Computing and Enhancing Power Handling in Bandstop Filters rmingyu/MTTs2007/6.pdf · 2007. 3....

2/28/2007R. V. Snyder, RS Microwave

Computing and Enhancing Power Handling in Bandstop Filters

ByR. V. SNYDER

RS MICROWAVEBUTLER, NJ

IMS2007 WorkshopJune 4, 2007


CONTENTS OF THIS TALK

-Power handling in bandpass vs. bandstop…similarities and differences-Passband vs. stopband-Peak power and average power-Some examples (quasi and full-elliptic bandstop with wide pass and stopbands)…expensive failures noted, analyzed and cured-Distributed vs lumped -Mixed circuits (lumped and distributed)-Limitations of each-How to improve power handling-Performance (and other) costs associated with improvement-Generally warranted conclusions


Bandpass and Bandstop

-Similar problems:

For voltage breakdown (peak power)... air is the enemy…avoid gaps

For thermal breakdown (average power)…dissipation is the enemy


Bandpass (peak power issues)

-Peak power problems for ladder-type bandpassare near network center/high-power input, if power is applied in the passband/stopband region

-Peak power problems for bandpass with finite frequency TZ’s are near the resonators providing the TZ’s.

-“Center” and “input” refer to physical location in network


Bandpass (average power issues)

-Passband area for bandpass is troublesome due to resonator impedance (reflections) and the effects of bandwidth (lower percentage BW means lower average power handling capability due to dissipation)

-Stopband area for bandpass is usually sensitive to peak power (reflection issues) but not average power, because dissipation is low in the stopbandarea


Bandstop (peak power issues, traditional thinking)[1]

Peak power problems for bandstop (Chebychev, narrow stopband) are near network high-power input/center, if power is applied in the stopband/passband region -[1]: Torgow and Collins, Bandstop Filters for High-Power Applications, Trans MTT, Sept, 1965..equations 12, 16, 17, 19 and 20-Peak power problems for bandstop (quasi or full-elliptic, wide stopband) are distributed throughout the filter, with the worst problems in resonators effectively providing the frequency of a particular transmission zero (TZ) - This wasn’t known when [1] was written because bandstop filters with wide stopbands were not possible

-“Center” and “input” refer to physical location in network


Bandstop (average power issues)

Passband area for the bandstop is usually the not a significant area of difficulty because dissipation and VSWR are minimized here-Stopband area for bandstop is difficult because of resonator impedance (reflections) and couplings to main through path


Bandpass and Bandstop (continued)

-Stopband reflections can cause problems for both bandpass and bandstop

-Passband edges for bandpass correspond to notch edges for bandstop

These are trouble regions because dissipation losses are significant in these areas and thus average power capacity is most affected here

Sounds the same, but “passband” for the bandstop is analogous to “stopband” for the bandpass


Bandstop example of interestWide stopband, wide passband, quasi-elliptic

Capacitively shortened coupled resonators [2]Short TL sections between resonators contribute to rejection and make response quasi-elliptic

[2]: R. Levy, R. Snyder and S. Shin, ‘Bandstop Filters with Extended Upper Passbands”, T-MTT-S, June 2006


Bandstop example of interestWide stopband, wide passband, quasi elliptic


Bandstop example of interestWide stopband, wide passband, quasi-elliptic

This filter safely handles 1000 W in passband regionBUT

It was observed that power levels of 100 W in stopbandarea selectively destroyed loading capacitors. Depending on stopband frequency, different capacitors were damaged. This was an expensive failure (a failure basically means throwing away a very costly filter) and it was decided to investigate via simulation.


Electric Field Plots and Power capacity

• E-field plots are compared with 50 watts average power applied ateach rejection pole frequency.

• Hot spots can be located from the E-field plots, showing high peak power at resonant frequency.

• Loading capacitors at high peak power area must be designed withdimensions below breakdown voltage limits. (Peak E-field intensity should be below the breakdown voltage of the loading capacitors)

• At passband frequencies nearest the rejection band, loading capacitors face higher peak power.– Highest peak power occurs at passband edge transition frequency.

