What are Bandpass Filters?

Delving into the intricate world of electronic signals, we often encounter a silent workhorse that shapes the way we communicate – the Bandpass Filter. These filters are used to isolate and eliminate unwanted noise, leaving us with the frequencies we need.

While most engineers know what bandpass filters are, there are some interesting facts about this technology that are less commonly known. For example, the foundation of modern filter theory and practice took place during World War II, and some filter types have roots from the 1800s.

As we dive into the world of bandpass filters, we explore their critical characteristics, varied designs, and the people and stories behind their evolution. Before we start, let’s define what a bandpass filter is.

What are Bandpass Filters?

A bandpass filter, like all filters, aids in removing unwanted noise from a signal.

These bandpass filters offer rejections of both low frequency and high frequency signals, to isolate a particular band referred to as the ‘passband.’

All bandpass filters have similar critical characteristics when designing. Here are a few examples, but keep in mind there are other variables depending on the specific application:

Center Frequency:

The center frequency is the frequency at the center of the passband.

Bandwidth:

The bandwidth is the difference between the upper and lower cutoff frequencies of the passband.

Passband Ripple:

The variation in the amplitude of a filter's output signal within the passband. Higher-order filters have less passband ripple than lower-order filters

Roll-off Rate and Rejections/Stopband:

The roll-off rate or rejection of frequencies outside the passband. A steeper roll-off rate means that the filter will attenuate out-of-band frequencies more quickly.

Q Factor:

Q factor generally describes specifications such as the steepness of skirts, or the selectivity, and how low the insertion loss is.

Insertion Loss:

The insertion loss is the reduction in output power, typically measured in dB, from input to output of the filter.

Input Power Handling:

Input power handling is how much input power can be applied to the filter before it is damaged or begins to degrade.

How are Bandpass Filters Designed?

Bandpass filters can be designed using different materials, each with its own advantages and disadvantages.

• Lumped Element
• Combline
• Ceramic
• Microstrip
• Waveguide

From Q Microwave’s Filter Brochure - Filter Technology Trade-offs Q Microwave Offerings in Green

The Different Types of Bandpass Filter Designs

There are three main different types of bandpass filter designs: Elliptic, Chebyshev, and Butterworth.

Elliptic

The elliptic filter is a signal processing filter with equalized ripple behavior in both the passband and the stopband. While this filter has the sharpest (fastest) roll-off of the three types, it ripples.

Did you know that a mid-1900s invention showed elliptic function band-pass filters using generalized coupling? It was patented by microwave filter pioneer Milton Dishal in 1951.

Butterworth

This filter is named after the British engineer and physicist Stephen Butterworth in 1930 when it was invented. This signal processing filter type has the flattest frequency response in the passband.

Chebyshev

Unlike Butterworth filters, these Chebyshev filters are analog or digital filters that have a steeper roll-off. They’re faster than Butterworth filters, but not as fast as the Elliptic. Fun fact: Q Microwave only creates and packages analog filters.

In many military and space applications, Chebyshev filter designs are used as they provide a quicker transition from passband to stopband, or rejections. This filter type is named after Pafnuty Chebyshev, who was an influential Russian mathematician in the late 1800s.

Visual Comparison of Bandpass Filter Designs: “Filter Design for Signal Processing Using MATLAB and Mathematica"

These three types of bandpass filters are used in a variety of applications, including radar, electronic warfare, and communication technologies.

Decoding the History of Bandpass Filters

While the bandpass filter is widely known within the engineering field, there are many facts about these everyday filters that are less known. At Q Microwave, we pride ourselves on constantly learning about RF technology to better support our customers.

Need a custom bandpass filter? There’s a reason Q Microwave has been trusted for 25+ years with low-cost production, first-class engineering, and responsive manufacturing.

Contact us to learn everything you need to know!