DESIGNING FILTERS TO WITHSTAND INIMICAL ENVIRONMENTS

Transcription

1 DESIGNING FILTERS TO WITHSTAND INIMICAL ENVIRONMENTS By R. V. SNYDER RS MICROWAVE BUTLER, NJ IMS 2005 Workshop WMB Filters II Practical Aspects of Microwave Filter Design and Realization

2 CONTENTS HOW DO WE DESIGN AND CONSTRUCT FILTERS TO TOLERATE THE FOLLOWING ENVIRONMENTS? Modeling Tried and True techniques Operating and non-operating environments discussed Altitude Humidity Salt spray Vibration and Shock Temperature Corrosive and penetrating liquids Space High power And the worst Degradation over TIME! Conclusions

3 Design Techniques 1. Modeling Performance over range of anticipated conditions Temperature and humidity: usually non-linear performance is experienced but static measurements enable connect the dots compensation using high and low expansion materials Vibration and Shock: Finite element packages available but accuracy depends on model complexity Power: Modeling techniques to be discussed later in this workshop

4 Design Techniques (continued) 2. Tried and true over the years Altitude: Seal the assembly Humidity: Redundantly seal the assembly Salt Spray: Proper choice of plating and seal technology Vibration and Shock: Avoid cantilevered parts and pot wherever possible Temperature: Compensating expansion coefficients, sometimes in more than one plane

5 Design Techniques (continued) 2. Tried and true over the years Corrosive and penetrating liquids: Proper finish and seal Space: Don t seal.vent internal gasses to avoid multipactor High power: avoid air gaps, design to accommodate high RF currents in low impedance regions, high RF potentials in high impedance regions, seal to retain sea level operating condition High RF currents: grounding techniques High RF potentials: radii must be large enough Elimination of multipactor (resonant breakdown) requires venting (not sealing) to remove internal gasses. This phenomenon occurs at low RF power TIME: Don t miss any of the above! Shelf Life Performance Life Allowable Degradation (warranty considerations)

6 TEMPERATURE Housing is Aluminum, resonators are copper, substrates are both ceramic and soft (both types plated). Ceramic has a positive TC while the soft substrate has a negative TC Commercial FE software can solve this problem but it is described by non-linear differential equations.

7 Low-Expansion Capacitive Substrates used in temperature compensation of filter Housing is aluminum, resonators are copper, substrate is plated ceramic. Ceramic TC is linear with respect to operating temperatures.

8 455 MHz Evanescent BPF, BW=10 MHz Typical Construction for narrow band, highly temperature stable BPF s Tried and true solution, partially cut and try! NEGATIVE TC CAPACITOR VARIABLE CAPACITOR POSITIVE TC CAPACITOR

9 Housing is aluminum, Resonators are brass, insulators are teflon. Resonators are stepped, with the stepping ratio adjusted for temperature stability. Teflon is non-linear with respect to temperature. This is no problem for broad band filters but is a problem for narrow band cases Folded Notch Evanescent BPF

10 Temperature/Humidity Compensation of cavity filters, assuming initial D/L = 1

11 One-Dimensional vibration/shock analysis example Finite element software is commercially available to perform 3 dimensional analysis I is the moment of inertia for the beam

12 How to prevent vibration and shock damage to filters Pot the structure! Potting compound has an effective dielectric constant of about It is poured in as loose granules, then baked at 140 C for several hours to form an encapsulated structure capable of withstanding 1000 s of g s.more than the housing can tolerate! Filters have to be conditioned at even higher temperatures before potting, to avoid detuning during the potting operation

13 SALT AND CORROSIVE LIQUIDS Finish and seal integrity are the keys to success Finish must include barrier layers, such as electoless nickel, gold or iridium As these barrier layers are not very conductive nor do they adhere well, the plating process is important Plating must include copper and silver for conductivity, usually over the very thin adherence and barrier layers, and then additional gold or chemical coating such as iridite applied as a final layer over the silver Typically, painting includes an epoxy layer or layers to further enhance resistance to salt and solvents. The marking must also resist the attack of solvents and thus must be epoxy based or permanently laser engraved Seal will be discussed on later slides

14 SALT AND CORROSIVE LIQUIDS PLATING IS FUNDAMENTAL!

15 TYPICAL PLATING ANALYSIS

16 Sealing Filters To prevent damage from salt, humidity, altitude (other than free space), corrosive liquids and over time, filters must be sealed to at least the Gross Leak level Sealing requires redundancy, using at least two of the following: O-rings Compliant RTV-based compound ( bathtub caulking ) Epoxy Soldering Welding Selection requires consideration of overall environment. For example, with lots of vibration, epoxy is essentially useless because it cracks! Seal must withstand multiple thermal shock cycles without cracking (can t be fundamentally epoxy based)

17 Use of O-rings (siilicone rubber gaskets) and silicone sealant Groove to be filled with silicone RTV Silicone Rubber gasket in groove also filled with RTV Potted area Sealed, similar unit using blind mate connections with glass to metal seals silver-soldered into filter body Filter is sealed to retain high power operational capability at 70,000 feet for at least 30 minutes (slow depressurization)

18 Leak Testing using hot Flourinert DUT, immersed in 125 C Florinert The hot flourocarbon increases internal pressure in the DUT, causing a tendnecy to force out a stream of bubbles. The inert liquid is easily removed from a failed DUT

19 What happens when plating and/or seal fail in the presence of lots of salt? Plating has degraded Salt has deposited due to seal failure. The salt, in conjunction with moisture, causes plating failure followed by catastrophic filter failure at high input power It s not a pretty sight!

20 High Power Notch Filter showing high RF current and voltage areas of concern (center conductor not illustrated) Low impedance end of resonator machined directly to ground. Entire unit is sealed (measures 40 x 6 x 2 ) High potential ends and edges machined with full radius

21 Prevention of Multipactor Use of an intentional path for venting internal gas Vent hole Diameter =.005 Gasses (atmospheric or internally-produced by material outgassing) are vented out of the filter to increase the power threshold for the resonant breakdown known as multipactor.

22 Multipactor Reduction using venting Sample calculation for vent hole diameter

23 Survival and degradation with time Warranty length is determined by time it takes for filter to degrade and not meet specification Degradations in loss, power handling capacity or even cosmetic appearance can be construed as unacceptable Typically, units must meet full operational capability for one year Multiyear warranty requires modeling, accelerated life testing to determine rate of deterioration or at least very good understanding of potential design or manufacturing flaws Extended life requires extreme attention to all of the details presented in this discussion

24 CONCLUSIONS (and they are related to each other) Filter design in the real universe requires awareness of much more than network and E-M theory Survival over time (for both the filter and the filter designer) requires attention to mechanical and environmental issues, as well as electrical details Global modeling will become increasingly important in failure prediction and mitigation, as filter complexity increases and both electrical and environmental specifications become more demanding Time-proven methods exist, and reverse-engineering these methods, using modeling, should provide a synergism for the art and science of design Materials science is a very important area and should be carefully studied as a part of the filter design process