• Not at 1st resonator…must be analyzed using field solver.– The resonator with the greatest sensitivity to the applied peak power

determines the power capacity of the filter.


Bandstop example of interestWide stopband, wide passband, quasi-elliptic (Movie clip follows)


JTIDS/MIDS Elliptic Band Rejection Filter


JTIDS/MIDS Elliptic Band Rejection Filter (movie clip follows)


JTIDS/MIDS Elliptic Band Rejection Filter (some slides from the movie clip now follow for purposes of visualization in the handout)


E-field at 1st Rejection Pole

• 50 W average power applied at 964 MHz, 1st

rejection pole

• Hot area:Resonators close to applied frequency and close to power imposed port, face high peak power.

Port 150 W

Port 2

Port 1 Port 250 W


• 50 W average power applied at 973 MHz, 2nd

rejection pole


Port 150 W

Port 2

Port 1Port 250 W

E-field at 2nd Rejection Pole


• 50 W average power applied at 999 MHz, 3rd

rejection pole

•Hot area:Resonators close to applied frequency and close to power imposed port, face high peak power.

Port 150 W

Port 2

Port 1 Port 250 W

E-field at 3rd Rejection Pole


•50 W average power applied at 1081 MHz, 4th rejection pole


Port 150 W

Port 2

Port 1 Port 250 W

E-field at 4th Rejection Pole


• 50 W average power applied at 1182 MHz, 5th rejection pole


Port 150 W

Port 2

Port 1 Port 250 W





Port 150 W

Port 2

Port 1 Port 250 W



• 50 W average power applied at 959 MHz, lower passband edge

• Hot area: Resonators (6th) close to applied frequency face the highest peak power

Port 150 W

Port 2

Port 1 Port 250 W

E-field at lower Passband edge


• 50 W average power applied at 1226 MHz, upper passband edge

• Hot area: Resonators close to applied frequency face high peak power

Port 150 W

Port 2

Port 1 Port 250 W

E-field at upper Passband edge


• 1000w average power applied at 750 MHz, passbandarea.

• Hot area:

At 750 MHz, 1 Kw, tight coupled line spacing area faces high peak power

Port 1500 W

Port 2

Port 1

1000 W

Port 2

E-field at Passband (1-1)


• 1000w average power applied at 1400 MHz, passband area.

• Hot area:

At 1400 MHz, 1 Kw, tight coupled line spacing area faces high peak power

Port 1500 W

Port 2

Port 11000 W

Port 2



• 1000 W average power applied at 1700 MHz, passband area.

• Hot area: At 1700 MHz, 1 Kw, tight coupled line spacing area faces high peak power

Port 150 W

Port 2

Port 11000 W

Port 2



Distributed vs. Lumped (and mixed together)-Similar problems: For voltage breakdown (peak power)... air is the enemy…avoid gaps

For thermal breakdown (average power)…dissipation is the enemy(The problems listed are common to all filter types)

It is easier to avoid air gaps with most distributed circuits than with lumped-Coiled inductors are difficult to fully insulate-Component connections are weak points

-Distributed element Q tends to be greater than lumped element Q-Bandwidths can be more narrow and thus breakdown is a bigger problem-Because bandwidths are typically more narrow, dissipation remains as an

issue even with higher Q -Very difficult to model in E-M domain; experimental results match expectation from [1], because rejection bands are typically narrow-Easy to model in circuit domain but breakdown doesn’t show up without making assumptions as per [1]


Distributed vs. Lumped (and mixed together)

Lumped-Distributed mixture…lumped series lowpass mainline capacitively-coupled to short circuited coax resonators-updated version of [3][3] R. V. Snyder, “Quasi-elliptic Compact High-Power Notch Filters Using a Mixed Lumped and Distributed Circuit”, TMTT-S, April 1999


Distributed-Lumped-1030/1090 MHz dual notch filter

Dual 1030/1090 MHz notch filter; 1000 W peak, 5% duty in stopband, 300 W peak, 40% duty in passband. We have not had a power failure in over 1000 units tested, and so no damage photos are available. These are simply too expensive to force into failure!


