How to Select High-Frequency Capacitors for RF Circuits

When some designers start getting into RF design, they might find that some off-the-shelf components do not operate as rated in their datasheets. Components that will be used in RF systems must be designed and rated to operate at very high frequencies that will be accessed in the system. This might sound obvious on the surface, but it is easy to forget when searching for components, and not all components will be able to meet the specifications of high-frequency systems.

In the case of passive components, there are high-frequency versions of discrete components that have higher-than normal operational specifications reaching up to very high frequencies. Some of these applications include as discrete passives on printed RF circuits, and compensation for impedance mismatch, and as filter or impedance matching networks on unique emitters.

About High-Frequency Capacitors

High-frequency capacitors are marketed as such due to their ability to retain ideal capacitive behavior up to very high frequencies. Capacitors will not exhibit ideal behavior up to the intended operating frequencies in RF systems, even if they are marketed as “high-frequency” or “RF” components. First, it’s important to note that both the construction of the capacitor itself and the PCB will create the non-ideal behavior observed in these systems.

High-frequency Capacitor Specifications

In addition to the actual capacitance value, there is a short list of specifications to look at when selecting capacitors for high-frequency systems.

• Case size: Smaller case sizes tend to have higher self-resonance, and they can access smaller capacitance values (see below).
• Temperature stability: The capacitance and other ratings of the component will change when temperature changes. These capacitors are usually ceramics, and some ceramic dielectrics like NP0/C0G have very high stability.
• Self-resonant frequency or ESL: These values might be specified on a design curve or provided directly in the datasheet. They could also be determined from an impedance curve.

While there are always other specifications required for selecting these components, the above list contains the most important specifications for most applications. The non-ideal behavior of capacitors can be examined from their equivalent circuits, which need to include PCB effects.

Equivalent Circuits for RF Capacitors

The equivalent circuit for a capacitor is well-known, especially by high-speed digital designers working on PDN impedance engineering. The equivalent circuit for a capacitor is generally modeled as a simple series RLC circuit, which gives a minimum in the impedance curve for the capacitor.

When we are operating above the typical digital range of off-the-shelf capacitors that would be used in digital systems, we also have to consider the parasitics of the pads and nearby reference planes. This means the equivalent circuit for an RF capacitor would function as shown in the model below.

Capacitor circuit model that is used at high frequencies.

Here we have the standard set of parasitic elements that appear in the typical capacitor model (ESR and ESL); these determine a capacitor’s impedance curve and its self-resonant frequency. Capacitors marketed specifically for RF systems also have these parasitic elements, but they are specifically engineered so that the self-resonant frequency is very high. This model is well-known among digital designers who have to use a large number of decoupling/bypass capacitors for power delivery.

The typical self-resonant frequency is picked out from a curve like the one shown below. Note that there is some dependence on case size when determining the capacitance of a high-frequency capacitor. Note: the example graph shown  below does not consider any of the parasitics in the above model.

The above curve is typically used by digital designers to pick out capacitors from a particular product family. For example, in the above product family, you can expect a 10 pF 0201 case size capacitor to have ideal behavior up to about 2 GHz. Smaller capacitors that are built with the vendor’s design curve shown above can reach higher self-resonant frequency values and would be more appropriate for use in very high frequency systems.

This illustrates one of the challenges in high-frequency systems: getting to very high capacitance. If there was some application where you need to have very high capacitance at very high frequency, it can be challenging and may require paralleling many capacitors to reach the target capacitance value.

Parasitics in Real Circuits

What’s different from the digital systems model is the presence of inductance and capacitance associated with the traces and pads on the leads of the capacitor (Cg and Lc respectively). If you need discrete capacitors in a very high frequency board, then you need to account for these values in your circuit model. These values are determined by the following factors:

• The size of pads and traces connecting to the capacitors
• The thickness of the dielectric in the PCB
• The distance to the reference plane below the circuit

The result is that the above curve is not necessarily observed once the components are placed on a real PCB. The parasitics modify the impedance of a circuit, which then modifies the input impedance, and these modify the S-parameters for the circuit being designed.

The typical values one would expect for parasitics around high-frequency capacitors are shown below. These values come from the fact that the pads and any intervening traces connecting to pads are like small transmission lines, so they have some self-capacitance and self-inductance that determines their performance in RF circuits. These values can be used with a desired capacitor in SPICE simulations to verify circuit behavior.

High-Frequency Passives On-Die

If you look at some production systems, reference designs, and evaluation products, you may not see high-frequency passive components placed in the typical locations for AC coupling, DC coupling, series impedance matching, or shunt impedance matching. There are very good reasons for this.

In RF integrated circuits, most impedance matching networks are integrated onto the semiconductor die; they will not be placed as discrete components outside the IC package unless absolutely necessary. This is done because:

• Equivalent elements in the RF circuit on-die have much smaller parasitics
• The smaller parasitics push non-ideal behavior out to higher frequencies

Very simply, by placing everything on-die, the RF integrated circuit designer significantly reduces the parasitics associated with discrete capacitor placement on the PCB.

This is another reason you will see many circuits that are placed with small passive integrated circuits instead of being built from passive components. By integrating all the passive components on silicon, many of the internal parasitics are suppressed and do not become obvious until much higher frequencies. The only remaining parasitics in these cases are the pads and connecting traces, which makes design and PCB layout much easier.

These integrated circuits are available in standard packaging and will normally be terminated to the system impedance. Examples include:

• Attenuators
• Passive component arrays
• Couplers/isolators
• LPF, HPF, and BPF circuits
• Power dividers/combiners

Although these components are highly integrated and will include passive elements on-die, there will still be some parasitics associated with the pin and pad transition onto the PCB. The magnitude of this depends on the component being used in your design. For this reason, simulations and tests are often needed to ensure designs with these integrated components or with discrete RF capacitors will operate to specification.

Other High-Frequency Passive Components

Due to the challenges with parasitics mentioned above, it is not so easy to just take any passive component off the shelf and use it in an RF circuit. Capacitors get the most attention because of their high-frequency characteristics in determining PDN impedance, as well as their use in RF filter circuits. However, parasitics in the pad and trace placement for these components always modify the component rating.

The other two main passives (resistors and inductors) also have high-frequency versions that are known to operate as rated into GHz frequency ranges. For example, there are wirewound inductors that are designed to operate above 10 GHz with nH or uH inductance values. These inductors are often used as filtration components and impedance matching components on power amplifiers and antenna feedlines.

Small-case wirewound inductors with the above structure can operate reliably with high Q factor up to 10’s of GHz.

Another example is high-frequency resistors, which will operate with rated resistance and minimal capacitive/inductive coupling up to very high frequencies. This occurs because of the much simpler internal construction of these components. When some point resistance is needed in an RF circuit, such as power dividers or couplers, these components can be used and can operate reliably up to much higher frequencies than RF inductors.

The FC0402E50R0BSWS high-frequency resistor from Vishay is rated to operate up to 20 GHz.

Finally, there are discrete semiconductors that are rated as reliable up to very high frequencies. Always make sure to check component bandwidths if components will be used in RF systems. Even though the components may be rated to operate at these frequencies, the self-capacitance and self-inductance in traces and pads will still affect the operation of your circuits, and this might require some field solver simulation software to estimate the effects of parasitics on circuit operation.

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