One time we got a new batch of assembled boards that would randomly quit working. This was for an older product that we had been selling for years without issue. Turns out an IC supplier had moved their chip to a smaller process. This smaller chip had less gate capacitance and was making much sharper corners on it's square waves.
It was slow data, like 10mhz or something. We were safely under the 1/10 wavelength rule with the old, softer corners. With the new sharp corners we were over the 1/10 wavelength rule and the circuit was ringing so hard it would cause nearby chips to latch up. We added some termination resistors and it was fixed.
Also note that this kind of thing can be hard to diagnose, because when you put the scope probes on, some of the ringing energy goes into the scope and it tends to improve the situation as long as the probe is there.
>Also note that this kind of thing can be hard to diagnose, because when you put the scope probes on, some of the ringing energy goes into the scope and it tends to improve the situation as long as the probe is there.
When you're in the thick of it trying to debug, these absolutely make you want to scream. But once you figure it out and have a minute to catch your breath, they are oh so satisfying that you did figure it out.
There are properties like resistance, effective capacitance, impedance and reactance that would not intuitively be associated with a pure conductor, but neglecting little things like this can contribute to behavior that might raise its ugly head in unforseen ways if you are not aware.
What about back-emf in the antique world of vacuum tube audio working at hundreds of volts where you are driving what's known to be a very reactive but low-impedance load?
As the signal from the amp into the voice coil displaces the speaker cone from its resting position, the spring action of the cone surround works to return the cone to the non-energized point. Constantly, this has always happened.
But with tubes the impedance and voltage are so high that a major step-down audio output transformer was used to isolate the high voltage from the speakers as well as properly match the impedance.
No audio transformer is needed for solid state amplifiers since the transistors work at relatively safe voltages and impedance is not a big issue.
Either way as the speakers spring back, their voice coil moving on its own across the magnetic field, it generates a pulse of electricity from the speaker itself that appears at the output of the amplifier but did not actually come from the amp.
At high power and especially with square-wave type distortion often seen in musical instrument amps, this back-emf from the speakers can be stepped up to over 1000V on its reversed way back to the tubes through the "step-down" output transformer. When the tubes are only rated for a few hundred volts this is not ideal, and if the tube does not suffer internally, it can still cause a spark to jump between two adjacent pins on the tube socket. Once this happens both the tube and the socket can be ruined due to conductive carbon formation within the bakelite tube base and/or tube socket.
This may be an extreme example, but wires are not perfect conductors and circuit boards are even less perfect as insulators.
I learned the hard way recently that my attempt at visualizing pin states on prototype boards using LEDs without current limiting resistors was a bad idea: it caused the LED to light up when the output pin was turned HIGH, but the downstream receiver of the pin didn't register the HIGH because the LED stole too many volts
I only figured this out after building and tearing down my prototype and reassembling each of the modular bits, verifying they worked, and then noticing it didn't work after integration until I removed the LEDs. I have since learned I also could have used a buffer driver or LED driver, alkthough that adds more complexity to my simple prototype.
(in case you're wondering, yes, I know you're supposed to use a current limiting resistor, but I've also observed that my 5mm LEDs work just fine when given regulated 5V, they end up dropping 4.6V and consuming 40mA, which is about double the current they are rated for.)
How bright do you need the LEDs to be? Even 20 mA is a huge amount for modern electronics. On my boards I have 100 kohm resistors with green LEDs and they are very visible even at some 20 uA of current
Not really bright at all (sitting next to the device in a room where I control the lighting). My current LEDs are 5mm white, Vf 3V, max 20mA and I just hooked one up to my variable power supply and it looks like I can run it at 3mA and it's still super bright.
At times I have run them much lower, to the point where the light is just barely visible. If I set my power supply to cap out at 2.6V instead of 3, the current reading is 0.000, which I think must be below 1mA, and it's still quite visible.
(i'm not an ee expert so I frequently make thinkos related to voltage and current, but I think I've mostly got LEDs down).
> when you put the scope probes on, some of the ringing energy goes into the scope and it tends to improve the situation as long as the probe is there.
The software equivalent is so-called "load-bearing printf"s, where for example `printf("broken_var: %i\n",broken_var);` causes broken_var to not get optimized out and so start working correctly. But in hardware that happens even when you (effectively) inspect broken_var in a attached debugger, because the closest thing you have to a debugger (eg, oscilloscope) effectively just is a load-bearing printf.
Generally, a scope probe is best represented as a very small capacitance to ground, followed with a rather large resistance. That capacitance tends to shut most high frequencies to ground.
Of course, as things are with the black arts of RF engineering, there are possible situations where that same probe would make things worse, or appear inductive.
I wonder if the quantum physics equivalent is heisenberg's uncertainty principle. Probably not, but I have observed "observing a detail of a system often interferes with the running of the system"
In a past life, I worked in a video post house with a very competent engingering department. Without fail, if we had a problem that required an engineer, they would come in, see the problem, and fix it. Unless, if the only engineer available was the director of the group. The problems were never repeatable when he was present, and everything worked fine. This is why there was a constant request to have a life sized cutout of him to leave in the room to ensure the equipment behaved properly
It was slow data, like 10mhz or something. We were safely under the 1/10 wavelength rule with the old, softer corners. With the new sharp corners we were over the 1/10 wavelength rule and the circuit was ringing so hard it would cause nearby chips to latch up. We added some termination resistors and it was fixed.
Also note that this kind of thing can be hard to diagnose, because when you put the scope probes on, some of the ringing energy goes into the scope and it tends to improve the situation as long as the probe is there.