PASSIVE INTERMODULATION AND PEAK INSTANTANEOUS POWER: THE IMPORTANCE OF TEST TECHNIQUES FOR OPTIMIZED ANTENNA DESIGN AND PERFORMANCE.

Transcription

1 PASSIVE INTERMODULATION AND PEAK INSTANTANEOUS POWER: THE IMPORTANCE OF TEST TECHNIQUES FOR OPTIMIZED ANTENNA DESIGN AND PERFORMANCE Date: Authr: July 13 th, 2015 Dr. Jawad Abdulnur, Directr f Engineering

2 1 TABLE OF CONTENTS PASSIVE INTERMODULATION (PIM)... 2 PIM verview... 2 Active vs passive... 3 Main causes f PIM... 3 Cmprd s intermdulatin test prcedures... 4 Cmprd s PIM test cnditins... 5 Cmprd s measures t avid PIM generatin... 6 PEAK INSTANTANEOUS POWER (PIP) PIP verview... 7 Insertin lss effect... 8 Reflectin effect... 8 Cmprd s measures t avid PIP breakdwn... 9 CONCLUSION ABOUT THE AUTHOR TABLE OF FIGURES Figure 1: Attenuatin and reflectin in linear systems... 2 Figure 2: High-rder frequency cmpnents generated by nn-linear systems... 2 Figure 3: PIM test setup... 4 Figure 4: Type f filtering system used in the PIM setup featuring the X-Pass Expandable Transmit/Receive Multicupler technlgy... 4 Figure 5: PIM test equipment fr VHF... 5 Figure 6: Cmmercial PIM test equipment... 5 Figure 7: Cmprd s PIM testing anechic chamber... 6 Figure 8: Impact f mdern mdulatin schemes... 7 Table 1: Example f calculatin... 8 Table 2: Reflectin effect n PIP... 9 Figure 9: 3D simulatin t represent the distributin f electric field arund a flded diple... 10

3 2 PASSIVE INTERMODULATION (PIM) PIM OVERVIEW: \ Multitne intermdulatin distrtins (als referred t as Passive InterMdulatin r PIM) are caused by the nnlinear behavir f devices when tw r mre signals are present at the input. \ In linear devices, the utput is linearly prprtinal t the input. When tw signals at different frequencies F1 and F2 are mixed, the result at the utput is tw signals at F1 and F2. Ideally, n ther frequency cmpnents are generated. (see figure 1 belw) FIGURE 1: ATTENUATION AND REFLECTION IN LINEAR SYSTEMS \ In nn-linear devices (see figure 2 belw), when tw signals at different frequencies F1 and F2 are mixed at the input, the result is a series f harmnics and high-rder frequency cmpnents at the utput: nf1± mf2; n,m = 0,1,2,3,4, FIGURE 2: HIGH-ORDER FREQUENCY COMPONENTS GENERATED BY NON-LINEAR SYSTEMS

4 3 These new frequency cmpnents becme a surce f interference and therefre need t be carefully cntrlled. \ The 3 rd rder intermdulatin (2F1-F2) is the strngest prduct. The 5 th rder PIM prduct is abut 15 db lwer than the 3 rd rder and the 7 th rder is lwer by an additinal 15 db. The PIM figure increases as well with increased input pwer levels by an apprximate 2:1 rati. (when the input pwer is increased by 3dB, the PIM figure will be increased by apprximately 6dB) \ When the 3 rd, 5 th r higher rder mix fall within the RX band, the level must be belw the squelch pint. If nt, it will cause significant desense (receiver desensitizatin) issues. ACTIVE VS PASSIVE: \ Active devices such as amplifiers require surces f energy (e.g. DC biasing) in rder t perate whereas passive devices d nt. Typically, active devices are nn-linear and thus, the main surce f intermdulatin distrtins and spurius emissins in RF systems. \ Under certain cnditins, the linear passive devices that are suppsed t be linear shw nnlinear behavir that results in minr distrtins, cmmnly referred t as Passive InterMdulatin (PIM) distrtins. Previusly, PIM was f little cncern t the telecm system engineers, but it is nw presenting majr challenges t the wireless industry. Mdern systems indeed require much tugher frequency plans, and with the use f higher transmitter pwer levels and mre sensitive receivers, PIM manifests as an interfering signal which may degrade the perfrmance f the receiver. MAIN CAUSES OF PIM: \ Passive InterMdulatin (PIM) ccurs anywhere in the fllwing systems: Antennas Cuplers Filters Feeders and cables Cnnectrs Lightning arresters \ Envirnmental influence and elements utside the system that are generating PIM: Twer mdules and cmpnents Blts Brackets Nearby metallic bjects and bstacles

