RF Board Miniaturization: A SMT Component Manufacturer's Perspective
The trend of miniaturization in electronic components is a given, as it is well documented that consumers want smaller, lighter electronic products with more functionality. This is achieved by either making components on the board physically smaller, using undertab terminations, or by modifying the design to reduce the number of components required to achieve the performance goals. The concept of an array or module can typically save more space than simply using smaller discrete parts, as well as increase performance characteristics.
For example, thin film couplers have recently appeared as a design solution to reduce the number of components in a design by eliminating the isolator. In the past, designers used isolators at the PA output to the antenna in order to transmit the forward signal and reject any reverse signal coming from the antenna because of mismatch. Input and output impedances are typically 50 Ohm. Many designers would like to get rid of isolators because they are bulky, expensive and add error because of temperature variations and ~ a 1db insertion loss. Thin film couplers are an ideal replacement part, as they are available in case sizes as small as 0603 and feature a 3w power rating. Thin film isolators have a low insertion loss ~ 0.25dB and high isolation ~ 16dB.
Another example is thin film capacitors.
Thin film capacitors exhibit extremely repeatable frequency responses can be used to replace some Band Reject filters. A typical Band Reject Filter (BRF) might utilize a total of six components and can be potentially replaced by a single high performance thin film capacitor. Advantages of the thin film are a significantly small footprint, extremely high performance and ease of end use implementation.
Yet another option for band pass filter miniaturization is to use an integrated passive device. This component can integrate the capacitors and resistors used in a high performance twin T filter (for example). The design can be maximized for electrical performance and shrunk into a single package due to thin film fabrication. Thin film integration of passive component naturally lends itself towards modularization trends within the RF world
Thick film integration has a role to play as well particularly in higher power applications. One way to modify a design concept in order to save board space is by replacing discrete LC T configured filters in a low Q filter with FeedThru filters. A FeedThru filter can replace 2 discrete ferrites and a capacitor in a package as small as 0805. FeedThru filters are designed to be a broadband filter – typically boasting of a –30dB frequency attenuation across a 300 mhz bandwidth. They are ideal in filtering voltage to transistors & RF PAs. Integrated thick film FeedThrus are becoming common in a variety of circuits due to their low cost, small size and impact on reliability and system manufacture.
New materials have played a large role in the ability for designers to replace numerous components with a single part. For example, AVX’s sub pf multilayer varistor (MLV) is constructed with a zinc oxide material to build a bi-directional transient voltage suppressor – essentially a back to back zener. Unlike the back to back zener , the sub pf MLV can have its capacitance minimized and still maintain an ability to withstand repetitive ESD pulses and relatively large in rush current. In the ‘off’ state, the sub pf MLV acts like an intermediate ‘Q’ capacitor to filter noise and in the ‘on’ state the sub pf MLV acts like an ultra fast turn on time bi-directional transient voltage clamp. It saves board space (0402 case size) by replacing a zener set as well as a discrete capacitor.
Design engineers have several options to help them minimize the PCB footprint. By utilizing advanced components, design engineers can reduce the size of their design, while improving the performance characteristics, and the functionality of their design.