Digilent’s OpenScope

 

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.

Digilent Analog Discovery 2 USB Oscilloscope

I have recently learned about a new USB oscilloscope, Digilent’s Analog Discovery 2, which seems to be a step up from the BitScope BS10 USB oscilloscope that I currently own. Digilent’s offering has differential inputs, 14-bit ADC (instead of 8), 100MS/s (instead of 40MS/s), and much nicer-looking user interfaces (no more black background!).  It costs a little more ($279 vs. $245, both costing more to get BNC connectors for higher-speed oscilloscope probes), but Digilent has an academic program that reduces the cost to only $179, so that even with an extra $20 for the BNC adapter and $20 for scope probes, the price is still lower than the BitScope.

I’m considering getting the Analog Discovery 2 scope (if I qualify for the academic discount), but I’ll probably wait until I replace my laptop.  The free Waveforms 2015 software runs on a wide range of Windows versions, but only 10.9 or newer on Mac OS.  (It also runs under some versions of Linux).  I’m still running Mac OS 10.6.8 on my laptop, and I don’t want to “upgrade” to a newer OS on the old hardware—I’m planning to replace the laptop this year, but I’m waiting to see whether Apple comes out with a usable MacBook Pro in 2016, or whether they’ve gone all in for connector-less laptops, in which case I’ll probably have to switch to a cheaper, but clunkier Linux laptop.

One of the things I like about Digilent’s marketing is that they have a very thorough reference manual online, which goes through the design of the hardware, explaining the schematics and some of the design choices for what chips they used. The online reference manual for Waveforms 2015 seems decent, but not as thorough as the hardware manual.

I’m curious whether any of my readers have tried the Digilent USB oscilloscopes.

Santa Cruz Mini Maker Faire went well

The first Santa Cruz Mini Maker Faire seemed to go well.  I did not get to see much of it, since I was busy at my booth most of the day, though I did get a break for lunch while my assistant Henry manned the booth, and I made a quick tour of the exhibits during that break, to see what was there, though with no time to chat with other exhibitors.

I understand that about 1800 people bought tickets to the Mini Maker Faire, which probably means there were over 2000 people on-site, including volunteers and makers.  I hope the food vendors did OK—I ate at the Ate3One truck, since I never have before, but my opinion afterwards was that CruzNGourmet and Zameen have better food (both of those trucks are frequently on campus, and I’ve eat at each several times).

My day went pretty well, though I had one annoying problem, having to do with my pulse monitor display. When I set up the booth Friday evening, the pulse monitor was not working, and I thought that the phototransistor had somehow been broken in the rough ride in the bike trailer, so I brought the pulse monitor home, replaced the phototransistor and tested in thoroughly.  Everything worked great, so I packed it more carefully for transport in the morning.

When I got everything set up Saturday morning, I found I had no electricity, though the electricity had worked fine the night before.  After I finally tracked down a staff member with the authority to do anything about it, he suggested unplugging the other stuff plugged in and switching outlets.  I turned out that the only problem was that the outlets were so old and worn out that they no longer gripped plugs properly—taping the extension cord to the outlet box so that the weight of the cord didn’t pull out the plug fixed the power problem.

Once I had power, I tested the pulse monitor, and it failed again!  I used the oscilloscope to debug the problem, and found that the first stage transimpedance amplifier was saturating—there was too much light in the room, and even shading the pulse monitor didn’t help. By then, my assistant for the day (and my group tutor for the class on campus), Henry, had arrived and gotten the parking permit on his car, so I raced home on my bike to get resistors, capacitors, op amp chips, multimeters, hookup wire,and clip leads to try to rebuild the pulse monitor from scratch on the bread board.

When I got back to Gateway School, I tried a simple fix before rebuilding everything—I added a pair of clip leads to the board so that I could add a smaller resistor in parallel with the feedback resistor in the transimpedance amplifier, reducing the gain by a factor of about 30.  This reduced gain kept the first stage from saturating, and the pulse monitor worked fine.  Rather than rebuild the amplifier, I just left the pair of clip leads and the resistor in place all day—they caused no problem despite many people trying out the pulse monitor.

I think that I want to redesign the pulse monitor with a logarithmic first stage, so that it will be insensitive to ambient light over several decades of light.  That should be an easy fix, but I’ll have to test it to make sure it works. I don’t think I’ll have time this weekend or next to do that, but I’ll add it to my to-do list.

I’ll need to think about whether to include having a logarithmic response in the textbook—that is certainly more advanced than what I currently include (just a transimpedance amplifier), which is already pushing students a bit.  A transimpedance amplifier is a pretty common component in bioelectronics, so I really want to leave one in the course.  I’m not sure a logarithmic amplifier is important enough or simple enough to include at this level (I don’t currently cover the non-linearity of diodes).

 

Here is the booth display with my assistant, Henry. I was permitted to use painter’s tape to attach the banner to the whiteboard.

The magenta laptop on right (which my family refers to as the “Barbie laptop”) was a used Windows laptop that I bought for testing out PteroDAQ installation on Windows. It was set up with PteroDAQ running all day, recording a voltage from a pressure sensor and a frequency from a hysteresis oscillator (as a capacitance touch center).

Just to the left of that was a fairly bright stroboscope, using 20 of my constant-current LED boards. To its left is my laptop, displaying the current draft of my book. Behind (and above) the laptop is my desk lamp, which uses the same electronic hardware as the stroboscope, though with only 6 LED boards, not 20.

In front of the laptop is the pulse monitor, which includes a TFT display in an improvised foamcore stand. I used just a half block for the pulse sensor, relying on ambient light (sunlight and the desk lamp) for illuminating the finger.

