECSIW resonators, thus both coupling components are taken into account when extracting the inter-resonator coupling coefficient. Since the aim of this work is to provide a new coupling mechanism that enables for the design of wide-band BPFs, such aspect has not been studied and presented in this manuscript.
4. In Figure 12, Can you comment on the reason why the difference between the S21 simulation and the measured value in stop-band is as large as 15-20 dB?
Reply: As we briefly mentioned in the manuscript, the probe station setup was responsible for the important difference between simulations and measurements in the filter stop-band above 4 GHz, mainly when the rejection is higher than 40 dB. Specifically, we have detected a damage of one GSG probe that we consider to be responsible for such discrepancy. We wish to repeat the measures as soon as possible with a new set of probes. Unfortunately, nowadays, we are not able to perform such new measurements.
Minor comments
1. In line 92, it is written as ”visible in Figure 2-(b)”. However “(a)” and “(b)” is not found in Figure 2.
Reply: Thank you for the suggestion. The reviewer is right. We have modified the sentence on line 92 as follows:
‘’ … , and Cff is the capacitance due to the fringing effect of the field between the edges of the plates, which are visible in right part of Figure 2’’.
2. Although the value of MF in Table 2 is "-96.8%", according to the definition of equation (8), it seems a mistake of "+96.8%".
Reply: Thank you for the suggestion. The reviewer is right. We have corrected the table and introduced the value +96.8% in the proper column.
3. In line 106, there is a description of susceptance slope parameter is b = 86.5 mS. So “Coaxial b = 0.085” in Table 2 seems to be a mistake of 0.0865.
Reply: Thank you for the suggestion. The reviewer is right since there was a mistake. However, as explained in the following, the parameters of the proposed CSIW resonator configuration have been recalculated and the correct values have been introduced in the manuscript.
4. In line 79 and 88, the "green tape" is probably a trademark, so it seems better to use a general “dielectric”.
Reply: Thank you for the suggestion. We have replaced ‘’ green tape’’ with ‘’dielectric tape’’ in the manuscript.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript describes an implementation of filters that uses combline resonators placed inside below cut-off SIW cavities. The authors propose an LTCC filter with a novel coupling (at least in this field) that allows a quite high coupling level between cavities. This coupling is similar to tap used in standard combline filters. The paper is clear and well written.
In the following there are some minor suggestions/corrections.
-) In equation (1) the length of the coaxial post is equal to ‘h’, which has been defined as the height of the stack-up (Figure 1). However, this length should be shorter (i.e. equal to ‘hp’ defined at line 151). Please correct the equation (1) and Table I accordingly.
-) In the coaxial post the vias are not staggered. Is it a possible problem for the deformation of the filter or mechanical tolerances?
-) Line 167: are you sure that ‘less economical’ is correct?
Author Response
Review Response File
Article Title: “A Novel Magnetic Coupling for Miniaturized Bandpass Filters in Embedded Coaxial SIW”
The authors would like to thank the reviewers for their valuable comments and suggestions, which have contributed towards the improvement of the quality of the new manuscript version. The edited text and the answers to the reviewer’s raised questions are marked in red in this letter. In the revised manuscript, the corresponding revisions are also marked in red for easy reference.
ASSISTANT EDITOR
Comments to the Authors:
It has been reviewed by experts in the field and we request that you make major revisions before it is processed further. … .Please revise the manuscript according to the reviewers' comments and upload the revised file within 10 days. … .Please provide a cover letter to explain point-by-point the details of the revisions in the manuscript and your responses to the reviewers' comments. Please include in your rebuttal if you found it impossible to address certain comments. The revised version will be inspected by the editors and reviewers.
Reply: Thank you for handling the publication of this manuscript. The paper has been revised and edited based on the comments made by the reviewers. Every numbered comment has been addressed in detail in the resubmitted revised version, and this cover letter explains all changes performed.
REVIEWERS' COMMENTS:
Reviewers' Comments to Author:
Reviewer: 2
The manuscript describes an implementation of filters that uses combline resonators placed inside below cut-off SIW cavities. The authors propose an LTCC filter with a novel coupling (at least in this field) that allows a quite high coupling level between cavities. This coupling is similar to tap used in standard combline filters. The paper is clear and well written.
Reply: Thank you for appreciating the contents of the submitted manuscript.
In the following there are some minor suggestions/corrections.
-) In equation (1) the length of the coaxial post is equal to ‘h’, which has been defined as the height of the stack-up (Figure 1). However, this length should be shorter (i.e. equal to ‘hp’ defined at line 151). Please correct the equation (1) and Table I accordingly.
Reply: Thank you for pointing out the error. In fact, the length of the coaxial line embedded in the substrate corresponds to hp=1.792 mm. Thus, taking into account this value, we have corrected the error in equation (1), Table I and we have also recalculated the CSIW resonator properties and updated those values in the manuscript. The new values are:
hp=1.792 mm
b = 0.1055 S
Cl = 10.75 pF
Θ0 = 8.89º
Z0 = 61.1 Ω
-) In the coaxial post the vias are not staggered. Is it a possible problem for the deformation of the filter or mechanical tolerances?
Reply: Thank you for the comment. As the reviewer correctly comments, the inner post via holes have not been staggered. Indeed, we wanted to keep the coaxial inner conductor section as much homogeneous as possible so that it could be modelled as a transmission line without discontinuities. Such discontinuities could introduce alteration of the EM field of the fundamental TEM mode, which could be prejudicial to the correct working of the coaxial SIW topology. On the other hand, the via staggering technique has been used for the via fence (i.e. via holes that form the SIW cavity walls) in order to avoid mechanical deformations of the device due to the accumulation of the via hole filling paste (i.e. via posting).
Since the inner coaxial do not extend along the entire stack-up (but only from the first to the eighth layer) and those via holes are short-circuited with the big inner conductive disk (i.e. inner loading capacitance plate), there is a very limited risk to introduce deformations on that internal conductive disk. Moreover, to minimize as far as possible via posting problems, special care has been taken in the via filling process by performing several tests, which also include optical measurements on the filled dielectric tapes.
As the results of the ECSIW resonator show, the agreement between measurements and simulations is excellent because the capacitive component of the resonator is dominant, and the impact of any elongation of the inner conductor post due to the via posting is minimized.
-) Line 167: are you sure that ‘less economical’ is correct?
Reply: Thank you for the suggestion. Indeed, the correct sentence, which we have corrected in the manuscript, is ‘’manufacturing cost reduction’’. Thus, the sentence is now:
‘’… thus allowing for a stack-up simplification and manufacturing cost reduction.’’
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The author responses and paper update satisfy my concerns.
