RF/Microwave Filters

What are Filters?

Filters are very specific circuits that perform signal processing functions, they can either remove unwanted frequency components from the signal or enhance wanted ones. In some cases, both are desirable and can be achieved. In the RF/microwave world, they are filters but operate in the GHz range, which means that traditional filter design techniques do not apply to them. Other than typical broadcasting or receiving of high frequency signals, RF/microwave filters are also commonly used as building blocks for duplexers and diplexers to combine or separate multiple frequency bands.

Filter Categories

1. Band-pass filter: select only a desired band of frequencies
2. Notch filter: eliminate an undesired band of frequencies
3. Low-pass filter: allow only frequencies below a cutoff frequency to pass
4. High-pass filter: allow only frequencies above a cutoff frequency to pass

Filter Response Types

Because of we can make certain trade-offs to enhance certain aspects of a filter, there is more than a single way to implement a filter. For example, if we sacrifice for some band ripple, we can achieve very steep roll-off for a better bandwidth performance.

Here are some common filter implementations:

1. Chebyshev (equal-ripple): The Chebyshev filter is arguably the most popular filter response type. It provides the greatest stop-band attenuation but also the greatest overshoot. It has the worst for group delay flatness (OK for CW applications such as a frequency source).
2. Bessel-Thomson (maximally flat group delay): Best in-band group delay flatness, no overshoot, lowest stop-band attenuation for given order and percentage bandwidth (ideal for receiver applications such as image-rejection filters).
3. Butterworth (maximally flat amplitude): Best in-band amplitude flatness, lower stop-band attenuation than Chebyshev, better than Chebyshev for group delay flatness and overshoot (usually used as a compromise).
4. Gaussian: This filter provides a Gaussian response in both frequency and time domain. It is useful in IF receiver matched filters for radar.

Filter Technologies

Generally, most filters are implemented by sharing some circuitry with resonators because they contain capacitors and inductors at their core. However, there are also other technologies that can be used to implement filters.

Here are the two dominant filter technologies:

Lumped: These are filters using discrete components such as capacitors and inductors. However, this technology is limited at extreme lower and higher frequencies, as inductors are either one million turns or less than a turn to provide the inductance needed.

Distributed: As shown in the “Transmission Line Theory” post, lines can be designed to achieve equivalents of circuit components. A distributed filter is usually implemented using microstrip transmission lines, but the trade-off is the bandwidth, as transmission line theory show that the device would only work at very limited frequencies.

Reading More

1. http://www.microwaves101.com/encyclopedias/filters
2. http://www.radio-electronics.com/info/rf-technology-design/rf-filters/rf-filter-basics-tutorial.php