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J.R. Buchanan


DSO 138 Oscilloscope Review

December 2017

Picture of Oscilloscope

Recently my son has started showing interest in electronics as a hobby. I've been getting him some tools and equipment. Recently on the web, I've been seeing mentions of inexpensive digital oscilloscopes with low specs that are said to be actually useful despite their limitations. So I bought him a DSO 138 on Amazon. It's made by JYE Tech Ltd, and on Amazon I only paid $21.00. I got a fully assembled item, it's also available as a partial kit with the through-hole components not installed. Oddly enough, on Amazon, the kit is not much cheaper, and if you're not careful, more expensive than the assembled item. Looking at the complexity and parts count, I can't see how the price can be this low. Perhaps it's subsidized?

The package contains the 'scope, on a bare board with no case, a "probe" that's simply a BNC to alligator clip cable, the sort of thing you'd use on the output of a function generator, and a manual. The wall wart to power it is not included. You'll need to find an 8V to 12V, nominally 9V DC wall transformer with a 5.5/2.1mm barrel jack, center positive. That's the same as an Arduino Uno BTW.

First, we hooked the probe up to the provided 1KHz, 3.3V peak to peak test output. It worked fine. The input attenuation is rather unusual, consisting of two three position switches, one for 1V/div, 0.1V/div, and 10mV/div. The other one divides the sensitivity by 1, 2, or 5. This gives you 9 attenuation levels between 10mV/div, and 5v/div. This means a maximum input voltage of +/- 20V. You can use a (not in the package) 10x probe to get that up to 200V (be careful!). With two switches and a 1x/10x probe (not included), it can be a little confusing to read a voltage from the graticule and calculate what it really means. Fortunately, there is an overlay at the top of the screen which provides many measurements on the waveform at a glance. I especially liked the frequency display, the peak, and peak to peak voltages, as well as the RMS voltage. Remember that if you do get a 10x probe and use it in the 10x mode, you'll have to multiply any voltage readings by 10 to get the actual voltage. All of the functions other than the voltage and the AC/DC switch are accessed by pushbuttons and the on-screen display.

Does the 'scope live up to its claim of 200KHz bandwidth? No, but it's not really a big problem, the actual bandwidth is quite good enough for a beginner who's short on money. I set the waveform generator in my Picoscope to 100KHz, the highest it will go, and set it to 1V peak to peak. The Picoscope read 1V peak to peak, the DSO 138 reported 0.56V peak to peak. The bandwidth of a 'scope is usually defined as the frequency where the indicated signal level has dropped by 3dB. The loss in dB at 0.56/1 is 20log(0.56/1)=-5dB. What is the frequency where the output displayed is actually 3dB down? The voltage ratio that describes a 3dB loss is 0.707/1 Experimenting, we find that this occurs at about 40kHz. Not the advertised 200kHz. Still useful at audio frequency, but don't expect square waves to look square as the frequency rises. Experimentally, I noticed that square waves started looking wonky somewhere below 10Khz, still well within the audio range.

Any other anomalies? As the frequency of the input (sine wave) is increased, the display on the graticule and on the on-screen information drops down from the actual DC level of the signal. At 1Khz, the offset is -20mV, by 100KHz, it drops down to -100mV. Irritating.

Another minor annoyance is that the triggering is pretty jumpy. If it gets too irritating, you can put it in single-shot mode. There is no trigger level control, only rising/falling slope. Also, the on-screen display is very useful, but it covers a lot of the waveform displayed. It would be nice if it could be turned on and off.

Interestingly, the device comes with a full schematic. It does not come with a real probe, however. I can't imagine working with the clumsy alligator clips and stiff cable of the provided "probe." Another limitation is that it's single channel. Another channel would make comparing two waveforms a lot easier, a situation that comes up often in the real world. To be honest though, it would add to the cost of the 'scope, which is definitely built to a price point.

The build quality is fairly good, reasonable solder joints all around. The did leave out the 1KHz test waveform jumper, easy to put in. Not having a case makes me a little nervous, but cases are available online, and, if you have a 3D printer, many cases are available on Thingiverse. We printed one, it looks like it's going to work when we clean up the surfaces a little with an X-Acto knife, a small file, and some sandpaper. The buttons and switch slider extensions don't fit quite right. It's also possible that some changes in printer settings could help.

Is this the last/best 'scope you'll ever need? No, it is not. But for a beginner, working with lower frequencies, it will get the job done, and it will do it just well enough. When my son and I were learning how it operated, we used it to troubleshoot a real circuit, http://www.buchanan1.net/md_1.html (third circuit). The cats had gotten to it and pulled some wires and components out. After we replaced them, it did not work. We looked at it using the DSO 138, and despite the fact that we were pushing its real-world bandwidth capabilities, it helped us greatly. I feel that this is a useful device for the beginner, and at an affordable price, a stepping stone to more expensive units. I personally like USB 'scopes that use a computer's screen for the display and for all the controls. The use very little bench space. I use a Pico Technology 2204A and have for a few years ($139, with 2 probes, at the time of this writing). I like it well enough that I just ordered the much more expensive 2208A ($679, with 2 probes, at the time of this writing). https://www.picotech.com/oscilloscope/2000/picoscope-2000-overview