Doug's picture
High Frequency Measurements Web Page
Douglas C. Smith

 Address:  P. O. Box 1457, Los Gatos, CA 95031
 TEL:      800-323-3956/408-356-4186
 FAX:      408-358-3799
 Mobile:   408-858-4528
 URL:      www.dsmith.org
 Email:    doug@dsmith.org



Technical Tidbit - February 2010
Damping Board Resonances Using Discrete Resistors

1mm spaced boards

Figure 1.
Test Setup For Measuring The Resonance of Two Copper-clad Boards Connected by a Wire

Abstract: When circuit boards are connected by conductors, a resonant circuit results from the inductance of the conductors and the capacitance between the circuit boards. A simple way to damp such board resonances is discussed. The method can be very useful if the designer is restricted to only one connection point between two boards and applies both to a pair of boards or a single board mounted over a metal chassis.

Discussion: Figure 1 shows the test setup used for this Technical Tidbit. The test setup was also used in the January 2010 Technical Tidbit on this site where the focus was on measuring the resonance of the boards. For this article, the focus is how do damp the resonance that results when two boards (or one board and a chassis) are connected by conductors.

The setup is composed of two copper-clad boards connected by a wire about four inches (~10 cm) long, a Fischer TG-EFT pulse generator, two current probes, and a scope. The two boards are separated about one mm by a plastic bag. One current probe, a Fischer F-62b, was used to inject pulses and the other, a Fischer F-65, was used to monitor the current. The setup is described in more detail in the January 2010 Technical Tidbit on this site.

Figure 2 shows the measured current in response to the injected pulse from the Fischer TG-EFT generator. Note the ringing on the current waveform at about 34 MHz has an amplitude of about 500 mA. This data was presented in the January 2010 Technical Tidbit but the question now is what to do about it. One could connect the boards at multiple points to reduce the inductance between the boards and thus raising the resonant frequency, hopefully high enough to not be a problem. But sometimes, just moving the resonant frequency of a structure is not enough or a designer is constrained to connect two circuit boards at only one location. What can one do then about the resonance?

1mm resonance plot expanded

Figure 2. Current Measured Between Two Copper-clad Boards at 1 mm Spacing
(Vertical scale = 200 mA/div, Horizontal scale = 20 ns/div)


One possible answer is shown in Figure 3. A resistor is connected between the two copper-clad boards. Figures 4, 5, and 6 show the resulting current waveforms that resulted for resistor valuses of 360, 51, and 22 Ohms respectively. Note that 360 Ohms provides some damping and 51 Ohms more, but 22 Ohms damped the ringing in less than one cycle. The initial peak is about the same in all cases, just the damping has been changed by the resistor value.

The data suggests that the capacitive reactance between the copper-clad boards and the inductive reactance of the wire between them is about 20 to 30 Ohms, a typical value encountered when mounting a circuit board over a metal chassis at resonance. In that case, the larger copper clad board in Figure 1 becomes the chassis.

damping resistor near probes

Figure 3. Close-up View Showing a Resistor Connecting the Two Copper-clad Boards


resonance with 360 Ohm damping resistor

Figure 4. Current Measured Between Two Copper-clad Boards at 1 mm Spacing With 360 Ohm Damping Resistor
(Vertical scale = 200 mA/div, Horizontal scale = 20 ns/div)


Resonance with 51 Ohm damping resistor

Figure 5. Current Measured Between Two Copper-clad Boards at 1 mm Spacing With 51 Ohm Damping Resistor
(Vertical scale = 200 mA/div, Horizontal scale = 20 ns/div)


Damping with 22 Ohm resistor

Figure 6. Current Measured Between Two Copper-clad Boards at 1 mm Spacing With 22 Ohm Damping Resistor
(Vertical scale = 200 mA/div, Horizontal scale = 20 ns/div)


A slight variation of the experiment is shown in Figure 7 where the resistor has been moved to the opposite side of the boards from the current probes and pulse injection. Figure 8 shows the result.

Damping resistor opposite probes

Figure 7. Test Setup Modified to Place the Damping Resistor Opposite the Current Probes


Resonance witih 22 Ohm damping resistor opposite probes

Figure 8. Current Measured Between Two Copper-clad Boards at 1 mm Spacing With 22 Ohm Damping Resistor Opposite Current Probes
(Vertical scale = 200 mA/div, Horizontal scale = 20 ns/div)


The waveform in Figure 8 is essentially the same as in Figure 6. Placing the resistor on the opposite end of the board made no difference. This is to be expected when the size of the boards are small compared to the wavelength at the frequency of interest, 34 MHz in this example.

The resistor could have been added directly in parallel with the wire connecting the two boards with the same result. This technique can be used to damp board resonance when the board is constrained to a single point of connection to another board or a system chassis.

Summary: Current probes and pulse injection were used to investigate the resonance between a circuit board and nearby metal (chassis or another board) connected by a conductor. In particular, the resulting resonance was shown to be damped by a resistor of a few tens of Ohms between the circuit board and the nearby metal. The damping was independent of the position of the resistor because the copper-clad boards used were small in comparison to a wavelength at the 34 MHz resonant frequency of this example.

Additional articles on this website related to this topic are:
  1. Current Probes, More Useful Than You Think (~170K)
    (1998 IEEE EMC Symposium paper)
  2. January 2006, A Small Change Can Have a Large Effect
  3. October 2007, Using Noise Injection for Troubleshooting Circuits
  4. November 2007, Measuring Structural Resonances in the Time Domain - Part 1
  5. December 2007, Using Current Probes to Inject Pulses for Troubleshooting
    (Current Probes, More Useful Than You Think - Part 2)
  6. March 2009, Using Current Probes to Inject Pulses for Troubleshooting - Part 2
    (Current Probes, More Useful Than You Think - Part 3)
  7. April 2009, Construction of a Series 50 Ohm Termination
  8. May 2009, Using a Series 50 Ohm Termination With a High Voltage Transient Generator
  9. January 2010, Using Current Probes to Inject Pulses for Troubleshooting (Board Resonances) - Part 3
    (Current Probes, More Useful Than You Think - Part 4)
Equipment used in this Technical Tidbit:
  1. The scope used in this Technical Tidbit is an Agilent Infinium 54845a scope
  2. Fischer Custom Communications F-62b and F-65 current probes
  3. Fischer Custom Communications TG-EFT high voltage pulse generator
If you like the information in this article and others on this website, much more information is available in my courses. Click here to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems. Click here to see upcoming seminars in Newport Beach, CA.

Click here for a description of my latest seminar titled (now also available online as a WebEx seminar):

EMC Lab Techniques for Designers
(How to find EMC problems and have some confidence your system will pass EMC testing while it is still in your lab).


D-104 mike
Check out my new site, CircuitAdvisor.com containing tutorials, tech news and more! Just click on the microphone to go there. Content is added every week on technical topics so check back frequently.
Top of page
Home

Questions or suggestions? Contact me at doug@dsmith.org
Copyright 2009 Douglas C. Smith