Mini Coax+ goes UMTS

In the multimedia age of constantly increasing data rates and ever greater integration of electronic components, the importance of radio-frequency transmission characteristics, e.g. cross-talk, insertion loss, reflection behaviour, is growing all the time, especially relating to electromechanical components.

Mini Coax+ goes UMTS

For every future- and technology-oriented component manufacturer who is not satisfied to simply reproduce the state-of-the-art, it is essential to have highly qualified personnel, measuring and simulation facilities and in-house know-how.

Especially in the radio-frequency range, sophisticated measuring technology and comprehensive simulation tools accompany new products from their conception through development, design optimisation and on to readiness for use in the customer application.

For some time now, HARTING has been supplying a miniaturized coaxial connector - a "blindmate" RF PCB connector of the press-fit type - which operates reliably far into the gigahertz range and satisfies the most rigorous physical requirements.

The challenge

The challenge

The press-fit technique, where advantages - of physical robustness, easy handling, reliable contact making even without soldering - are undisputed, can prove to be an unacceptable bottleneck at data transmission rates which are beyond those currently demanded. For very rapid RF signals the plated through hole (PTH) acts as a kind of barrier. A large proportion of signals entering the connector are reflected or the signal passing through is distorted on arrival at the receiver. But the signal path within the connector is also significant. Unfavourable geometries such as sharp-edged angles can have a negative effect on uniform characteristic impedances of the RF link.

In cellular radio base stations, the principal application for Mini Coax connectors, maximum signal frequencies of 1.8 gigahertz have been used up to now. In the new, much more compact stations, signals at maximum frequencies of up to 3.7 gigahertz are transmitted. The higher the frequencies, the stronger the interfering signal reflections will be. Since the pre-amplifier output is at low impedance in the transmitting state, returning energy can lead to a complete overload - and even damage the amplifier beyond repair. Even if he amplifier can be stepped down, this at least produces a reduction in the useful signal level.

 

The plated-through holes (PTH) required for the press-fit technique have a diameter of approx. 1 mm, whereas the two microvia types (blind and buried via, see fig. 1) have diameters that are around 5 to 10 times smaller. If RF signals are to be transmitted in a defined manner, one or more signal return conductors must be connected to ground at low impedance with an exact diameter ratio between signal lead and mass lead, without the impedance of this coaxial structure changing. For the level of impedance, the following applies with adequate precision:

With a conventional PCB material, the internal diameter of a ground pad would have to be more than 6 mm in order to ensure the characteristic impedance of 50 ohm which is usually required. This is inconceivable with the termination technique of current connectors.

Cross-section showing the design of a multilayer PCB with the tree most important types of through hole for placing components

Furthermore, a ground recess of this size would promote cross-talk between adjacent signal layers on an unacceptable scale.

Project background

The specification for designing the new connector was that the reflection losses, including PCB effects, must not exceed 10% over the entire frequency range up to 3.7 GHz. (Comparative measurements confirmed that when the press-fit technique is used, losses of 30% or more must be expected.) As the PCB, a 26-layer backplane made from low-loss material (GeTek) 6.5 mm thick was taken. To utilise the reliability and robustness of the predecessor, the "mating pattern" of the ground terminations was retained. In this way, quasi backward mating compatibility was achieved for the new model.

Reflection and insertation

Proposed solution

For reasons of consistent handing, the ground terminations are now designed not as press-fit-contacts but for the PiHIR technique (Pin-in-Hole Intrusive Reflow). The signal connections - surface mountable and also reflow suitable - are mounted on SMT pads with integrated micovias, and the significantly smaller diameter of the microvias ensures that the necessary insulation distance from the ground pins is maintained. The lack of coplanarity in relation to the PCB surface can be a problem with SMT. The requisite tolerance compensation is provided by a slide mechanism of the internal lead female contact, which is given a U-shape. At the same time, this concept provides adequate heat supply between PCB surface and connector floor during soldering.

RF Design

Side view of the angeld Mini Coax connector

Although mating compatibility with the previous design sets certain geometric conditions, nevertheless the transition to the SMT technique demands a largely new connector development since scarcely a single detail remains unchanged. Starting with a fundamental analysis of the existing Mini Coax connector with press-fit technique, the development involves exporting the 3D CAD design data into an RF computer simulation program which also operates 3-dimensionally and uses the finite element method (FEM). In this computer simulation program, a volume networking of the connector into numerous small cells (the finite elements) takes place, for which the electromagnetic propagation processes are then calculated. The available simulator is capable of both static 2D and dynamic 3D simulations (full-wave-analysis). By means of calculations of the electromagnetic field distribution and also the line parameters, those parts of the connector and/or PCB which cause deviations from the normal impedance can be specifically analysed and optimised. The simulator also permits automatic design adaptation if one of the target variables, e.g. impedance, is mandatory.

In the present case, simulations resulted in the multiple angles of the female contacts used up to now being replaced by only two less pronounced angles. The die-cast zinc cavities, which partly form the external conductor, were optimised, as was the contact area. Here, the slide mechanism proved to be the most demanding detail. Finally, the SMT solder bucket had to be adapted to the microvia and the ground recesses.

Result

FEM volume networking of the connector

In short, the required specification - a maximum of 10% reflection losses for the entire system, including PCB effects - was achieved without any limitation (see fig. 2) The microvias were optimised so that they are virtually "invisible" for the RF signals and therefore no longer represent a barrier. All the simulation results were confirmed by wideband RF measurements (network analysis). The HARTING Mini Coax+ with SMT technique was designed to give maximum RF performance by means of consistent and systematic utilisation of the development-accompanying design tools. This product makes it clear that it is possible to reconcile the often contradictory physical and handling requirements and transmission requirements, fulfilling both in equal measure.