Integrated capacitors for high frequencies
In our communications-dominated electronics world, with the focus on ‘connected’ and wireless portable devices, high frequency design and integration have become critical factors.
The integration of digital, analogue and mixed-signal circuits, continues apace, with RF and microwave circuitry more often included. Notably, however, the integration of associated passive RF and microwave components has progressed far more slowly. These components, principally inductors and capacitors, which form such a small part of a circuit diagram, can now occupy most of the physical space on a printed circuit board implementation.
Clearly, increased integration of microwave components, such as filters, baluns and couplers, promises significant advantages.
These integrated passive devices combine several discrete devices in a single package, and our approach at Syfer is to combine a proven low temperature co-fired ceramic (LTCC) technology with established manufacturing techniques to design and produce complex passive RF and microwave components as single chip devices.
LTCC products are based on a glass-ceramic composite. A device comprises several ‘green’ layers each down to 50[micro]m thick, upon which the required circuits are printed. After printing the layers are stacked then fired in a single process.
A firing temperature below 900°C allows silver conductor material to be used, offering the best possible electrical conductivity and thus the highest Q. Once sintered, the material forms a highly reliable, structurally and electrically stable component.
The combination of a high Q and the high thermal conductivity of the composite material ensures that the size reduction benefit is not offset by an undesirable reduction in power handling capability.
Contact is made between layers using conducting vias; this allows the creation of inductors. Thus it is possible to produce capacitive, inductive and stripline elements with ground planes and stripline inter-connects. The manufacturing process is a three-dimensional development of the standard process for ceramic capacitors, and retains all of its advantages: low cost, high reliability and good temperature performance.
But the design process is, of course, far more complex, and requires skills not normally found within the ceramic surface mount device industry.
A key advantage to the OEM is that with these integrated passives, board design is greatly simplified. When designing an RF filter using discrete components, the chip capacitors and printed or chip inductors are interconnected by printed tracks on a circuit board. These tracks are microstrip transmission lines, and must be designed for the correct impedance with the microstrip phase length, loss and dispersion all taken into account. The discontinuities between the microstrip lines and the discrete components, the dimensional and electrical tolerances of the PCB, and component placement tolerances, must all be included in the analysis. With integrated filters, the electrical and mechanical tolerances associated with the PCB and with the placement of discrete components, no longer directly affect filter characteristics.
Integrated filters can deliver improved performance in several respects. The interconnections between circuit elements are physically smaller and thus electrically less significant. They are also in stripline, not microstrip, thus less dispersive, less lossy and more easily analysed and synthesised. The discontinuities within the filter are less severe, and component proximity effects can be accounted for in dedicated 3D analysis software.
The finite difference time domain method of simulation used in our 3D modelling package provides the most accurate of designs in the shortest development time and with the minimum number of design iterations.
Implementing many circuit elements within a single package obviously much reduces assembly time, component cost, and circuit dimensions. Less obvious advantages are the technical design and cost benefits of reliably reproducible performance.
The reduced effort involved in in-house design and development of a discrete element filter, also reduces cost and improves time to market. Specifying an integrated surface mount filter effectively subcontracts all this design work to the filter supplier, where its cost is amortised across multiple designs and high volume manufacture.
This low temperature co-fired ceramic technology, combined with proven surface mount device manufacturing techniques and specialist design skills deliver integrated filter components for high frequency applications to give the competitive advantages of low cost, high performance and reduced time to market.
Dr John Yelland specialises in RF applications and products at Syfer.