First test will be conducted to establish the polarity of Pbat for the condition where the battery energy is depleting. Once known, we will have a baseline to compare against our measurements on the actual circuit.
DSO Limitations
While taking some measurements I noticed that the "mean" of any trace will have a slight variance in value depending on where it is placed vertically on the scope screen. The degree of variance is enough to be of a concern, so I made a video illustrating this particular limitation one should be aware of when using DSO's. I've also outlined a method to virtually eliminate this slight flaw and get the most out of your critical measurements. Tektronix gives you the ability to add an offset to any trace, and doing so seems to eliminate this measurement artifact. I am not certain if Lecroy or Agilent, or any other DSO brands have this feature.
I will be utilizing this method whenever I can in the upcoming measurements.
The video will follow shortly.
Here's that video demonstrating the variance of the mean measurement value on a trace as it transitions from bottom to top of the screen. Assigning a counter-DC offset to the trace seems to correct this scope artifact.
http://www.youtube.com/watch?v=4p4EwIW5YWU
This video demonstrates the use of a simple DC circuit to establish the polarity of the battery power measurement as per the connections on the schematic. In this case the polarity is POSITIVE. This circuit of course exhibits a battery whose energy is DEPLETING.
As such for the tests which will follow, a battery power measurement that is positive will indicate that energy is being depleted in the battery, and a measurement that is negative will indicate that the energy is being increased in the battery.
http://www.youtube.com/watch?v=iTZA4iY38Ds
Attached is a drawing of the circuit I'll be testing with added protection and CSR components.
The following video clearly demonstrates how parasitic circuit inductance can and will skew the battery power measurement "Pbat" when taken using a DSO oscilloscope.
Of note to remember for the video:
- A "positive" indicated Pbat tells us that the battery energy is depleting.
- A "negative" indicated Pbat tells us that the battery energy is increasing.
http://www.youtube.com/watch?v=pnZLwA2Uohs
The following segment "4" video illustrates essentially the same as segment 3 did, in that parasitic inductance between CSR probe tip and probe ref will significantly skew the Pbat measurement. There was concern expressed that I did not have the Vbat probe reference connected, and therefore it is connected in this redo. In this segment I only compare the CSR probe measurement in two positions rather than 5. The variance is dramatic still and changes from roughly -1.02 to +0.91 by simply moving the probe tip and ref 1" away on either side of the CSR body.
Video Link: http://www.youtube.com/watch?v=SietCns2r-M
The two attached pictures relate to those two probe positions and their resulting Pbat values.
Here again is the same test as segment 4 but with 20 cycles in the scope display. There is no significant difference in the measured Pbat values.
http://www.youtube.com/watch?v=CGDzic26qQs (http://www.youtube.com/watch?v=CGDzic26qQs)
Here I've done a video segment illustrating an often-encountered problem when doing power measurements on switching type circuits...GROUNDING!
A test involving a function generator and an oscilloscope is going to be troublesome, particularly when wanting to measure multiple components in the circuit. The two pieces of equipment must be isolated by some means; either by lifting the earth ground on the FG or using differential scope probes. I chose the former as will most folks attempting these measurements.
With the FG reference lead isolated from the oscilloscope's reference leads, the scope probes can be placed anywhere in the circuit in order to accurately and safely perform specific power measurements.
http://www.youtube.com/watch?v=0rtuSwfs-90
This next segment illustrates the correct location for the FG's reference lead. Previously it was connected in such a way to bypass the CSR and would therefore not capture any current the FG may be passing through the circuit.
http://www.youtube.com/watch?v=e_ZK6OPJLl4
In this segment I do yet another Pbat measurement, or 3 rather.
With the FG earth ground isolated and its reference located at the proper connection point in the circuit, we explore 3 different Pbat measurements; 1) with parasitic inductance between the CSR probes (negative mean power), 2) without parasitic inductance affecting the CSR measurement (positive mean Pbat and closer to actual value), and 3) the correct measurement without parasitic inductance affecting the CSR and Vbat measurements (positive Pbat and correct value).
http://www.youtube.com/watch?v=WXdsXvq6QAs
Two potential sources of measurement error are examined; parasitic inductance (we've looked at previously) and induced emf in the scope probe ref leads.
http://www.youtube.com/watch?v=aQKm0qVUCkk (http://www.youtube.com/watch?v=aQKm0qVUCkk)
Here we attempt to get a more accurate measurement for Pbat by using a makeshift Kelvin probe across the original CSR located close to the circuit.
Two outcomes: 1) the mean Pbat changes from negative to positive, even when the Kelvin probe is widely-spaced across the CSR, and 2) the magnitude is high but close to the actual value. The remaining discrepancy is most likely due to the parasitic inductance between the battery + terminal and the actual TP3 measurement point for Vbat.
http://www.youtube.com/watch?v=wctTbyqTj1A (http://www.youtube.com/watch?v=wctTbyqTj1A)
Next we obtain the measurement of power from the function generator (FG).
http://www.youtube.com/watch?v=s1ytR1uu8TM