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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
Tags: , , , , ,

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.)

2017 January 29

Thermistor lab graded

Filed under: Circuits course — gasstationwithoutpumps @ 22:14
Tags: ,

I just spent my entire weekend grading 37 design reports for the thermistor lab—it has not been a fun weekend.  The coming week or two will be grading hell, as I have homework due for the 72-person class Monday, Wednesday, and Friday (with another lab report due next Monday), and no grader or TA.

This lab report was the first of the quarter, so there were a lot more problems with the submissions than I expect to see on future lab reports.  I’ve tried to collect some of my notes on the more common writing errors for this blog post, with the intent of trying to work them into the chapter on lab reports in the textbook:

  • Some students had wordy introductions. I want reports to start with a clear, concise statement of the engineering goal, not a dump of any random factoid that might be vaguely related to the report.
  • Report should be standalone—not referring to homework. If something in the homework is needed, incorporate it!
  • Use paragraphs with one topic each. Every paragraph should start with a topic sentence, and the rest of the paragraph (if there is any) should support and amplify that topic sentence. It is better to have one-sentence paragraphs than to ramble from topic to topic without a paragraph break.
  • Fit your model to your data, not your data to a model. You should never be changing your data to make it fit your theory—you should be changing your theory to fit your data.If you say you are fitting your data to your model, you are claiming to commit scientific fraud.
  • Best-fit curves are not necessarily lines—students don’t have a “line of best fit” in this lab, because the models we’re fitting are nonlinear.
  • Figure captions should be paragraphs below figure, not noun phrases above figure. Any anomalies or interesting features of the figure should be pointed out in the caption.  Most of the crucial content of the report should be in the figures and captions, because that is all 90% of readers ever look at in a science or engineering paper.
  • Refer to figures and equations by number, rather than “schematic below” or “equation above”.
  • Don’t use screenshots for schematics or gnuplot output—export graphics properly as PDF files and incorporate them into the report so that they can be printed at full resolution even when scaled.
  • Many students use way too much passive voice.  Using passive voice is a way to hide who did something or deny responsibility (see Nixon’s “mistakes were made”) and should not be necessary in a design report.
  • Use past tense for things that have been done, not present tense.  Also, “would” is not some formal version of the past tense—it is a marker for the subjunctive mood in English, which has a whole lot of different uses.  In technical writing, the most common use of subjunctive is for “contrary to fact”.  If you say “I would put the thermometer in the water”, I immediately want to know why you don’t—I expect to see the sentence continue with “, but I won’t because …”
  • “Software” is an uncountable noun, which means that it can’t be used with the indefinite article “a”.  There are a lot of uncountable nouns in English, and there isn’t much sense to which words are countable and which aren’t—even closely related languages with similar notions of countable and uncountable nouns mark different nouns as uncountable.  I’ve only found one dictionary that marks countability of English nouns—the Oxford Dictionary of American English, which is available used for very little money.
  • Equations are part of a sentence (as a noun phrase), not random blobs that can be sprinkled anywhere in the paper.  No equation should appear without a textual explanation of its meaning, and the meaning of its variables.
  • There was a lot of misuse of “directly proportional” and “inversely proportional”: A directly proportional relationship plots as a straight line through zero. The voltage output in the thermistor lab is not directly proportional to temperature—it is increasing with temperature, but the function is sigmoidal, not linear.  Similarly, an inversely proportional relationship between x and y is a direct relationship between 1/x and y. It plots as a hyperbola. The resistance of a thermistor is not inversely proportional to temperature, as the resistance is proportional to e^{B/T}  not B/T.
  • Read the data sheet carefully!  A lot of students claimed that their thermometers were good to 150°C, but the data sheet said that the thermistor they were using had a maximum temperature range of  –40°C to 105°C, not 150°C.
  • Students need to use the right metric prefixes.  For example, “kilo” is a lower-case “k” not an upper-case “K”.  This becomes even more urgent for “micro” (µ), “milli” (m), and “mega” (M).  At least one report needs to be redone because the students claimed a value around 200MΩ, when they (probably) meant 200mΩ.  What’s a factor of a billion between friends?
  • Some students are clearly not used to using the prefixes, because I saw a lot of values around 0.0001kΩ, which should have been written 0.1Ω (or even 100mΩ).  Even worse, a lot of students just wrote 0.0001, with no indication what the units were (that triggered a number of “redo” grades on the reports).
  • “Lastly” is not a word—”last” is already an adverb. The same goes for “first”, “second”, and “third”. Perhaps it is easier to keep this in mind if you think of “next”, which is in the same class of words that are both adjectives and adverbs. For some reason, students never write “nextly”.
  • The ×  symbol (\times in LaTeX) is only used for crossproduct, not for scalar multiplication (except in elementary school). The normal way to show scalar multiplication is juxtaposition of the variables, with no operator symbol.
  • “Before” and “after” make no sense in the voltage divider circuit. You can sometimes use those terms in a block diagram that has a clearly directed information flow from inputs to output, but not for talking about the two legs of a voltage divider.

 

 

 

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