WMH-8 Design of Low PIM Diplexers - University of Waterloomingyu/MTTs2007/8.pdf · Background...

BackgroundTheoryGeneral principles for PIM reductionCoaxial Diplexers

StructuresExamples

UHF Interdigital Diplexer for Spacecraft TestCellular Combline Coupled Cavity

References and Acknowledgements

WHM IMS 2007 Honolulu © 2007

Design of Low PIM Coaxial Diplexers Chris Radcliffe

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First noticed in High Power Military Ground Station Antennas in the late 60sCaused many problems on early communication satellitesMoved into the mainstream of the microwave industry with advent of multicarrier Cellular networks in the 80s and 90s.

Tens of thousands of diplexers and cables had now to be intermod free


PIM - Background

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Passive Intermodulation products (PIM) are generated when 2 or more RF signals pass through a non linear medium.The current level and the magnitude of the non-linearity determines the PIM level.

Ferromagnetic materials have highly non-linear hysteresis of their magnetic domain boundaries and produce much higher PIM levels than non-magnetic materials. [1]Polar dielectric materials have much higher PIM levels than non-polar materials. [2]

Surface and junction effects are very important.Thought to be caused by semiconductor effects and tunnelling


PIM - Theory

4WHM IMS 2007 Honolulu © 2007

Effects of Non-linearities

–The voltage transfer characteristics of a non linear mechanism can be described in terms of an infinite polynomial:

–Vout = a0 + a1Vin + a2Vin2 + a3Vin

3 + a4Vin4 + ...

–where the terms Vin and Vout represent the voltage of the input and output signals respectively. The coefficients a0, a1, a2, etc., relate directly to the type of mechanism and its electrical context and, it will be seen, have an important impact on the variation of power between input and output signals.

– For a simple two carrier case, the input voltage can be described as:

–Vin = V1 cos(ω1t) + V2 cos(ω2t – φ2)


Effects of Non-linearities

This input voltage can be substituted into the polynomial and the terms expanded to the nth degree, reducing the terms to single powers of cos(x). This allows all terms of the same frequency value to be gathered together - that is, all terms generating the same PIM frequency.

It can be shown that the amplitude of a single intermod frequency depends on all the coefficients a1 .....an. These coefficients in turn can depend on many separate effects.

Hence the output power variation of a third order intermod does not normally follow the expected third order 3dB/dB relationship.[5], [6]

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Avoid Ferromagnetic materialsMinimise current density at junctions [3]

Use choke coupled junctions if possible [4]Provide high contact pressures at junctions where high current densities cannot be avoidedAvoid the interconnection of dissimilar metals

Especially those with a large electro potential difference

Minimise the number of jointsEnsure solder joints are free from defects and stress free

Ensure contact surfaces are kept scrupulously clean.


Guidelines for Low PIM


Coaxial Diplexers

Now we will look at two very different coaxial diplexer examples; Interdigital Diplexer UHF High Power (>4kW) Low loss, high rejection and very low 7th order PIM

Combline Coupled Cavity Diplexer Cellular application – high volume production Moderate power - 2 x 40W Low 3rd Order PIM required


UHF Interdigital Diplexer



Used for ground test of UHF Military spacecraftMain CharacteristicsLarge 1.6 metre long 100kgLow loss 0.5dB 60mm GPSHigh Power 250W CW 4.5kW peakHigh rejection 120 dBLow PIM -160dBm for 7th order

10 or 11 section chebychef filters coupled by common transformerTap coupling for input and output



Designed by circuit theory with some optimisation of the common transformer. EM design of tap points



Designed for high power -Maximum voltage calculated,worst resonatorshown here;

2500V maxmeans3mm min gap(with 2 x safety factor)



Intermod design featuresAll internal parts of silver plated brass or pure copperSilver plating at least 5 skin depths thickNo exposed threads inside - resonators tuned by extending length - susceptance tuners are posts Number of solder joints minimised – only 2 critical onesResonators bolted to side walls under high pressureHousing bolted together - mating areas very clean with pressure ridge - high tensile bolts every 20mm



Intermod design featuresExtending length resonators



Intermod design featuresExtending length resonators – voltages across gaps and at bottom inside resonator calculated using EM solver