Dual notch filter example.1000 W peak, 5% duty (stopband), 300 W peak, 40% duty (passband)


1030 MHz notch example (accidental failure)

Similar single notch, F0=1030 MHz, 800 W peak (25% duty)


1030 MHz notch example (before failure)

Similar single notch, F0=1030 MHz, 800 W peak (25% duty)Simulated and measured data, before applying too much power in reject band


1030 MHz notch example, high power in reject band

1200 W applied, 1030 MHz, at 80,000 feet….first two resonators burned…air gap between resonator end and insulator caused arc, which then propagates


1030 MHz notch example, high power in reject band

1200 W applied, 1030 MHz at 80,000 feet….first two resonators burned, air gaps visible


1030 MHz notch example, high power in passbandIntentionally forced to failure

1030 MHz single notch filter, 1 Kw peak (5% duty) in passband, 300 W (40% duty)


1030 MHz notch example, high power in passband

Basic structure, shown prior to applying excessive high power…note it is another mixed lumped-distributed network



1030 MHz single notch filter, 1 Kw peak (5% duty) in passband, 300 W (40% duty)……measured response prior to high power



1250 W peak, 100 W average applied at 1000 MHz, 80,000 feet (specification is 1000 W peak, 50 W average)



1250 W applied, 1000 MHz at 80,000 feet….center resonators burned, air gaps visible


How to improve power handling for bandstop (or any other) filters

1. Analyze problem using field solver to identify hot spots in structure. These will depend on the frequency of the high power energy.

2. Breakdown voltage for elements in hot spots must be greater thanapplied RF voltage found in step (1).

3. Eliminate air gaps…much easier said than done, because insulators expand and contract as temperature changes, causing gaps to form. However, shrink fitting insulators, taking advantage of temperature rather than being subject to thermal changes, and preferentiallyinsulating those areas most subject to breakdown….all helpful

-It is important to note that little air gaps are worse than big air gaps-electrons, like fleas, jump more readily over the small gap

4. Be sure to heat sink to remove thermal flux resulting from dissipation of high average power

5. All the above are obvious, but the failures are so expensive andembarrassing that it pays to pay attention!


Performance (and other) Costs for power handling enhancement

1. Increased insertion loss is a typical penalty. Elimination of air gaps sometimes requires inclusion of compliant fill material, with loss tangent higher than that of the insulators.

2. Volume….it usually takes increased dmensions because achieving loading capacitance or required resonator diameter or space from housing…or other, simply takes volume.

3. Design time…field solvers take time and money, both to validate appropriate models and simply to translate results into practical dimensions.

4. Manufacturing cost: This is an “always” penalty, because eliminating gaps, ensuring flat surfaces for heat sinking…these are labor-intensive processes and thus are costly.


Generally warranted conclusions

1. Failures based on application of high power are usually “terminal” for the device under test. The DUT becomes a throw-away.

2. Design for high power requires understanding of the network on adetailed, internal examination basis. Where are the regions of sensitivity to high voltage, high current, thermal expansions, etc?

3. Given validated models, field solvers find the problems but overcoming the problems requires careful design and manufacture.

4. Obtaining validated models requires some controlled failures to ensure that predicted failure modes are real. Prepare to spend money!

5. Finally, bandstop and bandpass filters have very similar failure modes, but some critical differences. These have to be appreciated in order for successful design and test phases to be completed.


Acknowledgement

Thanks to my colleague, Sanghoon Shin, for his excellent work in model testing and model validation.

Thanks to the technical staff at RS Microwave for the design andfabrication of excellent high power notch filters.

And…thanks to you, the audience for sitting here so patiently.

Computing and Enhancing Power Handling in Bandstop Filters rmingyu/MTTs2007/6.pdf · 2007. 3....

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