5 4 \ The fllwing can play a rle in generating PIM by acting as dides in mixing signals. This is mainly caused by: Pr cntact junctins Materials that exhibit levels f hysteresis (ferrmagnetic materials) Cntaminatin Lse cnnectins Crrded bjects COMPROD S INTERMODULATION TEST PROCEDURES: \ PIM cannt be predicted by simulatin. The nly way t find ut the level f PIM generatin is t measure it. FIGURE 3: PIM TEST SETUP FIGURE 4: TYPE OF FILTERING SYSTEM USED IN THE PIM SETUP FEATURING THE X-PASS EXPANDABLE TRANSMIT/RECEIVE MULTICOUPLER TECHNOLOGY

6 5 FIGURE 5: PIM TEST EQUIPMENT FOR VHF COMPROD S PIM TEST CONDITIONS: FIGURE 6: COMMERCIAL PIM TEST EQUIPMENT \ PIM testing is perfrmed using tw 20 Watt (43dBm) transmitters. The signals are filtered and cmbined. The level f intermdulatin prduct is measured with a spectrum analyzer. \ The X-Pass Expandable Transmit/Receive multicupler technlgy is used fr VHF and UHF frequencies. The test beds fr these frequencies are develped and manufactured by Cmprd. Fr higher frequencies (e.g. 700/800/900 MHz), a cmmercial test setup is used. \ All parts, cmpnents, cables and cnnectrs in the test setup are certified lw PIM. PIM perfrmance f the setup is verified prir t each measurement by replacing the antenna with a lw PIM lad. Our standard specificatin fr lw PIM antennas is -150 dbc and we always ensure that ur test bed has a residual InterMdulatin (IM) that is 10 db belw this value in rder t ffer a reliable measurement. \ Passive devices such as filters, cable/cnnectr assemblies, etc. (where the signal is cnfined within the structure) are very lightly affected by electrmagnetic signals existing in the envirnment because f the shielding structure that is prtecting these devices.

7 6 \ By cntrast, because f radiative prperties, antennas are highly affected by external surces. A strng islatin frm the envirnment is therefre required - hence the imprtance f using an anechic chamber. FIGURE 7: COMPROD S PIM TESTING ANECHOIC CHAMBER \ Cmprd s antennas are tested in an anechic chamber which is equipped with custmdevelped absrptin panels specifically designed fr PIM testing. Ferrite materials are nt allwed. In additin, special shaping and cnfiguratin are used t handle the pwer at frequencies as lw as 138 MHz. This is achieved by maintaining a suitable size fr each absrber. As the shielding structure des nt cntain any ferrmagnetic materials, it results in a shielding level that is better than 100 db. \ Omnidirectinal, quasi-mnidirectinal and directinal antennas with very high directivity can be tested in this chamber, frm 138 MHz up t 18 GHz. COMPROD S MEASURES TO AVOID PIM GENERATION: \ Cmprd implements a series f measures, frm design all the way t manufacturing, t avid r reduce t the lwest pssible level the ptential surces f PIM: Careful selectin f materials Use f high-perfrmance cables and cnnectrs Highest quality f welding as well as apprpriate trque and alignment during the assembly prcess t avid pr mechanical cntacts r lse mechanical junctins Cmplete eliminatin f cntaminated surfaces and gaps (parts cleaned in an ultrasnic bath) Perfectly smth metal surfaces that are free f cracks, distrtins, flakes r shavings Sealed pints f cntact s as t avid crrsin

