Gas station without pumps

2017 February 18

Digilent’s OpenScope

Filed under: Uncategorized — gasstationwithoutpumps @ 10:08
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Digilent, which makes the excellent Analog Discovery 2 USB oscilloscope, which I have praised in several previous post, is running a Kickstarter campaign for a lower-cost oscilloscope: OpenScope: Instrumentation for Everyone by Digilent — Kickstarter.

I’m a little confused about this design, though, as is it is a much lower-quality instrument without a much lower price tag (they’re looking at $100 instead of the $180 or $280 price of the Analog Discovery 2, so it is cheaper, but the specs are much, much worse). The OpenScope looks like a hobbyist attempt at an oscilloscope, unlike the very professional work of the Analog Discovery 2—it is a real step backwards for Digilent.

Hardware Limitations:

  • only a 2MHz bandwidth and 6.25MHz sampling rate (much lower than the 30MHz bandwidth and 100MHz sampling of the Analog Discovery 2)
  • 2 analog channels with shared ground (instead of differential channels)
  • 12-bit resolution (instead of 14-bit)
  • 1 function generator with 1MHz bandwidth and 10MHz sampling (instead of 2 channels 14MHz bandwidth, 100MHz sampling)
  • ±4V programmable power supply up to 50mA (instead of ±5V up to 700mA)
  • no case (you have to 3D print one, or buy one separately)

On the plus side, it looks like they’ll be basing their interface on the Waveforms software that they use for their real USB oscilloscope, which is a decent user interface (unlike many other USB oscilloscopes).  They’ll be doing it all in web browsers though, which makes cross-platform compatibility easier, at the expense of really messy programming and possibly difficulty in handling files well.  The capabilities they list for the software are much more limited than Waveforms 2015, so this may be a somewhat crippled interface.

I would certainly recommend to students and educators that the $180 for the Analog Discovery 2 is a much, much better investment than the rather limited capabilities of the OpenScope.  For a hobbyist who can’t get the academic discount, $280 for the Analog Discovery 2 is probably still a better deal than $100 for the OpenScope. The Analog Discovery 2 and a laptop can replace most of an electronics bench for audio and low-frequency RF work, but the OpenScope is much less capable.

The only hobbyist advantage I can see for the OpenScope (other than the slightly lower price) is that they are opening up the software and firmware, so that hobbyists can hack it.  The hardware is so much more limited, though, that this is not as enticing as it might be.

Some people might be attracted by the WiFi capability, but since power has to be supplied by either USB or a wall wart, I don’t see this as being a huge win.  I suppose there are some battery-powered applications for which not being tethered could make a difference (an oscilloscope built into a mobile robot, for example).

Going from a high-quality professional USB scope to a merely adequate hobbyist scope for not much less money makes no sense to me. It would have made more sense to me if they had come out with OpenScope 5 years ago, and later developed the Analog Discovery 2 as a greatly improved upgrade.

2017 February 6

Every second counts

Filed under: Uncategorized — gasstationwithoutpumps @ 21:03

I’ve been enjoying the videos at http://everysecondcounts.eu/, which were started by a Netherlands comedy show in response to Trump’s America First speech.  They made a fake tourism video, with an excellent Trump voice impersonator, arguing for making the Netherlands second.

Other comedy shows soon took up the challenge creating their own mock tourism videos (I particularly liked Denmark’s entry and Germany’s).

There are now nine videos, with more undoubtedly in the pipeline.

Hysteresis oscillator is voltage-dependent

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 20:42
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Today in class I did a demo where I tested the dependence of the frequency of my relaxation oscillator board on the power supply voltage.

The demo I did in class had to be debugged on the fly (it turns out that if you configure the power supplies of the Analog Discovery 2 to be low-speed waveform channels, then you can’t set them with the “Supplies” tool, but there is no warning that you can’t when you do the setting), but otherwise went well.

One surprising result (i.e., something else that hadn’t happened when I tested the demo at home on Sunday) was that the frequency appeared to go up instead of down when I touched the capacitive touch sensor.  This I managed to quickly debug by changing my sampling rate to 600Hz, and observing that the 60Hz frequency modulation was extreme at the podium, taking the oscillation frequency from 0Hz to 3MHz on each cycle.  Grounding myself against the laptop removed this interference and produced the smooth expected signal.

Anyway, when I got home I was much too tired to grade the lab reports turned in today (I’ve got a cold that is wiping me out), so after a nap and dinner, I decided to make a clean plot of frequency vs. power-supply voltage for my relaxation oscillator.  I stuck the board into a breadboard, with no touch sensor, so that the capacitance would be fairly stable and not too much 60Hz interference would be picked up.  I powered the board from the Analog Discovery 2 power supply, sweeping the voltage from 0V to 5V (triangle wave, 50mHz, for a 20-second period).

I used the Teensy LC board with PteroDAQ to record both the frequency of the output and the voltage of the power supply.  To protect the Teensy board inputs, I used a 74AC04 inverter with 3.3V power to buffer the output of the hysteresis board, and I used a voltage divider made of two 180kΩ resistors to divide the power-supply voltage in half.