To the left of the pulse monitor was a stack of business cards for my book and sheets of paper with my email address and URLs for this blog and the book.  I should have included the PteroDAQ URL as well, but I had forgotten to do so. I did tell a lot of people how to find PteroDAQ from the navigation bar of my blog, but putting it on the handout would have been better. Ah well, something to fix next year (if Gateway is crazy enough to do another Mini Maker Faire, which I hope they are).

I also had all my bare PC boards that I had designed and not populated, plus my two Hexmotor H-bridge boards, behind the business cards. One of the amplifier prototyping boards was displaying in the Panavise that I use for soldering.

On the far left of the table is my Kikusui oscilloscope and two function generators, set up to generate Lissajous figures.  I let kids play with the frequencies of the function generators, take their pulse with the pulse monitor, and play with the pressure sensor and the capacitive touch sensor.

My booth was not the most popular of the Faire by any means (certainly the R2 Makers Club in the next booth was more popular), but I was kept busy all day and I talked with a lot of people who seemed genuinely interested in what I was doing, both with the UCSC course and as a hobbyist.

YouTube closed captions are awful

I looked at the automatically generated closed captions on the Oscilloscope video and they were awful—pretty much just word salad. Speaker-independent voice recognition is obviously not a solved problem at Google.

I could not figure out from the captions what the text was supposed to be, so I’m definitely going to have to take an hour to edit the captions.  For your amusement, here are the automatically generated captions (with time stamps removed):

this

results go and circuits lab

truthful

showing this witness during a self-test
internal calibration began to overshadow

while doing this is the oscilloscope
trunk of an actual protest rally we can

make our nation of this gradually goes
to click here to lower celebration

finals where

whatever settings the previous first
layout which is probably not what you

want once it’s finished it off with
oppressed peoples historical back three

people said these are falling also know
what you want

matter whatever the last person in our
class there will help now also passed a

law received

craftspeople a suitable for calibration
single heads because we were so

impressed you’re the slap on a single
figures out what comes out is to view

that clearly see that is where color
associated with it

channel one yellow peril to this
political channel 39 channel for each

row has little to show you which channel
is for example channel two minutes later

colored chance i’m not respecting the
settings now able to accept this coupled

with 30 measures signal felt craft so
this is where signal from this marker

which is just changed it as a compliment
but this month now throws away and easy

going to take the last year which will
just show us that trust is on the screen

like this property

now to see this listing all we won’t be
using that much bandwidth and landlords

are very very important shows little
light switch when she can be set when

one asks the folks at matthews
telescopes for example tax and you still

think so it will bring up the menu by
pressing his body is still scope sent

along acts now here on the same set to
tax

understand same problem check this
before now also watch now deciding where

around his father is cold six hits and
two bowls bowls permission to just see

they didn’t grade office he’s facing
they’re going about two-and-a-half

divisions so too cold for division

viable single with the slower larger
call for example a change is now five

balls for division and we can see the
same no only goes up by one division in

the other direction to one whole
provision

signal goes off you might rather
increased number of major producing

science able to close at present the
many long now playing here next 10

microseconds

horrible for his own vision for vertical
sets for all channels

provision a single all this way for
division last thing they apparently

because the other way to increase I’m
provision shrinking the way we did not

see one single this way every one
division so this is one killers we’re

also do this population 44122 color
calibration on the east coast is

slightly off

is it about fifteen years

channel as well you just said to
channels on here in great numbers if we

wanted to try to channel off push button
again while standing around a nice

things

orders probe is not properly compensated

just get there is very little space
heater which we can trust is a very

small screwdriver slotted over shooed
and other causes around the corner we

get it just right now include role is

channel on it for

which will help us all based on the
sound levels in the road below ground

connection between the microphone
amplifier board here has the microphone

in silver amplifier chip in black and
some connections on the screw terminal

we have power

5 volts red and brown and black it is
very important to always use this

convention for power and ground so that
you don’t actually connector on things

we don’t have the output signal here in
purple and everything is connected to

these heavier bands which have alligator
clips to the power supply we’re not

going to connect our protocol but we
could have to ground lead to the ground

and probably get here to center panel
which is the signal that is a kind of

difficult connection to make now it’s
connected and we can see on the screen

the output of the mic channels now just
a distraction in the range goals

overseas buyer has your rivals lot we
can also notice that occasionally take

notice cause they saw your head

only when the signal go from below 212
above 2.4 liter just this the trigger

man but other than that shows a trigger
which means it is based on a single

going from their low point for both
point to low is actually separate slope

is rising at going global

another option

trick or treating just normal several
times which will only trigger this is

more useful to wear a lot of things that
you don’t know the exact true that many

levels

sure the level at which is to tax low on
the screen

levels for single trigger a tad bit
lower

usually get a trigger but it’s a free
set a closer Trevor so we had a simple

Update 2016 April 6: After about 5–6 hours of work, I got the closed captions fixed, so these amusing automatic ones are no longer on the video.  Editing the captions is even more work than editing the video, so I can see why so many people don’t bother!

Oscilloscope tutorial video

I’ve released an intro tutorial on using the digital oscilloscopes in the lab:

The tutorial is fairly short (12 minutes, 23 seconds), and so only covers the basics of using the oscilloscope, not the myriad of features that the scopes in the lab have.

Incidentally, it took us about 2 hours to shoot the video, 2 hours for me to edit it, 2.5 hours to render it, and 5 hours to upload it to youtube.  (I would have uploaded it to Vimeo, where I already had an account, but they limit non-paying customers to 500MB a day, and the 1080p video was 1.6GB—it’s not worth paying $10 a month to upload one video.)

I’ll probably do more videos this summer, if students find this one useful, but since the production takes about 50 times the running time, don’t expect a lot of videos from me!