Filters for High Frequency Systems
By Ron Demcko, AVX

Today’s miniature, portable high frequency communication devices require added consideration when it comes to filtering. Portable high frequency communication operating voltages are typically low, frequencies and data rates are high, and government regulations are complex and ever more demanding. These combinations make effective filtering critical to a system’s performance.

For ease of discussion, we will break “filtering” into two areas of discussion:

• Power supply filtering
• RF (signal) filtering

Power Supply Filtering
The quality of voltage is the foundation of the design. The capacitors chosen for power supply filtering should have low equivalent series resistance (ESR), low equivalent series inductance (ESL) and stable capacitance. Many times, a combination of high capacitance tantalum (or electrolytic) and MLCC capacitors are used in parallel to improve the frequency response of the filter. See Figures 1A and 1B.

The parallel combination of capacitors allows designers to easily tailor an effective filtering solution for each specific application. Most capacitor manufacturers have simulation software that allows the engineer to perform frequency analysis on their web sites. The ease, cost and effectiveness of this filtering method ensures its broad and continued implementation.
The use of miniature SMT FeedThru filters is also common practice at the Vcc of transistors and field-effect transistors (FETs).

SMT FeedThru filters offer a simple to implement, miniature, single component solution for broadband filtering. A typical filter may offer a -40 db attenuation across a frequency spectrum bandwidth of over 500MHz. See Figure 2.

FeedThru filters are low Q filters capable of broadband EMI filtering. They are non-ferrous based LC T filters that have up to 5 amps of FeedThru current capability in a 1206 package. In comparison to discrete LC T filters, FeedThru filters can provide significant advantages, including ease of use, small size and performance.

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 and 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.

RF Signal Filtering
As briefly referenced earlier, IEEE 802.16 and 802.11 families of standards specify broadband and high data rate wireless connectivity. The higher the data rate of a transmitted signal and the more bands used within a system, the higher the signal-to-noise ratio required for the system. Add the consistently evolving government emmission regulations and the requirements for filtering become very complex and demanding.

To improve the signal-to-noise-ratio, insertion losses must be minimized in addition to developing bandpass and reject definitions that are clear, mathematically significant and constant with system age and operating environmemts.

Filters
In order to minimize physical size, cost and layout complexity, end manufacturers use a limited number of components to perform filtering.

One very useful type of filter that has evolved over the years is the SMT Band Reject Filter. The design of miniature bandpass filters (BPFs) can be accomplished through the use of thin film manufacturing techniques and computer aided design models. These methods have allowed manufacturers to create an incredibly efficient capacitor based filter rather than the traditional, larger, more complex twin T filter network. The resulting capacitor based filter exhibits a very narrow band notch with high attenuation. Additionally, the BPF has minimal insertion loss out of band.

One of the main advantages of a capacitive based BPF is that an unexpected problem can be fixed with relative ease once the pcb design is in place. Most of the time the exact value of capacitor needed is experimentally determined by placing different capacitors on the pcb and then monitoring the filtering response. The trial and error method is used since the filter’s performance is affected by the surrounding environmemt and pcb. The most common area of implementation is between the PA and antenna matching network.

Other uses of a BPF might be to improve receiver desense problems in a multiband system. In this particular case, a BPF can be used in environments that have simultaneous RF frequencies operating at once and thereby improve receiver sensitivity through filtering an appropriate frequency spectrum.

Another area of use could be to filter unwanted intermodulation products. The high Q characteristics of the thin film, capacitor based BPF are ideal to selectively eliminate the IMD spectrum of concern.

Why Thin Film BPFs Work
Most BPFs are based either on discrete components or on an integrated ceramic structure that is a fired ceramic technology. In that fired ceramic, layers of ceramic dielectric material and metal alloy electrodes are interleaved and then sintered at high temperature. This technology potentially exhibits component variability in dielectric properties (losses, dielectric constant, and insulation resistance) as well as variability in electrode conductivity and physical size.

Thin film BPFs eliminate these variances. The same thin film technology that is commonly used for producing semiconductor devices is used to create the BPF. Applying this technology to the manufacture of BPFs has enabled the development of components where both electrical and physical properties can be tightly controlled. For example, line width variations are less than 1um and layer thickness can be controlled to 100Angstron.

These manufacturing features create a thin film capacitive based filter that has repeatable frequency response.

Steps to Implement Thin Film BPF
It is very easy to implement a thin film capacitive based BPF in a design. First, determine the center frequency that needs to be eliminated. Next, select a case size and use self resonant frequency charts and S21 plots for that case size to determine the BPF for evaluation.
Thin film capacitive BPFs save system size, weight and complexity while improving system reliability, manufacturability and performance.

Materials
New materials have also played a large role in the ability of 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 high voltage, repetitive ESD pulses of 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 (0201 or 0402 case size) by replacing a zener diode package as well as a discrete capacitor. Implementation is very easy since the MLV can be placed on the EMC capacitors’ solder pads. A board performance evaluation can occur without pcb redesign.

Design engineers have several options to help them minimize the PCB footprint while meeting the filtering requirements of portable electronic applicatitons. By utilizing these innovative components, designers can reduce the size of their design, improve performance characteristics, and enhance the functionality of their design.

AVX Corporation
www.avx.com
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