Sunday, July 3, 2016

Reviewing the DSO112A Coral mini oscilloscope

Why an oscilloscope

I hadn't really looked at oscilloscopes for decades, because I really didn't have much use for one. I only recently started doing electronics again, and tend to stick with things centered around microcontrollers and specialized digital modules. A circuit with even as many two analog components is pretty rare for me.
So I was surprised to see a reference to a $20 oscilloscope. Didn't think you could get one for even an order of magnitude more than that. So I investigated them, and found a slew of micro-controller based oscilloscopes ranging in price from that $20 - for a kit - to a little over $100. They typically have one channel, a small display, and a a very low bandwidth limit. But what do you know - I might even have a use for one!

The elephant on the web

If you start looking at reviews of these things, they divide into two types. Either they are inexpensive tools that don't do much, but don't cost much either so are worth it, or they are not real oscilloscopes, but toys that won't teach you anything about oscilloscopes, and in general worthless junk, with the latter opinion typically being held by more experienced users.
So let's talk about that. Are they real oscilloscopes? Well, they appear to have the basic functions of an oscilloscope, even if very limited. On the other hand, they aren't nearly as capable of what I think of as an oscilloscope, so let's allow they aren't for the sake of the argument. That's ok - I don't want an oscilloscope, I want a tool for examining signals in my microcontroller and rc projects. If one of these will do that, I don't care what you call it. I'm going to keep using the phrase real oscilloscope to refer to the bulky, multi-channel oscilloscopes costing hundreds of dollars, but only as a convenience. No slight to USB and mini DSOs is intended.
A toy? Well, yeah - I'm going to add it to the set of things I play with on a regular basis, like rc transmitters, microcontrollers, multimeters and logic analyzers. I won't argue if you want to call it a toy.
And yes, they won't teach someone unfamiliar with oscilloscopes anything about oscilloscopes as I used to use them. I don't care. If they are fun and help with the things I want to do, I'll learn about oscilloscopes if I ever need one of those. If you want to learn about real oscilloscopes, maybe they aren't for you.
Finally, are they junk? Well, something is junk if it can't do what you bought it to do. That could happen because it just doesn't have the capabilities you need, which is why I think these get called junk. But it could also be because the quality is so bad they can't do what they claim to be able to do, which is why you should read reviews.
So, what do I need to do? For most of my projects, I use a multimeter or an inexpensive logic analyzer in a USB dongle and the sigrok software. What I really want is something to fill the gap between them - so a simple way to log analog values to the computer, or a multimeter with a graphical display. I'd been looking at some low end multimeters that had these kinds of features, but some of these inexpensive oscilloscopes can do both and are in the low end of the price range for these multimeters.
The one thing I've wanted to do recently that an oscilloscope would be useful for is dealing with the RF signals used in RC transmitters and receivers. Checking with an RF engineer, I find that the correct way to deal with this is with a demodulator, which brings them out of the RF domain and into the digital domain where a logic analyzer is the tool of choice. Even then, a high end signal would be something like 16 12 bit channels in an 11ms frame. In round numbers, that's 1.6 channels in a millisecond, or ~20 bits in a millisecond, meaning a 20Khz signal if you make a lot of simplifying assumptions that a real RF engineer would fire me for, but generally raise the sample rate. Such signals are well within the range where these things work.
Of course, I could stay in the RF domain. But for working with 2.4GHz FHSS signals, spending hundreds of dollars on a good entry-level "real" digital oscilloscope will get me something that's junk, because it's capabilities have been exceeded.
So if they're junk, they're junk because the quality sucks. And that's what I'm going to address in the rest of this review.

Why the DSO112A

After looking over the available offerings, the DSO112A had almost all the features of all the oscilloscopes in this price range, the exceptions being an FFT display mode, and the ability to transfer a capture of the screen display. Since it seemed to be unique in being able to operate as a USB oscilloscope, those two features seemed superfluous.

