Frequency & Microwave
Choosing blocking capacitors – it’s more than just values
This article explores improving Radio Frequency (RF) performance, but with less capacitors that, in their ideal form, block DC current and pass AC current. This makes capacitors a fundamental building block in RF and microwave systems. They are often used to create filters, generate DC protection, and to create bypass networks. Often designers use rules of thumb or approximate equations to link capacitor values to final RF performance. As system requirements constantly require higher performance these assumptions are no longer valid. Here at Knowles Precision Devices DLI facility, we looked in detail at real world performance that can’t be predicted by design theory, and then took measurements with common capacitor bypass networks to support our analysis
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n the case of bypass applications, capacitor values are carefully chosen to provide a low resistance ground path for unwanted noise signals generated by switching power supplies or high frequency noise coupled into the system. Using the ideal capacitor impedance: Eq. 1
(Where f is the operating frequency and C is the capacitance value in Farads). An engineer or system designer can easily calculate the theoretical capacitor values needed to provide a low resistance path to ground at a given frequency. For designers looking to have broadband RF isolation, capacitors are the go-to components to complete the task. In practice, actual capacitors are modelled as a combination of capacitors, inductors, and resistors. At resonance the parasitic inductor and capacitor cancel out and an impedance low is realised. Above resonance the response of the inductor begins to dominate and cause the positive impedance slope as shown in Fig. 1. A common design rule is to consider three or four capacitors in shunt to ground, each
with different values to provide broadband RF isolation. The end result becomes clear looking at the dashed red curve in Fig. 1. This shows four capacitors curves superimposed, providing a broader frequency range of low resistive paths to ground for unwanted noise signals. Many designers go this route based on
the recommendations made by the reference designs from the active device manufacture. Fig. 2 is borrowed from a major manufacturer of amplifiers and shows the approach they recommend. Bypass capacitors are identified with yellow bubbles.
In this case the manufacturer is
recommending 10 capacitors per MMIC to provide adequate grounding for DC supply lines. When taking such an approach one needs to consider the RF performance of the system as a whole. Rather than looking at the capacitor in isolation and relying on the performance predicted in Fig. 1, we should ask if the RF performance of the capacitors in practice is optimal for a design.
One aspect of capacitor RF performance is driven by parasitic inductance as discussed earlier in the article. The parasitic inductance causes the positive slope after resonance and drives real world capacitor performance away from what is predicted by the ideal values of eq. 1. The majority of unwanted inductance in the plots previously shown in Fig.1, arises from the contact pad geometries.
One can calculate Fig. 1: Capacitor impendance curves 20 June 2018 Components in Electronics
approximate values if an assumed current (I) flows through the capacitor:
Fig. 3: Broadband RF response of V-Series capacitors in shunt to ground
1. Under this assumption the flux is calculated with Ampere’s law presented in eq. 2:
2. Where the magnetic flux density (B)
around a closed path is equal to the current (I) times the free space permeability constant ( _0). Once flux density is calculated the total flux through a surface (ie contact pad) is determined with eq. 3:
3. And inductance is then related to flux through eq. 4:
4. Through the magic of math and some algebraic elbow grease one can arrive at an equation that shows inductance is inversely proportional to contact pad geometries.
In other words, if we can increase the contact pad size we can reduce the parasitic inductance. Using new manufacturing processes, DLI has managed
to provide a larger pad area in equivalent capacitors footprints, without compromising on the voltage rating.
Equations and datasheet Fig. 2: Typical bypass capacitor recommendations
recommendations only get an engineer so far. At some point the “rubber must meet the road” as the saying goes. This is where DLI’s state of the art RF and microwave test lab verifies RF performance through a series of RF measurements of the V- Series Capacitors and guarantees performance. Bench measurements show that by optimising a blocking capacitor for RF performance, superior rejection can be achieved. Furthermore, that RF
performance can be optimised for different system needs. Fig. 3 shows the S Parameter response of DLI’s 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 per cent efficiency. Using Measured Bypass Network results, DLI took the investigation one step further to consider what the performance of these devices could achieve in minimising recurring harmonics
from a hypothetical tone. To block frequencies below 100 MHz, they compared a traditional capacitor bypass network pulled from an industry datasheet (consisting of three capacitors ranging from pF to nFs), V-Series Capacitors, and a DLI UX series capacitor in combination with the V-Series part. They then ran a few different tones through the devices to simulate noise sources and compared the results. Interestingly, the RF performance was not necessarily correlated to the choice of capacitor’s capacitance value, and verifying RF performance is the only way to guarantee performance. The other finding from this work is
related to the space efficiency of this RF performance minded approach. In the sub 100MHz range one V-Series capacitors provided measured performance equal to or exceeded that of the standard multi- capacitor approach. This is an 82 per cent reduction in footprint area without a compromise on performance. For designers whose costs rise substantially with increased footprints and component count, a solution with the V-Series capacitors can minimise components counts on bill of materials (BOM).
www.knowlescapacitors.com www.cieonline.co.uk
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