8 7 PEAK INSTANTANEOUS POWER (PIP) PIP OVERVIEW: \ In mdern digital multi-carrier cmmunicatin systems, amplitude and phase mdulatins are cmbined in cmplex envelp wavefrms. The peaks f in-phase signals will be added and the result is a significant rise in vltage ccurrences. This creates a serius Peak Instantaneus Pwer (PIP) handling issue in devices and mdules such as antennas. The peak can be high enugh t create arcing acrss junctins and gaps in sensitive areas. \ The rati f the signal peaks t the average pwer level is usually expressed as Peak t Average Pwer Rati (PAPR). Higher data rates lead t higher PAPR. \ The fllwing diagram shws the impact f mdern mdulatin schemes n the PAPR which is increasing frm apprximately 3.5 db in the 3G wireless netwrks and up t 8.5 db in the 4G netwrks. The average transmitted pwer level generally remains the same, as this determines the distance (range) f the RF transmissin. FIGURE 8: IMPACT OF MODERN MODULATION SCHEMES Fr N channels with V vltage amplitude each, the PAPR is given by the fllwing expressin: The previus frmula leads t:

9 8 As a result, PIP is given by: \ If we knw the Peak t Average Pwer Rati (PAPR in db) f the mdulatin scheme as well as the Cntinuus Wave (CW) pwer f each carrier (Pcarrier in Watts), this frmula gives a gd apprximatin f the peak instantaneus pwer in Watts. INSERTION LOSS EFFECT: \ The insertin lss culd cme frm the cmbining netwrk, feeders r ther passive cmpnents. The fllwing frmula allws taking int accunt the effect f this lss. (In this frmula, the Insertin Lss IL shuld be a negative value in db) REFLECTION EFFECT: TABLE 1: EXAMPLE OF CALCULATION (FOR A SITE WITH 12 CHANNELS COMBINED, 20 W POWER PER CHANNEL, 3.2 DB INSERTION LOSS, 1.3:1 ANTENNA VSWR, THE PEAK INSTANTANEOUS POWER PIP GENERATED IN AN ANTENNA FOR DIFFERENT MODULATION SYSTEMS) Residual reflectin due t mismatch ccurs in all cmmunicatin systems. It is represented by the reflectin cefficient r which is related t the pwer and Vltage Standing Wave Rati (VSWR) in the fllwing frmula:

10 9 The amunt f peak pwer reflected (in Watts) is given by: \ Fr an antenna f 1.4:1 VSWR, arund 3% f the incident pwer is reflected. This pwer shuld be added t the incident pwer when analyzing the PIP handling f the antenna. \ Fr an antenna f 1.5:1 VSWR, arund 4% f the incident pwer is reflected. This pwer shuld be added t the incident pwer when analyzing the PIP handling f the antenna. \ If we take the PIP value f ur example and add the reflectin effect, we btain tw different VSWR values: TABLE 2: REFLECTION EFFECT ON PIP COMPROD S MEASURES TO AVOID PIP BREAKDOWN: \ PIP is respnsible fr breakdwn (arcing) in the antenna. This is different frm the Cntinuus Wave (CW) pwer respnsible fr temperature rise. When arcing ccurs in the antenna, the surface is damaged and the interir gets cntaminated. This is a destructive prcess which leads t majr failure in the antenna. \ Knwing that breakdwn in the air ccurs at 3000 KV/m, Cmprd s engineers develped 3D simulatin techniques t represent with precisin the distributin f electric field strength radiated by a diple. The mechanical cnstructin f the diple is reprduced in this simulatin and surce excitatin nrmalized t 1 Watt is cnsidered.

