I am following with great interest the On-line Newsletters of High-Speed Digital Design. In one of your recent responses regarding the measurement of power/ground planes, you are suggesting not to connect the probes to nearby points.
[Editor's Note: Please see Newsletter Vol. 2 Issue 14]
Can you elaborate somewhat further on your reasoning that 'local crosstalk will pollute your measurement'? Also, can you give some more reasoning behind your suggestion of using the resistance divider theorem in this case? Do you mean here the magnitudes of impedances?
Measuring the power and ground impedance on a complex board is never easy. I of course always welcome any other comments on the subject, pro or con.
Regarding the comment below about "polluting your measurement", the issue is that we are trying to measure a very small signal (the Vcc noise voltage). Any direct crosstalk between the signal source and the probe may overwhelm the signal you are trying to measure.
Example: suppose we have a 50-ohm sine wave source, and a 50-ohm probe.
Adjust the sine wave source so we see 1V p-p when the source is plugged straight into the probe (no power system connected).
The total driving point impedance at the probe point is 25 ohms (fifty from the source in parallel with fifty from the probe).
Now connect both probe and scope to the power system on your bare board (no active parts installed - just the bypass caps).
If the Vcc-ground impedance is 0.1 ohms (a reasonable value for a good power system), then the Vcc-ground impedance will reduce the voltage received at the scope by a ratio of (0.1)/(25) -- that's the resistor divider theorem. (In the analysis below I just converted this number to a dB figure.)
The voltage at the scope will now be about 4 mV. This is a small, but clearly visible and easily measured amount of noise.
Now let's look at the direct coupling between the source and the probe.
Couple both source and probe to the board using coax (I like to use RG-174, because it's thin, flexible, and easy to solder to the board). Let's say that at the source attachment point, the distance between the coax shield attach point (to board ground) and the coax signal attach point (to board Vcc) is about 1/2 inch, and that the coax signal conductor stands up about 1/8 inch above the surface of the board. The total exposed area of this loop between the signal conductor and the ground conductor is (1/2)*(1/8) = 0.0625 square inches.
Assume the same for the probe connection.
If the source and probe coax cables are connected to points separated by about 1 inch, the mutual inductive coupling between these two loops will be on the order of 0.02 nH. The total di/dt flowing through the source loop, times the mutual inductance between loops, will couple crosstalk noise voltages directly into the probe loop.
At a frequency of, say, 500 MHz, the total noise voltage will be:
- = Lmutual*(di/dt)
- = 0.02nH*2*(pi)*500MHz*(1V/25ohms)
- = 2.5 mV
This noise voltage is almost as large as the signal you are trying to measure. Keep the source and probe cables separated by at least a few inches, or put them on opposite sides of the board, and keep the length of the exposed coax signal conductor as short as practical.
Also, at frequencies in the multiple-megahertz range and beyond, you will notice that the impedance is a function of the position of the source and probe cables, due to various resonance effects between the power and ground planes. If you're interested in that topic, check out the cool simulations from SIGRITY (and probably many others) on the subject.
Thanks for taking the time to write.
Dr. Howard Johnson