introduction the increased demand from the mobile com- munication industry to provide greater channel capacity coupled with the increased sensitivity of receivers has exposed a condition within rf coaxial connectors referred to as intermodulation distortion (imd). this condition occurs when non-linearities within the connectors act as imperfect diodes to generate other frequencies known as intermodulation products (imp). some of these frequencies appear within the receive band and effectively block the channel. the purpose of this application note is to outline the basic causes of intermodulation and the techniques m/a-com has undertaken to minimize this condition. im basics modern developments in base stations for gsm, dcs 1800 and pcs 1900 have necessitated the use of ?-16? ?.1/9.5?and ??connectors due to the increased power requirements. the requirements for performance are typically in the order of -160 dbc to -163dbc (when working in dbc) or -120dbm (when working in dbm), both with 2 x (+43dbm) tones. the requirement is so stringent because the connectors are used in post- filtering sections of the transmit path (between the diplexer and the antenna) and also because the system is a full duplex system where the multiple-carrier transmit path is also the receive path. in a truly linear system, the output is directly proportional to the input, following the form of y=mx+c (see fig. 2). coaxial connectors have traditionally been viewed as following this pattern. in reality, there have always been non-linearities present in coaxial connectors. these were not readily apparent as the resultant im products were significantly below the noise floor of the system due to relatively weak carrier signals. this situation becomes apparent when the incident power is raised above 30 dbm. the small non-linearities have a characteristic similar to a square-law (see fig 3). the distortion to the waveform is evident, the positive 1/2 cycle being significantly greater in amplitude than the negative 1/2-cycle. when converted to the frequency domain, this waveform consists of the desired fundamental plus a decaying series of related harmonics that, in themselves, interact with other carriers present on the transmission line. id1018 intermodulation in rf coaxial connectors v3.00 m/a-com, inc. north america: tel. (800) 366-2266 n asia/pacific: tel. +85 2 2111 8088 n europe: tel. +44 (1344) 869 595 fax (800) 618-8883 fax +85 2 2111 8087 fax +44 (1344) 300 020 1 specifications subject to change without notice. fig. 1 m/a-com intermodulation test lab linear vs. non-linear response 12 10 8 6 4 2 0 120 100 80 60 20 0 non-linear linear fig. 2 fig. 3 40
the ef fect of this interaction pr oduces additional fr equencies, some of which occur wher e they ar e least wanted (see fig. 4). the 2f1-f2 (3r d or der imp , imp 3 ), 3f1-f2 (5th or der imp , imp 5 ) and 4f1-f2 (7th or der imp , imp 7 ) pr oducts can all manifest in the r eceive band and, if suf ficiently lar ge, ef fectively block a channel by making the base station r eceiver think that a carrier is pr esent when one is not. potential causes of im in coaxial connectors ther e ar e numer ous factors which can af fect intermodulation performance in rf coaxial connectors. identified below ar e the most likely sour ces of concern: contaminated plating solution insuf ficient plating thickness corr osion dissimilar metals in intimate contact magnetic materials in the signal path low contact pr essur e less than 360-degr ee contact poor surface finish debris and dust within the connector convoluted signal path remedies for im in rf coaxial connectors t o combat the above identified im sour ces, m/a-com undertakes pr ecautions during the design and manufactur e of the pr oduct, as summarized below: high quality plating to 6 m for im-sensitive pr oducts the plating must also be fr ee fr om contaminants and pr operly passivated with a chr omate passivate. silver has been the pr eferr ed plating material as it possesses the lowest practical r esistivity ther eby minimizing interface contact r esistances. m/a-com also of fers a unique white br onze plating finish which pr ovides excellent durability , tarnish r esistance and non-magnetic pr operties ideal for low inter - modulation. during testing with a system noise floor of -145 dbm, the dif fer ence in performance between silver -plating and m/a-com? new white br onze finish is not discernible (r efer to white br onze application note id1014). ? restrict materials to copper and its alloys. this ensur es maximum plating adhesion and minimum electr ochemical potential dif fer ence be - tween the base materials and their over -platings. a void the use of stainless steel, nickel, ferrites, etc. in the signal path magnetic and para-magnetic materials will only compound non-linearites and give poor er interface contact r esistances. during experimentation, m/a-com discover ed a degradation in performance of 20db when nickel plate was used. the pr esence of magnetic or para-magnetic materials will also cause the forwar d imp figur e to dif fer fr om the r everse imp . ?quality machining surface finish is paramount. the signal pr opagates within a ?kin?if this skin is too r ough, the signal will r epeatedly transition thr ough metal and surface oxide layers, ther eby cr eating the same ef fect as a poor panel contact (see fig. 5). for im-sensitive designs m/a-com ensur es 0. 