Mini Coax+: Beyond 2 GHz

With UMTS, the "Universal Mobile Telecommunications System", a standard has been defined which is regarded as the European candidate for the worldwide FPLMTS (Future Public Land Mobile Telecommunications Standard). This standard permits both in-house communication and worldwide networking via satellites.

The frequency bands necessary for the systems of the future have already been allocated: 1.970 GHz -2.026 GHz and 2.110 GHz - 2.200 GHz. UMTS will offer transmission rates from 8 kbps up to 2 Mbps, and will therefore be suitable for a very wide range of applications, such as voice and audio Services, data, text and image transmission, and also video. The performance increase which this new system offers can be illustrated as follows: At 2 Mbps, Ernest Hemingway's classic novel, "For Whom The Bell Tolls", can be transmitted in full in around 20 seconds, whereas an ISDN channel only just manages one page per second.

New cellular radio standards for multimedia

Introduction of the cellular radio standards

The new possibilities, even including video transmission from mobile phone to mobile phone, will arouse the enthusiasm of millions of people. However, the transition from today's voice transmission via mobile phone to a mobile multimedia experience has a price: significantly higher data rates. Apart from higher-performance user equipment and network components, multimedia also demands a completely new concept of frequency utilisation and data transmission. The ambitious target of global networkability - being reachable anywhere in the world - can only be achieved with common international standards. This challenge was recognised at an early stage, and the International Telecommunications Union (ITU) acted accordingly, by establishing the internationally applicable IMT-2000 standard. The key points of this Specification are the aforementioned frequencies, a band-width of 5 MHz and a maximum data rate of 2 Mbps. How, and in what form, the data is transmitted is defined in the cellular radio standards WCDMA, CDMA-TDD, CDMA-2000, UWC-136 and DECT. In February 1998, the ITU approved these 5 different standards for the third generation of mobile communications. In this bundle, CDMA-TDD and DECT are the interface standards for in-house communication and stationary point-to-multipoint radio relay links through to every subscriber's home.

Specification of base station

Block diagramm of a base station

The system to which every mobile phone establishes radio contact is the so-called base Station (Base Transceiver Station, or BTS for short). It is the central point of every cellular radio cell and handles the radio traffic with the mobile subscribers as well as monitoring the physical radio link. The base station does not have a switching function. Several transceivers in such a base station modulate the user information to the carrier frequency in the gigahertz range. This newly obtained signal is amplified and emitted via the associated antenna. Incoming mobile phone signals are demodulated to obtain the actual information from the radio-frequency signals. The necessity of a compact and modular design combined with Optimum service-friendliness has essentially led to all the manufacturers in the market designing base stations as rack systems with backplane and daughtercards.

RF-Links within the base station

Signal path of a connector

The newly defined radio standards require higher transmission frequencies, bandwidths and data rates from the system hardware. For base stations this means in particular homogeneous signal transmission on PCBs, PCB transitions and cables. Each section of such a signal path can be described in terms of its characteristic impedance Zw. If the characteristic impedances Zw1, Zw2, Z, Zwn, of various line sections are equal (i.e. Zw1 Zw2 = … = Zwn). the signal transmission is homogeneous and ideally causes no interference such as signal reflection or signal attenuation. The greater the difference between the characteristic impedance of the line sections, the greater the signal reflection and signal attenuation will be at the interfaces. Every connector is also part of such a signal transmission and due to its interior, it constitutes a series of several line sections.

Fig. 3 shows a cross-section of the established Mini Coax connector together with the interfaces of the daughtercard and backplane. With this measuring set-up, the RF signals are injected at points A and G. The time domain reflectogram in fig. 4 represents the course of the characteristic impedance Zw as a function of the place along the signal path.

Time domain reflectogram

The centre line around which this characteristic moves is the 50 Ohm mark. For very high frequencies, the size of the characteristic impedance of a signal transmission is derived by first approximation from the root of the line-specific inductance to capacitance per unit length. Consequently, a collapse of the characteristic impedance is attributable to capacitive influences (so-called capacitive discontinuities) and an increase in characteristic impedance is attributable to inductive discontinuities.

To sum up, it can be stated that extreme demands are placed on radio-frequency links. They have to be small, stand up to harsh physical treatment and compensate for tolerances between daughtercard and backplane, must not cause any appreciable signal reflection or signal attenuation, and must guarantee reliable contact for at least 15 years. It is the combination of these requirements, which constantly sets new challenges for the manufacturer of suitable connectors.

The Mini Coax+

Angled module of the new Mini Coax+ for daughtercard placement

HARTING has been successfully developing the Mini Coax connector for more than 8 years now. Millions of them have already been installed in the base stations of leading mobile communications companies. Thus, HARTING was the first company in the market to establish coaxial multiline connectors for board-to-board RF links.

For base stations of the 3rd cellular radio generation HARTING is presenting a coaxial multiline connector (see fig. 5) which meets even peak requirements far in excess of the framework specifications of the IMT-2000 standard:

Signal transmission characteristics

The most important measure for drastically reducing reflection and attenuation of high-frequency signals up to 4 GHz is the avoidance of interfering discontinuities. This has been achieved principally by changing from the press-fit technique to an SMT design for the signal conductors (Surface Mount Technology).

 

Robustness

When plugging the daughtercard into the backplane and, to a certain extent, also during Operation, shear forces can act on any connector. For this reason the established "Pin-in-Hole" concept was adopted for the ground pins. They are realised in SMC design (Surface Mount Compatible).

Fully automatic assembly

The combination of SMT and SMC selected by HARTING ensures the fully automatic assembly and soldering with the reflow process.

In this way, the Mini Coax+ combines economical PCB assembly and robust design with excellent RF characteristics, thus making it the perfect candidate for applications specifically in the 3G telecommunications market.

HARTING's "Forum High Frequency" at the electronica 2000 offers all interested parties a platform for discussing various task definitions in the area of PCB RF transmission. We look forward to your visit.