High-pass Filter 101

by Peter McNeil | Jan 23, 2025

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High Pass Filter

Frequency selection is a key aspect of RF system design. There are often times where it is desirable to ensure only a signal energy from a limited spectrum enters or progresses through an RF signal chain. In the case where frequencies below a certain cutoff point should be diminished in the signal chain, Waveguide High Pass Filters or Coaxial High Pass Filters are essential. High-pass filters present a high level of reflective or absorptive attenuation to all frequencies below the cutoff frequency. Based on the quality and design of a high-pass filter, nearly all of the signal content and even noise of frequencies under the cutoff can be removed.

As all RF filters present loss in the passband, high-pass filters also result in some loss in the passband. Other performance metric considerations are the extent of the filter rejection, or out-of-band attenuation. No filter is ideal, and high-pass filters don’t have a perfect cutoff under the frequency limit. The filter attenuation gradually increases over frequency under the cutoff at a rate that depends on the filter design, complexity, and quality. More expensive and specially designed filters will present better filter response. However, for many high-pass filter applications, high performance isn’t required, and adequate attenuation and filter response steepness can be achieved without extensive complexity. There are a variety of high-pass filter design approaches that can be achieved with a simple RC/LC lumped element circuit, resonator, or relatively simple active circuits.

High-pass filters are often used when there is low frequency signal content that may enter the signal path. For instance, there may be a variety of lower frequency systems on a platform or congestion at lower frequencies that is beneficial to remove before attempting to apply further conditioning to a signal. A key benefit of high-pass filters for some applications is that they block DC and very low frequency signals, such as analog or power signals, and allow RF, microwave, and millimeter-wave signals to pass. Depending on the operating frequency range of a system, a high-pass filter may be used to remove interference and noise for all frequencies below the operating frequency, which removes the concern of the most common naturally occurring interference/noise phenomenon.

As there are many common forms of interference at lower frequencies, such as power line noise, electrostatic discharge (ESD), high pass filters are used extensively in testing and practical RF circuits. A high-pass filter may also be useful in very wideband systems where a wideband bandpass filter may be less desirable to greater complexity and possibly higher passband attenuation.

In RF systems, there is often the need to provide or enhance the isolation between different signal paths. A key component used for this purpose are RF Isolators. Isolation is the degree in which two or more RF paths are coupled. Given the nature of electromagnetics and RF coupling, any and all conductive pathways are coupled to some degree. In some cases, this isn’t an issue, as the frequency ranges or filtering components can handle undesirable signal content or interference. There are many cases, especially in precision communications and sensing applications, it is critical to have high levels of isolation between specific signal paths.

Isolation is generally measured as the difference in power level between an injected signal and the leaked signal that is coupled into another port on a different signal path. The figure-of-merit, Isolation, is most often expressed in decibels (dB), so a higher negative value means that there is less coupling, or more isolation if the value is expressed in positive polarity. In many cases, the isolation between a switch when closed is on the order of -20 dB to -65 dB, with -65 dB being exceptional Isolation.

The role of an RF Isolator, such as a RF Coaxial Isolator based on a RF Circulator, is to only allow for signal energy traveling in one direction to pass through the Isolator with minimal insertion loss. For signals traveling in the other direction, the signal energy is terminated in a matched load, or terminator, and absorbed. This function prevents the injection of signal energy traveling in any other direction than the desired signal path direction from entering through a node in the signal path.

A prime example of the use of an RF Isolator is to place the Isolator at the output of a device or node that may experience damage or desensitization to substantial reflections of the signal energy. This can be the case in a radar or communications system where the transit and receive paths are switched to the antenna. When switching off from the transmitter there may still be some high power transmit signal energy at the input of the switch may be reflected back to the transmitter power amplifier (PA) and signal chain. Having a RF Isolator at the output of the PA prevents a significant portion of the reflected signal energy from entering the PA output node.