11 10 \ This enables us t accurately calculate the amplitude in V/m prduced by the peak pwer in the system. \ When designing antennas and feed netwrks, we reduce the mismatch t the lwest pssible level. \ Design and manufacturing prcesses are carefully mnitred in rder t eliminate any residual imperfectins and avid charge accumulatin as well as high-density currents in critical areas. \ Our PIP-rated antenna is designed t supprt a maximum f 25KW peak pwer. A margin is als cnsidered t vercme the effects f temperature, humidity and altitude. FIGURE 9: 3D SIMULATION TO REPRESENT THE DISTRIBUTION OF ELECTRIC FIELD AROUND A FOLDED DIPOLE \ This simulatin shws the effect f the gemetry n the intensity n electric fields. This kind f simulatin is perfrmed n all Cmprd s antennas t prevent the arcing with a cmfrtable margin fr imprved PIP rating.

12 11 CONCLUSION Understanding and minimizing the detrimental impact f Passive InterMdulatin (PIM) and Peak Instantaneus Pwer (PIP) is critical t ensure the ptimal perfrmance and efficiency f yur RF netwrks. Special cnsideratins fr testing, preventin and measurement f these tw parameters are therefre needed t ensure that yur key cmmunicatins netwrk cmpnents, such as antennas and filtering slutins, d nt cntribute t netwrk perfrmance degradatin. As a leader in RF slutins, Cmprd has leveraged its frty years f experience in the field t establish and apply a set f best practices - frm the careful selectin f materials and sphisticated RF design practices, cmbined with strict quality cntrl during the manufacturing prcesses all the way thrugh t rigrus testing prcedures perfrmed using its wn anechic chamber as well as field testing - t deliver unmatched lw PIM characteristics prducts (better than -155 dbc) and guarantee sustained prduct specificatins ver time. In additin, Cmprd emplys pwerful 3D simulatin design techniques t accurately assess the impact f PIP, and thus prevent arcing and pssible premature cmpnent failures in antenna systems. Designed t supprt a maximum f 25KW peak pwer, its PIP-rated antennas are manufactured t the highest standards s as t eliminate any residual imperfectins and avid charge accumulatin as well as high-density currents in critical areas. We hpe that this verview will serve as a useful guide and that it will assist yu in the selectin f antenna cmpnents in the planning and design f yur wireless netwrk. Please cntact us at sales@cmprdcm.cm shuld yu wish t receive additinal infrmatin r discuss yur specific requirements. ABOUT THE AUTHOR Dr. Jawad Abdulnur jined Cmprd in 2013 as Directr f Engineering. In his rle, he is respnsible fr verseeing all aspects f R&D and new prduct develpment. Mr. Abdulnur has ver 20 years f experience in the wireless industry, having previusly wrked fr leading micrwave and RF slutin prviders such as EMS Technlgies and Mitec Telecm. Prir t jining Cmprd, he was Directr f Engineering and CTO at SDP Telecm, where he played a key rle in driving the prduct strategy and grwing the business.

13 12 During the curse f his career, he has acquired expertise in the field f satellite technlgies and participated in a majr prgram t develp satellite transpnders and RF mdules fr the Internatinal Space Statin. Mr. Abdulnur hlds a Bachelr f Science (B.Sc.) degree frm the Lebanese University in Beirut, a Master f Science (M.Sc.) degree frm Quebec University in Tris Rivières as well as a Ph.D. frm the Natinal Institute f Scientific Research f Quebec. As a Pstdctral Fellw, Research Assciate and Visiting Assistant Prfessr at Écle Plytechnique and Écle de Technlgie Supérieure (ETS) in Mntreal, he has authred ver 50 scientific articles and cnference papers n electrmagnetic thery and related numerical methds.