The DSO112A

First, a quick overview of the DSO112A. According to the specs, it's a 1 channel oscilloscope with a 2MHz analog bandwidth capable of making 5 million 8 bit samples a second, recording either 512 or 1024 samples. Resolution ranges between 2mV and 20V and 1us to 50s per division.
Steve Jobs would have approved of the hardware design as it only has one button. The other visible hardware is the screen and the connectors, and they fell down on the connectors. It has micro USB instead of USB C, and MCX connectors instead of BNC. Where the cheaper options require a USB serial bridge device to upgrade or capture data, it already has one. Likewise, it not only has an internal battery, but a built-in charger for it. These last two are optional or missing on many of the alternatives, but it's touches like this that made the DSO112A my choice. The test signal generator is made available by a standard connection, which can then be used to provide an external trigger. It also supports test frequencies from 1Hz up to 1MHz, which my logic analyzer says are spot on. Finally, there's an led in the case that lights up when it's charging.
The software feature list is rather lengthy. One of the nice features is visible on power on, when you get a screen of 24 presets along with a counter. You can select one of those, wait for the counter to expire or touch the Skip button to get to the oscilloscope screen.
The presets are another nice touch, given that it's got a lot of settings. It's not as comprehensive as a real oscilloscope, but it's still a lot of things to keep track of. The complete list for sampling is voltage sensitivity, offset and coupling, time unit, triggering mode, slope, position, level and source as well as the record length.
The features that don't affect sampling besides the presets are a the ability to hold the current sample, save that sample to EEPROM to examine later, or send it out the USB port as a CSV file. You can get the frequency of the displayed curve calculated and displayed on screen, at least for some of the sampling frequencies. You can also get the maximum, minimum, average, peak-to-peak and true RMS voltages displayed. In addition, you can turn on on-screen cursors to measure either time and voltage between your two selected points. And of course, it can act as a USB scope.

The hardware

It came in a simple white box, and included two identical - and identically cheap - probes, a short USB cable (as if anyone needs another one), a double-sided, multicolor one page fold out quick start guide, as well as a nice cloth bag that all of the above fits in.
While the quick start guide works, there's a proper manual available online, including complete documentation of the USB mode.
The case is mediocre quality plastic. Clearly, they were engineering to a cost point. Given that low cost is an important feature for these devices, I understand this, but it is unfortunate for a number of reasons.

Using it

For the most part, the user interface is well designed. The presets are going to be important; I created two (one for PWM and one for S.BUS signals) in the first 24 hours, just fooling around with it. It has some consistency issues (ESC? Not Done?). The settable values are displayed on the screen, and will open a menu of values when you touch them, which seems very intuitive. Scrolling the screen horizontally scrolls through the sample buffer, and scrolling it vertically moves the cursor on the side of the screen you're scrolling, which is either the vertical position or the trigger level. This solves a common complaint about JYETech's cheapest model, that it takes forever to scroll through the sample with buttons.
The thoughtful design is highlighted by the Hold function being accessible with a quick touch of the power button as well as an on-screen Hold button. At the other end, the on-screen cursors are a real pain to use. You have to pick these very small lines out of everything on the screen before you can move them, then again to stop moving them. A physical control - either a button to switch cursors and a knob or rocker switch to move the selected one - would have been a lot better.
Unfortunately, the cost cutting interferes with everything. The resistive touch screen - the less expensive option - doesn't always react well, which plays badly with the CPU being a bit under powered. So I'm often not sure that a touch registered, and will touch it again - only to hit whatever shows up when the first touch registers. Turning the time settings on and then off again happens all to often.
The cost cutting is particularly galling when it comes to the probes. Just a pair of alligator clips on apparently unshielded wires. The MCX connectors means you can't easily use standard BNC probes. There are probes available with MCX connectors, including some 10:1 attenuating probes, but none of them seem to be adjustable. BNC/MCX adapters are available, but may not work well with this, and will certainly cause issues with the provide bag.
A more serious issue the cost cutting creates is that, while it's a USB scope, there isn't any software that will work with it! JYETech's own software - JYElab - doesn't recognize it. They've released the source, and as far as I can tell it should work on the DSO112A, but neither they nor anyone else seems to have fixed this. On the other hand, I'd rather have it working in a cross-platform tool, so the source being available will help should someone tackle that.

Accuracy

Mostly, ok. Not great. Once the wave you're sampling starts turning into a bar, things get a wonky. The measurements are off, response lags, parts of the display get jittery, etc.
The display seems a bit glitchy at times, especially when you've got a full display. It's seldom a real problem, but it is annoying and a bit worrisome.
But when you've got a nice, clean signal, it works well. Checking measurements of a custom signal generator against my logic analyzer, they are in good agreement.

Summary

Yes, it's not a real oscilloscope. It doesn't have nearly the functionality they do. Even the capabilities it lists are exaggerated a bit. If you need the capabilities of a real oscilloscope, or want to learn how to use one, this is going to be a serious disappointment. I suspect that's true of the inexpensive 2-channel DSO's as well.
On the other hand, it costs a fraction of those oscilloscopes, and provides some nice functionality compared to things like multimeters and logic analyzers. If you've got a good handle on your needs and they are in this things capabilities, I think it's a good purchase.

You can see this oscilloscope in use in a youtube video of mine.