Five Key Filter Specifications

This article published by Knowles Precision Devices Blog simplifies filter selection by providing an overview and reference point for five of the most commonly used filter technology specifications.

The basic filter circuits were explained in this article Basic Filter Circuits Explained.

To select the ideal filter for your application, you first need to understand how to define your filtering requirements, as these requirements will ultimately determine the specifications of your filter.

While there are many possible filter specifications, this post covers the following five key specifications we feel are crucial to understanding:

• Center frequency
• Bandwidth
• Insertion loss
• Out-of-band rejection
• Selectivity

In Figure 1, each of these five metrics is called out on a plot for a typical bandpass filter response.

Figure 1. An example of a typical bandpass filter response with the five key filter specifications we are examining is called out.

Let’s dive into each of these specifications in more detail.

Center Frequency

The center frequency is the geometric or arithmetic mean of the bandpass filter’s upper and lower cutoff frequencies or 3dB points. The center frequency in the example in Figure 2 is identified as f₀.

Bandwidth

Bandwidth is the width of the passband of the bandpass filter and is expressed as the frequency difference between the lower and upper 3 dB points. When it comes to bandwidth, we can also look at the relative or fractional bandwidth of the filter, which is the ratio of a filter’s bandwidth to its center frequency. As shown in Figure 3, different filter technologies are capable of different fractional bandwidths.

As shown in Figure 3, it is possible to have a fractional bandwidth greater than 100 percent. For example, if your filter has a 2 – 18GHz range, the center frequency is 10GHz, and the bandwidth is 16GHz, making the fractional bandwidth 160 percent.

Insertion Loss

Insertion loss is the ratio of a signal level in a test configuration without a filter present (|V1|) to that when the filter is present (|V2|) and is calculated as shown in the equation below.

It is important to note that you should consider insertion loss as a specification on both the Tx side, since power is a system cost driver, as well as on the Rx side, because loss impacts the overall noise figure of the receiver.

Out-of-Band Rejection

A passband filter cannot allow interference from signals outside the bandwidth of interest. Therefore, your filter needs to have the ability to reject (attenuate) out-of-band emissions. These out-of-band emissions are far from the band of interest (refer back to Figure 1) but can still interfere with the signals within the passband through effects such as aliasing. More specifically, in their recommendation document on “Unwanted Emissions in the Out-of-Band Domain,” the International Telecommunications Union (ITU) defines out-of-band emissions as “Emission on a frequency or frequencies immediately outside the necessary bandwidth which results from the modulation process, but excluding spurious emissions.”

Selectivity

Selectivity measures a filter’s ability to pass or reject specific frequencies closer to the band of interest. Selectivity is sometimes described by talking about the size of the transition band necessary to get from the pass band to a certain rejection level. The transition band’s size is often expressed as a percentage of the center frequency. Thus, a filter’s selectivity can tell us how much of the total bandwidth needs to be dedicated to transition bands.

Smaller transition bands are needed if a filter has high selectivity, which means smaller guard bands are necessary, and less bandwidth is wasted implementing these features. Therefore, high selectivity is crucial in environments where adjacent channels are close together, as high selectivity enables RF system designers to use the available bandwidth most efficiently. Additionally, selectivity is a critical specification for determining a filter’s suitability for a given application because a system’s transmission and reception characteristics are given in terms of both insertion loss in the pass band and prescribed attenuation requirements in the stop band.

Putting it All Together: A Real-World Filter Example

While we started this post with a hypothetical example based on a typical bandpass response, Figure 4 shows these five specifications called out on an S21 plot for one of our 9.5GHz surface-mount bandpass catalog filters (example Knowles B095MB1S).

Figure 4. These five key specifications are identified on an S21 plot of one of the popular filters, B095MB1S.

Source: Knowles Precision Devices