4 m is the maximum. contact design this primarily af fects the connector interface. repeated matings can generate small amounts of plating fr om the individual parts. these oxidize and interfer e with the mechanical (and ther efor e electrical) mating of connectors. the oxidized debris gives further rise to metal and surface oxide junctions and consequently , higher im pr oducts. m/a-com, inc. nor th amer ica: t el. (800) 366-2266 n asia/p acific: t el. +85 2 2111 8088 n europe: t el. +44 (1344) 869 595 f ax (800) 618-8883 f ax +85 2 2111 8087 f ax +44 (1344) 300 020 2 specifications subject to change without notice . spectrum of products in a 2-tone system frequency (mhz) fig. 4 rx band tx band 3f2-f1 f1 f2 4f2-3f1 2f2-f2 2f2-f1 3f2-f1 4f2-3 f1 carriers % 120 100 80 60 40 20 0
ensur e, by design, a pr operly defined contact interface at connector , panel and contact interfaces insuf ficient contact for ce will give rise to metal to oxide junctions. the classic r ectifiers wer e metal oxide by composition. axial maximum material condition at the connector interface is critical in or der to ensur e minimum mismatch and maximum potential of a butt- contact. panel interfaces generally concern the physical contact of the connector body to the panel. in this case, it has been determined that a pr otr uding featur e as close as possible to the body bor e will give the best im performance. the applied mounting for ce is concentrated in the surface ar ea of the pr otr usion which, on engagement with the panel, punctur es the existing oxide layer to give a metal-to-metal, gas-tight junction. a voidance of crimps crimps, by natur e, can only give multiple point- contact rather than 360-degr ee contact and also cause a variability in the position of electrical contact during dynamic testing. im pr oducts will ther efor e be gr eater . it has been found that solder ed center contacts and clamp/solder outer contacts give the best static and dynamic im performance. impr oving im connector design m/a-com continues to pursue design techniques which impr ove intermodulation performance to addr ess emer ging telecommunication market needs. a state of the art intermodulation test facility and participation on the international (iec sc46d wg5) committee to develop standar d test practices ensur es our commitment to the understanding of intermodulation characteristics. this applied technology base is instr umental in developing innovative low intermodulation pr oducts for 7-16, t ype n, sma and osp interfaces. most commonly asked questions regar ding inter modulation why is intermodulation such a concern for cellular infrastr uctur e equipment? the primary concerns for cellular service pr oviders today ar e channel ef ficiency and clarity of transmission. gr owth in demand for mobile communications has cr eated a need to operate equipment at gr eater capacities and r eliability to service the competitive market. intermodulation degrades or limits the ability of the service pr ovider to operate at optimal levels of performance and may ultimately cause subscribers to experience poor call quality . intermodulation has become an important factor in system selection to ensur e the best possible network service. wher e is intermodulation most likely to occur in cellular infrastr uctur e equipment? intermodulation is typically of gr eatest concern between the filtering elements of the system and the antenna. the intr oduction of higher power levels for the transmit side of the equipment cr eates gr eater potential for intermodulation to occur . this is why the majority of focus for intermodulation concerns 7-16, type n, sma and 4.1/9.5 connector interfaces. is intermodulation a r ecent development? intermodulation has always been inher ently pr esent in rf coaxial connectors but may be r elatively imper ceptible in some devices for a variety of r easons. the amount of power applied to an rf connector determines the r elative im thr eshold which can be observed. intermodulation is ther efor e mor e likely to cause concern in a higher power system, for example, utilizing a 7-16 connector interface rather than an equivalent low power osx solution. the tr end towar d higher power digital cellular systems cr eates the need for gr eater intermodulation sensitivity . m/a-com, inc. nor th amer ica: t el. (800) 366-2266 n asia/p acific: t el. +85 2 2111 8088 n europe: t el. +44 (1344) 869 595 f ax (800) 618-8883 f ax +85 2 2111 8087 f ax +44 (1344) 300 020 3 specifications subject to change without notice . effect of surface roughness fig. 5 1.5 0.5 0 -0.5 -1.5 -1 1 oxide signal flow metal air 1. 2. 3.
