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Bypassing Effectiveness

I recently ran across an analysis by G4OEP of the effectiveness of multiple bypass capacitors. His work, available at http://g4oep.atspace.com/caps/caps.htm, looked at the effectiveness of parallel capacitors, measured by their transient response.

I thought it would be useful to repeat G4OEP's work in the frequency domain, by examining swept frequency responses of various bypassing configurations over the range 300 KHz to 3 GHz.

The technique I used is to measure the loss between two BNC connectors with various capacitor combinations shunting the signal to ground using a home made test fixture. The shunting loss is measured by an HP 8752B vector network analyzer. 0 dB loss corresponds to a BNC F-F adapter.
 

The test fixture. The height of the BNC center conductors above the ground plane is approximately 1/4" (6mm).

Fixture Reference Loss

The test fixture is on the crude side and itself shows significant loss above a few hundred MHz, as can be seen by the following sweep. The horizontal axis is log and the major divisions are 1, 10, 100 and 1,000 MHz. At 2 GHz, the loss in this simple test fixture is about 3 dB. If we take the 1 dB point as the cut-off for reasonably acceptable results, we can trust the data up to about 1 GHz.


 

82uF Electrolytic - Long Leads.

The first bypass we try is an 82uF electrolytic capacitor, with long leads (3/8", or about 9mm).

If we set 30 dB loss as the threshold of acceptable bypassing, the single electrolytic capacitor is effective up to about 7 MHz, with 25 dB loss achievable through 10 MHz.
 

82 uF Electrolytic - Shorter Leads

 

Our first attempt at improving the effectiveness is to reduce the lead length to the minimum possible, or about 1/8" (3mm) for this fixture.
Shortening the leads helps, with the 30 dB point now 20 MHz. At low frequencies where the lead inductance is negligible, both test configurations show about 42 dB shunt loss.

 


82 uF Electrolytic paralleled with 0.1uF.

A common strategy to improve bypassing effectiveness is to parallel large value capacitors with smaller value capacitors. One potential problem—the reason G4OEP conducted his transient tests—is a concern that the stray inductance will resonate with the smaller capacitor and thus reduce the bypassing to ineffectual levels near the resonant frequency. G4OEP tested for resonance by looking for ringing when the bypass filter arrangment was subject to a fast rise pulse; our examination will be more direct by looking for resonant effects in the frequency domain.
 

0.1uF shunting the electrolytic. The total lead length is under 0.25" (6mm).
We see definite signs of resonance in this configuration, with a notch at 8 MHz.

Still, paralleling the 0.1uF capacitor provides significantly more effective bypassing than the electrolytic standing alone. At 100 MHz, the single electrolytic provides about 17 dB shunt loss, whilst the parallel combination increases the shunt loss to about 26 dB at 100 MHz.  There's only a small increase in worst case bypass effectiveness with this combination.


82 uF Electrolytic paralleled with 0.1uF and 1000pF

If paralleling a 0.1uF helps higher frequency response, perhaps paralleling a 1000 pF will offer even more improvement.
 

Three paralleled capacitor bypass configuration.
We see the 1000pF has added a second resonance, around 50 MHz.

Comparing the three capacitor arrangement with the two capacitor configuration, we see at around 50 MHz, the two capacitor arrangement is slightly better. However, at 100 MHz, three parallel capacitors offer better performance.

The specific resonant frequencies and overall performance are, of course, highly dependent upon lead length and circuit layout, so take these measurements as only indicative.


4.7uF Tantalum Capacitor.

Tantalum capacitors offer less self-inductance than conventional electrolytic capacitors and should, therefore maintain bypassing effectiveness to higher frequencies, all else being equal.
 

4.7 uF tantalum capacitor installed with minimum possible lead length.
Since the capacitance value is only 4.7uF, we don't expect nearly as much shunt attenuation for low frequencies compared with the 82uF electrolytic. 

We see less than 30 dB attenuation throughout the frequency range, but note that 20 dB attenuation is possible up through 60 MHz.  This is a bit better than the 82uF electrolytic, and the tantalum is a much smaller package.


Surface Mount Capacitors - Series 0.22uF.

If lead inductance is part of our problem, suppose we try a leadless, surface mount capacitor. In this case, in order to physically fit, I've made a series stack of two 0.22uF 1206 surface mount capacitors, for a net of 0.11uF.
 

Not very pretty, but it works.
As expected, we don't see much attenuation at low frequencies, since the total capacitance is only 0.11uF.

There's a clear resonance at 9.5 MHz, and our arbitrary 30 dB cutoff point is about 43 MHz, with 20 dB attenuation available through 150 MHz or so.

 


Series Ferrite Bead

An alternative to shunting capacitance is series inductance. In this case, we'll try a multi-turn ferrite bead, Steward 28C0236-0EW-10, 3-turn device, with a series impedance of 998 ohms at 100 MHz. http://www.steward.com/web_info/CADPrints/Sales/28C0236-0EW-10-F.pdf. Ferrite beads are intentionally made of low Q material so that they offer both series reactance and resistance to high frequency signals and to minimize resonance effects.
 

The data sheet shows that at 100 MHz (the bead's self-resonant frequency), the bead appears as almost a pure resistance, with zero reactance. This is possible because the ferrite material used has high loss. You would not chose this material for a resonant circuit, but it's ideal for supressing EMI or bypassing undesired frequencies.

Above its 100 MHz SRF, the bead looks capacitive, probably due to turn-to-turn parasitic capacitance. Still, the data sheet demonstrates useful series impedance up to 1 GHz.

 

3-turn ferrite bead in series.
Not much effectiveness until we reach 50 MHz or so. The bead remains useful up through 1 GHz, however.

Bead with 2x 0.1uF bypass capacitors

In addition to the series ferrite bead, it's possible to bypass the leads with capacitors as well, thus combining shunt and series actions. We start with 0.1uF bypasses on both sides of the bead.
 

Ferrite bead with 0.1uF bypasses on both sides.
Now we are starting to get someplace—30 dB or more attenuation over the range 1 MHz through 400 MHz, with a best case attenuation exceeding 70 dB.

Bead with 2x 0.1uF and 2x 1000pF Bypass Capacitors

We can extend the parallel capacitor strategy by adding 1000pF capacitors to both sides of the ferrite bead. This should improve the high frequency loss.
 

Pi-filter with paralleled shunt capacitors and series ferrite bead.
Adding 1000pF capacitors improves the shunt loss to at least 30 dB up through nearly 1 GHz.

Conclusion

There's no single approach to bypassing, as we must start with an understanding of the frequency range over which we wish the bypassing to be effective. And, we must know the impedance of the circuit being bypassed. As the impedance drops, the bypassing loss of a given shunt capacitance decreases, but the effectiveness of series inductance increases.

The network analyzer plots presented here are taken with a 50 ohm impedance. If the circuit being bypassed is a low impedance power feed, then different component values will be necessary. Likewise, a high impedance control circuit may call for different component values. For example, component values for bypassing RS232 serial data lines require knowledge of the data speed as well as the RF frequencies to be protected against.

The data also supports the long accepted practice of paralleling different value capacitors to extend the effective bypass frequency range. Although in some circumstances resonant circuits can be created, the overall effectiveness still is improved over a single value part.

The long-held practice of keeping lead lengths as short as possible is certainly valid and should be followed, as well as using components suitable for the frequency band to be bypassed.

However, resonances and bypassing effectiveness are critically dependent upon circuit layout and associated strays. Accordingly, take these measurements as representing only concepts and suggestions, not hard and fast rules.