Figure 3. Loop Used for Noise Injection
Figure 4 shows a close-up of the loops coupled to the red "grounding"
wire. The mutual inductance between each loop and the wire is likely on
the order of about 10 nH of the total loop self-inductance of about 80
nH. The loops are separated to minimize direct coupling between the
pulse generator driven loop and the sensing loop connected to the
scope. Excessive direct coupling between the loops can result in too much
loop output to the scope from the initial pulse, making it difficult to
see the resonance without overloading the scope input.
Close-up of Loops Coupled to Wire Connecting the two Boards
Figure 5 shows the output of the loop connected to the scope. Since I
knew the resonance would be at a relatively low frequency, I used
the bandwidth limit on the scope vertical amplifier to filter out
the initial pulse from the generator and show only the resonance of the
boards and connecting wire. The amplitude of about 20 mV is of little
importance being determined by the generator output, the mutual
inductance between the loops and the wire, and the vertical amplifier
bandwidth limit used in the scope.
Resonance of Plates and Wire
In Figure 6, the first few cycles of Figure 5 are expanded to measure the ringing frequency. The scope indicates a frequency
of about 37 MHz (readout near bottom of screen). A frequency in this range is to
be expected given the circuit dimensions.
Frequency Measurement of Resonance, ~37 MHz
This method is especially useful for systems where boards and system
components are connected by wires having resonances below 100 MHz. One could also use RF injection
probes and current probes to do similar measurements on longer system
cables at lower frequencies. Higher frequencies will be the topic of Part 2 of this Technical Tidbit.