what is the best method of cable attachment for im sensitive cable assembly applications? soldering and clamping ar e pr eferr ed methods due to the 360-degr ee point of contact cr eated at the cable to connector interface. such intimate contact impr oves the overall contact r esistance leading to impr oved im characteristics. in addition, it is also better to solder the center conductor of the connector to the cable versus crimping due to the impr oved contact r esistance path and elimination of voids. ar e ther e ways to test for intermodulation in an rf coaxial connector? y es, very sophisticated methods ar e needed to test for intermodulation in rf connectors. the test system must utilize extr emely sensitive filtering or clean amplification so that the equipment itself has a very low intermodulation noise floor . ther e is not yet a standar dized appr oach to testing, although an international committee has been formed in the connector industry to addr ess the situation. m/a-com has a state of the art test facility wher e our designs ar e optimized for low intermod performance and wher e further analysis on the ef fects of this phenomena can be studied. is intermodulation in coaxial connectors fr equency dependent? no. because coaxial connectors ar e br oadband devices ther e is no fr equency dependency . some appar ent variability can be detected during testing but this is not due to the connector . the impedance matches of the output diplexer/triplexer and terminations ar e the causes of the variations and should not be incorr ectly attributed to the connector/assembly . m/a-com inter connect business unit has demonstrated that by varying the impedance match of the test station termination, a dut can show 15db better imp 3 than exists in r eality . imp 3 in mixers follows a 3db/db r elationship. what is it for connectors? the r elationship is identical. t aking the 3r d or der (2f1-f2): varying the power of f2 gives an imp 3 r elationship of 1db/db wher eas varying the power of f1 gives a r elationship of 2db/db as the imp is derived fr om the 2nd harmonic of f1. this gives a total of 3db/db when symmetrically varying both carrier powers. i am buying a complete cable assembly fr om m/a-com . ho w d o i interpr e t th e imp r esul t now? w ith caution! it is m/a-com? policy when testing devices to move away fr om the normal static test to a dynamic test wher e the cable termination interfaces ar e mechanically exer cised during live im conditions. it is also a good indicator to customers of the build quality of the assemblies. a dynamic evaluation has shown 15db degradation in imp performance for poor assemblies and even as much as 50db for bad ones. it is ther efor e str ongly advisable that im performance figur es ar e stated in the context of a dynamic measur ement. m/a-com, inc. nor th amer ica: t el. (800) 366-2266 n asia/p acific: t el. +85 2 2111 8088 n europe: t el. +44 (1344) 869 595 f ax (800) 618-8883 f ax +85 2 2111 8087 f ax +44 (1344) 300 020 4 specifications subject to change without notice . pr inted in u .s . 8/97 4. 7 . 8. 5 . 6. north america t elephone: 1-800-366-2266 fax: 1-800-618-8883 central america t elephone: 602-949-1642 fax: 602-941-1703 south america t elephone: 770-433-8228 fax: 770-433-8404 europe/middle east/africa t elephone: 44 (1344) 869 595 fax: 44 (1344) 300 020 asia/pacific t elephone: 85 2 21 1 1 8088 fax: 85 2 21 1 1 8087
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