Broadband Bypass Capacitors For High Speed Optical and mmWave

Posted by Peter Matthews on Feb 27, 2019 3:42:00 PM
Peter Matthews

Microwave and mmWave frequencies are found in RF applications and in optical communications. As we see increasing interest in mmWave for RF communications and move from 28Gbps to 56Gbps NRZ and 112Gbps PAM4 in the optical world, the system as a whole needs to maintain a high interconnect bandwidth.

To do this effectively without allowing noise into the system a truly broadband approach to noise filtering bypass circuits is required. Bypassing a system with very wide bandwidth can present challenges when selecting the right capacitor.

If DC lines are not properly decoupled or filtered, RF signals can inject RF power into other components within the system causing severe performance degradation. A Capacitor operating in shunt mode provides a broadband low impedance path to ground, acting as a filter for any unwanted noise. Looking closely at how these impedance varies with frequency is key to building an effective bypass filter.

In an ideal capacitor impedance falls with increasing capacitance, C:

 

capacitance

In an actual capacitor though we see the equivalent series resistance (ESR) due to loss mechanisms and the equivalent series inductance (ESL) from parasitics in electrodes, leads and other components. At a high enough frequency the ESL takes over as the defining factor in the impedance so that in the inductive region impedance rises due to ESL and we get the characteristic 'V' shape for the impedance vs. frequency, as seen in the plot below.

 

z vs f

Evaluating a bypass capacitor requires taking a closer look at the impedance of the device over the entire frequency range of interest, which depending on the transmission scheme can mean relatively high frequencies.

Bypassing broadband systems takes us to the inductive side of the curve - so slope is defined by inductance. By carefully selecting a capacitor that is designed to take both side of the curve into account, one can achieve broadband bypass filtering in a single component.

The following shows the S Parameter response of our V-Series parts in shunt to ground from 100MHz to 40GHz. Operating in shunt mode the capacitors provide a broadband low impedance path to ground, acting as a filter for any unwanted noise providing typical suppression values of -35dB or 98.22% efficiency.

 

broadband response of V series capacitors in shunt to ground

With a device like the V-series two advantages become apparent:

1, Replace multiple scaled capacitors with one device. Learn more about this in our article Choosing blocking capacitors – it’s more than just values.

2, Potentially, 'swap your 104s for a 103' - that is, utilize a lower capacitance capacitor that is specifically designed for broadband operation to replace a larger capacitance device that does not provide the same rejection in the inductive region. In the following plot we compare the rejection of a number of V-Series capacitors with the industry alternative. It is worth noting that the Knowles 10nF device approaches that of the alternative 100nF device.

 

broadband response of V series compared to alternative

 

 

 

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