Peter McNeil | Nov 02, 2022 | DC Blocks

A RF DC Block is a critical component of many RF systems where power is carried along the same paths as the RF signal. This function is essential for some devices. However, the presence of DC on a signal path is also detrimental to the performance of some DC components and devices. Hence, there is a need for a component that can be connected inline with the RF signal path that can block the DC voltage at that given point while still allowing for RF signals to pass through with minimal degradation.

Essentially, a DC block is a high pass filter, and are generally even constructed like them. The basic design of a DC block are capacitors in series with the transmission line conductors, either one of either or both. As many DC blocks are based on coaxial connectorized inline assemblies and modified coaxial transmission lines, the designations of the types of DC blocks are typically references to the inner and outer coaxial transmission line conductors. The inline capacitor can be in series with the inner conductor, outer conductor, or both. Inner DC Blocks have the capacitor inline with the inner conductor, while outer DC Blocks have the capacitor inline with the outer conductor. A DC Block with a capacitor inline with both the inner and outer conductor is commonly called an Inner and Outer DC Block.

Image of a DC block

Inner DC Block

An Inner DC Block is best suited to blocking DC and low-frequency audio currents, but only minimally impacts the transmission line performance. Outer DC Blocks directly prevent DC current on the outer conductor and can also be used to protect downstream RF devices and components from surges along the transmission line outer conductor/shielding. Inner/Outer DC Blocks are able to provide both benefits at marginally greater transmission line performance degradation than just an Inner or Outer DC Block.

In some cases, DC Blocks are also paired with attenuators or have different coaxial connectors on either side of the DC Block. This can be useful in reducing the BOM and footprint of a signal chain without sacrificing function or performance.

Just like with other passive RF components, there are a few basic performance parameters and specifications that are considered the most critical:

Block type
Characteristic impedance
Connector type (in series and out series)
Connector gender (in series and out series)
Minimum frequency
Maximum frequency
Maximum VSWR
Maximum Voltage (voltage withstand)
Operating Voltage (DC)
Input power maximum (CW)
Insertion loss

Peter McNeil | Feb 06, 2025 | RF Couplers

There are many instances in military/defense, aerospace, space, and naval applications where there is a need for sampling signals from high power transmission paths or monitoring the frequency of high-power signal paths. In these cases, the best RF component for the job is often an RF coupler. However, not all RF couplers are designed for extreme environmental applications indicative of the requirements for defense applications.

For instance, many RF couplers have coaxial connector ports either for the primary signal paths or the coupled ports. If the coaxial connector ports are coaxial types that use dielectric spacers, it may be that the dielectric material used can’t sufficiently perform under extreme temperature ranges specified for military grade coaxial connectors. Other environmental features include corrosion resistance, shock, vibration, humidity, altitude/pressure, etc.

Generally, standard waveguide flanges can meet all of these requirements, possibly with the exception of corrosion resistance. This is why many waveguides meant for defense applications are gold plated; this helps prevent corrosion while still maintaining high conductivity for low loss performance. A common standard for body plating specifications for waveguide couplers is MIL-DTL-5541.

Even with waveguide couplers, if the attachment methods of the coupled ports aren’t designed to withstand shock and vibration, the joint between the main waveguide and coupled waveguide may degrade or even fail. Hence, it is often imperative to ensure that even waveguide couplers are designed to meet MIL-Spec standards for ruggedness. For altitude/pressure performance, couplers often need to be sealed, either hermetically or with epoxy, to prevent outgassing or debris ingress to meet defense standards.

RF couplers designed for defense applications also tend to be specified to operate over certain useful frequency ranges and power levels for defense use cases. Examples of this include RF directional waveguide couplers designed to operate from 100s to 1000s of Watts and in key radar frequency bands, such as C-band, X-band, Ku-band, etc. These couplers will often have high peak and continuous wave (CW) input power ranges specified, as radar applications tend to have high peak power outputs.

All of these factors considered together likely indicate that a RF coupler has been designed to meet defense applications, even if it is not explicitly identified as such.

High-pass Filter 101

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