UHF Interdigital Diplexer000655-01 sn001 TX channell cjr 24/10/06

Frequency (GHz)0.310.30.290.280.270.260.250.240.230.220.21

IL &

RL

(dB

)

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

-100

-110

-120

-130



Solder jointscleaned and checked under10x magnification

This unit tested to better than -160dBm 7th order with two 250W input carriers


Combline Coupled Cavity Diplexer



Typical Internal structure


Combline Coupled Cavity DiplexerIntermod Challenges

High Volume – probably over 10,00 pa Low cost required - Parts need to be cheap but high quality where required - Assembly needs to be quick easy and foolproof Success or failure depends on - good design and well specified drawings - developing suppliers who can give the quality required in the right time-scales - well developed and well controlled processes



Intermod design features (1)

All parts internally of silver plated aluminium or copper Silver plating at least 5 skin depthsTuning screws have to be used – made as large as possibleUse capacitive coupling if possible (usually not possible)Minimise current density at tap points by making connections as large as possible.Number of solder joints minimised Resonators bolted to floor under high pressure or preferably machined into body


Combline Coupled Cavity DiplexerIntermod design features (2)

Lid bolted on - mating areas very clean - high tensile bolts every 20mm or less but no pressure ridge (too expensive)Antenna area critical. Connectors must be silver or white bronze plated.High pressure spring finger inner and outer connections7/16 or specially designed TNC best Connector flange should have pressure ridge and body flatness and surface finish needs to be good.



Intermod design features (3)

Avoid sharp features and cracks. Cavity bottoms should have radius to ensure proper platingCoupling strip edges should be rounded.Wires and strips should not have sharp bends where cracks might appear.

Cross-couplings may have lower field strengths than main-line couplings but same rules need to apply. In particular avoid unplated brass, zinc content causes problems.



Intermod Process Control (1)

Tapped holes in bodies for lid screws must be burr free and body must be flat over mating face - use chamfer on body tapped holes to avoid burrs affecting lid seating.Lids must be flat and scratch free on mating face.Lid tuning screw threads are critical and must be the correct fit and free from burrs. Rolled tapping best to ensure no burrs.Commercial fits between threads are too wide and need to develop own process.


Combline Coupled Cavity DiplexerIntermod Process Control (2)

Minimise transport of parts and handling.Get the packaging right so parts are well protected.Clean, clean and clean again. Use gloves for assembly.Don't change anything without testing first.Invest in at least two Intermod test benches with staff who know how to use them.Check the performance of the test bench frequently especially loads and reference diplexers.Never relax - be paranoid!


Design of Low PIM Coaxial Diplexers

References

[1] Bailey G.C. & Ehrlich A.C.: “A Study of RF Nonlinearities in Nickel”, Journal of Applied Physics, 50, 1979, pp. 453 - 461

[2] Khattab T.E. “An Investigation into the Passive Intermodulation Properties of Dielectric Materials in Space RF Components ”, PhD Thesis. University of Kent at Canterbury 1996

[3] Rootsey J.V., Gradisar A.A. & Bordenave J.R.P.: “Intermodulation Study (Intermodulation Products – Satellite Ground Antennas)”, AD-785 711, Final Test Report, Philco-Ford Corporation, 1973

[4] Petit J.S. & Rawlins A.D. “A Study of Multipaction in Multicarrier RF Components. Volume 2 WP2100; Anomalous Effects”, ESTEC contract No. 10873/94/NL/DS September 1996


Design of Low PIM Coaxial Diplexers

References (cont)

[5] G Macchiarella & A Sartorio; Passive Intermodulation in Microwave Filters: Experimental Investigation. MTT-S WMB7 2005[6] C Vinente & H L. Hartnagel; Passive Intermodulation at Junctions; MTT-S WHM7 2007

Acknowledgements

The theoretical part was provided by John Petit of ComDev EuropeMuch information on high volume production techniques was provided by Tim Jenkins and other previous members of Phase Devices and ComDev Wireless

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WMH-8 Design of Low PIM Diplexers - University of Waterloomingyu/MTTs2007/8.pdf · Background...

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