When I recorded a few cycles of the triangle waveform, using 1/60-second counting times for the frequency measurements, I got a clean plot:

At low voltages, the oscillator doesn't oscillate. The frequency then goes up with voltage, but peaks around 4.2V, then drops again at higher voltages.

At low voltages, the oscillator doesn’t oscillate. The frequency then goes up with voltage, but peaks around 4.2V, then drops again at higher voltages.

I expected the loss of oscillation at low voltage, but I did not expect the oscillator to be so sensitive to power-supply voltage, and I certainly did not expect it to be non-monotone.  I need to heed my class motto (“Try it and see!”) more often.

Sampling at a higher frequency reveals that the hysteresis oscillator is far from holding a steady frequency:

Using 1/600 second counting intervals for the frequency counter reveals substantial modulation of the frequency.

Using 1/600 second counting intervals for the frequency counter reveals substantial modulation of the frequency.

This plot of frequency vs. time shows the pattern of frequency modulation, which varies substantially as the voltage changes, but seems to be repeatable for a given voltage. (One period of the triangle wave is shown.)

This plot of frequency vs. time shows the pattern of frequency modulation, which varies substantially as the voltage changes, but seems to be repeatable for a given voltage. (One period of the triangle wave is shown.)

Zooming in on a region where the frequency modulation is large, we can see that there are components of both 60Hz and 120Hz.

Zooming in on a region where the frequency modulation is large, we can see that there are components of both 60Hz and 120Hz.

I could reduce the 60Hz interference a lot by using a larger C and smaller R for the RC time constant. That would make the touch sensor less sensitive (since the change in capacitance due to touching would be the same, but would be a much smaller fraction of the total capacitance). The sensor is currently excessively sensitive, though, so this might be a good idea anyway.

2017 February 5

Units matter

Filed under: Circuits course — gasstationwithoutpumps @ 11:37
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I was a little surprised by how many students had trouble with the following homework question, which was intended to be an easy point for them:

Estimate C2(touching) − C2(not touching), the capacitance of a finger touch on the packing-tape and foil sensor, by estimating the area of your finger that comes in contact with the tape, and assume that the tape is 2mil tape (0.002” thick) made of polypropylene (look up the dielectric constant of polypropylene on line). Warning: an inch is not a meter, and the area of your finger tip touching a plate is not a square meter—watch your units in your calculations!

Remember that capacitance can be computed with the formula C = \frac{\epsilon_r\epsilon_0 A}{d}~,
where \epsilon_r is the dielectric constant,  \epsilon_0=8.854187817E-12 F/m is the permittivity of free space, A is the area, and d is the distance between the plates.

The problem is part of their preparation for making a capacitance touch sensor in lab—estimating about how much capacitance they are trying to sense.

There is a fairly wide range of different correct answers to this question, depending on how large an area is estimated for a finger touch. I considered any area from 0.5 (cm)2 to 4 (cm)2 reasonable, and might have accepted numbers outside that range with written justification from the students.  Some students have no notion of area, apparently, trying to use something like the length of their finger times the thickness of the tape for A.

People did not have trouble looking up the relative dielectric constant of polypropylene (about 2.2)—it might have helped that I mentioned that plastics were generally around 2.2 when we discussed capacitors a week or so ago.

What people had trouble with was the arithmetic with units, a subject that is supposed to have been covered repeatedly since pre-algebra in 7th grade. Students wanted to give me area in meters or cm (not square meters), or thought that inches, cm, and m could all be mixed in the same formula without any conversions.  Many students didn’t bother writing down the units in their formula, and just used raw numbers—this was a good way to forget to do the conversions into consistent units.  This despite the warning in the question to watch out for units!

A lot of students thought that 1 (cm)2 was 0.01 m2, rather than 1E-4 m2. Others made conversion errors from inches to meters (getting the thickness of the tape wrong by factors of 10 to 1000).

A number of students either left units entirely off their answer (no credit) or had the units way off (some students reported capacitances in the farad range, rather than a few tens of picofarads).

A couple of students forgot what the floating-point notation 8.854187817E-12 meant, even though we had covered that earlier in the quarter, and they could easily have looked up the constant on the web to figure out the meaning if they forgot.  I wish high-school teachers would cover this standard way of writing numbers, as most engineering and science faculty assume students already know how to read floating-point notation.

Many students left their answers in “scientific” notation (numbers like 3.3 10-11 F) instead of using more readable engineering notation (33pF). I didn’t take off anything for that, if the answer was correct, but I think that many students need a lot more practice with metric prefixes, so that they get in the habit of using them.

On the plus side, it seems that about a third of the class did get this question right, so there is some hope that students helping each other will spread the understanding to more students.  (Unfortunately, the collaborations that are naturally forming seem to be good students together and clueless students together, which doesn’t help the bottom half of the class much.)

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