Squashing Bugs

VI Curve - capacitor

As you recall from the last post, we were able to get a few screen captures of a few components under test, but the pictures were a bit rough. Today, we still have rough pictures, but they are a lot better due to a little effort in reducing bugs and sampling intelligently.

Some of the bugs were found within Dispatch... which deserves a post of its own, but each was dealt with during a major refactor and testing was added to ensure that those bugs don't creep in again at a later time.

Capacitive Reactance

Today we were able to see some capacitive reactance as we changed the frequency. Note that the screenshots last time were all done at a particular frequency. Now, we chose a 0.22uF capacitor and varied the frequency. We can see the reactance go from nearly open-circuit to nearly short circuit.

We are adjusting the frequency by adjusting 'omega', which is the change in theta over time. As omega increases, so does frequency.

Capacitor, 0.22uF

Omega = 60, 11Hz

There is only the smallest deviation from 0mA. As each vertical division is 1.11mA, we are looking at much less than 1mA.

11Hz trace

Omega = 240, 44Hz

As we increase the frequency, we can see the ellipse beginning to emerge. Due to some hardware choices made earlier in the project, one can see a fair amount of noise on the trace. We can eliminate this in the next round.

44Hz trace

Omega = 480, 88Hz

Now we can clearly see a horizontal ellipse forming. The peak currents are ~1.1mA and the voltage is still able to traverse the entire +/-5V range.

88Hz trace

Omega = 1024, 188Hz

Now we have an open 'eye' which is about as close to an ellipse as we have available to us.

188Hz trace

Omega = 2048, 376Hz

And finally, we see the first signs of horizontal clipping. This is where the board can supply no more current and the voltage isn't making it to the ends of the x-axis.

376Hz trace

Capacitor, 4.7uF

We expect the 4.7uF to behaving similarly to the 0.22uF, but we should see the behaviors at lower frequencies. On this series, we are going to check a few more 'in-between' frequencies so that we can see what the component looks like throughout the cycle.

Omega = 60, 11Hz

11Hz trace

Omega = 100, 18Hz

18Hz trace

Omega = 120, 29Hz

We are seeing the beginnings of the short circuit on the left and right edges of the trace. This trend will continue as frequency increases.

29Hz trace

Omega = 160, 11Hz

11Hz trace

Omega = 240, 44Hz

44Hz trace

Omega = 2048, 376Hz

Note that there is still a narrow opening, but that if we continue to increase the frequency, the opening would continue to close until it was indistinguishable from a dead short.

376Hz trace

What did we Learn ?

As our capacitors are not making true ellipses, particularly at lower frequencies, the 200Ω of resistance in series with the path probably has something to do with it. It would be more ideal to directly connect the opamp output to the device; however, Microchip warned me that there was no current limiting protection on the internal opamps. That leaves me with having to limit the current in other ways. I chose a resistor. The next layout will likely have a dedicated set of opamps for the tasks.

We know that the Dispatch library is working well and that we can change the frequency on the fly by sending the omega to the omega topic via the serial port. It appears that we are at a point at which we should cut the next round of hardware. We can do more firmware development while waiting for hardware. Aside from a few reworks here and there, the harware had definitely proven that it has the base capability to perform the core operations of the curve tracer, but we have found some shortcomings of the hardware along the way that we would like to address. The next post in the series will highlight all of the changes that we wish to make to the form, fit, and function of the unit. After that, we will have the next boards on the way.

Stay tuned!