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dual-channel oscilloscope Jaycar QC1938 An oscilloscope is one of the most useful tools on an electronics workbench. Jaycar’s Digitech QC1938 100MHz DSO (digital storage oscilloscope) was released a few months ago, and they sent us a unit to try out. J aycar has recently started selling the QC1938 two-channel, 100MHz DSO with a 1GSa/s sampling rate and 8MSa (megasample) storage. Having digital storage brings many advantages that we will describe shortly. Some of its more noteworthy features include a dual display (zoom) mode, numerous serial protocol decoders and an arbitrary waveform generator. The display is a 7in (18cm) diagonal TFT LCD with a resolution of 800 × 480 pixels. It does not have a touchscreen interface. Our loaned review unit came in standard retail packaging, so our experience is the same as if we had bought it. It comes with a single 10:1 probe, a pair of BNC-alligator leads, an IEC power cable, a USB cable and a software CD. The software is also downloadable from the Jaycar website product page for those without an optical drive. The unit comes well-packed and is relatively compact at 32cm wide, 15cm tall and 11cm deep. The power cable plugs into a recessed receptacle 48 Silicon Chip on the left side, meaning you can push it back against a wall. The QC1938 feels sturdy. The numerous controls mean it could not be much smaller, except perhaps by being slightly less deep. A folding handle is recessed into the top, and it has ample vent holes. It would definitely be nice to have a second probe included, given that this is a dual-channel oscilloscope. Still, the BNC-alligator leads are usable for modest frequencies (eg, audio or lowspeed digital signals), but definitely not up to the full 100MHz. You can purchase a second probe from Jaycar (Cat QC1902) for $39.95, rated at 60MHz on the 10:1 setting or 6MHz on the 1:1 setting. Documentation We had a good read through the user manual to get an idea of what to expect. There are numerous features listed that we thought sounded handy. We do a lot of digital or mixed digital/analog Review by Tim Blythman Australia's electronics magazine designs, so serial protocol decoding is one feature we’d use a lot. The protocols include UART, LIN, CAN, I2C and SPI, which covers most of the protocols that we use. As we noted in our review of PicoScope 6426E (October 2021; siliconchip.au/ Article/15068), having an ample sampling depth makes it easier to decode longer communication sequences. Like many modern digital oscilloscopes, the QC1938 has a USB port; two, in fact. A USB-B socket at the back allows the oscilloscope to be connected to a host PC. We’ll investigate this feature and the accompanying software later. A USB-A socket on the front allows a USB drive to be connected. You can use this to save screenshots and perform firmware updates. It turns out that the QC1938 is actually a small Linux computer equipped with custom hardware allowing it to act as an oscilloscope. The waveform generator can deliver sine and square waves, noise, or even an arbitrary waveform of up to 4096 siliconchip.com.au The screen is large and bright, so much so that we had no trouble viewing it outdoors. Adjacent to the screen are the basic oscilloscope controls, while the other features are accessed from a variety of buttons along the top and side of the front panel. All the included accessories are shown here. While the alligator clip leads limit the usable frequency, they are sometimes easier to connect to circuit points than a proper probe. samples. The waveform can also be amplitude- or frequency-modulated (AM/FM). Another features that we think will be quite handy is the dual window (zoom) view, which shows the entire waveform on the top half of the screen and a zoomed subsample in the lower half. The waveform output socket is also labelled as the external trigger. This shouldn’t be a problem as long as you remember whether this socket is an input (external trigger) or output (waveform generator) at any given time. Still, it would be nice to have two separate connectors. The QC1938 has foldable feet so that the oscilloscope can be tilted up slightly and gives a much better view of the screen and access to the controls than it would on a flat bench. The rear feet are rubberised and are sufficient to keep it from sliding around. There is also a DEFAULT SETUP button that reverts the oscilloscope to its default settings, in case you have gotten it into a state where you can’t figure out how to change it back. Hands-on testing After spending a bit of time getting accustomed to the various controls, siliconchip.com.au we found that the QC1938 is very easy and intuitive to use. There is a delay of about 10 seconds after powering it on before it is ready, which seems reasonable. For example, the more expensive Rigol MSO5354 mixed signal oscilloscope takes up to a minute. We reviewed that unit in the February 2019 issue (siliconchip. au/Article/11404). A self-calibration routine is available via the UTILITY menu. When we first ran this, it appeared to need a few cycles before settling and completing calibration; subsequent calibrations took about two minutes. It’s recommended that this is done after the oscilloscope has warmed up and stabilised. The controls, visible in the front photo, have a variety of functions, but all work in a uniform fashion. Pressing one of the buttons shows a list of soft options alongside the F1-F5 buttons on the display. Items that use the soft menu include MATH, SAVE/RECALL, MEASURE, ACQUIRE, UTILITY, CURSOR, DISPLAY, TRIGGER, DECODE and WAVE GEN. This covers most of the features beyond the basic functions. Pressing F0 at any time hides the soft menu options, while F6 flips to Australia's electronics magazine the second page of options if available. The V0 knob allows numerical soft menu options to be dialled in easily. Several of the buttons light up, and where appropriate, they match their colour on the display. For example, the CH1 trace is displayed in yellow and the CH1 menu button is lit up in orange (near enough to yellow) when CH1 is active. CH2 is similarly green. The MATH-generated trace is purple and is controlled by the MATH MENU button, which lights up purple. MATH functions include FFT (fast Fourier transform spectral analysis), add, subtract, multiply and divide. The RUN/STOP button is either red or green to indicate whether continuous triggering is occurring. The WAVE GEN button lights up blue when it is active. The MEASURE button brings up over 30 parameters, such as frequency, amplitude and duty cycle, measured from the displayed waveform. While you might often be interested in a handful of these, being able to quickly see many different parameters is handy too. A small subset can be chosen to be displayed along the bottom of the oscilloscope display. The most difficult part of the learning curve for this oscilloscope is December 2022  49 becoming familiar with the menu locations of all the options, but they are all quite intuitive once you have found them. Serial protocol decoding Scope 1: despite the apparent noisiness of this 12MHz SPI signal, the QC1938 has correctly decoded it. The decoded date is in purple below the green trace, and uses lower case characters for hexadecimal digits. It is odd that the hexagonal ‘box’ containing the decoded data does not bracket the full eight clock cycles. Scope 2: this shows the oscilloscope decoding three consecutive serial bytes at 460400 baud, the fastest UART data it can decode. The protocol decoders can also trigger the scope on a data match, a handy way to synchronise it to other events. Scope 3: the zoom view offers a split screen and is a good way to view a small part of a waveform without twiddling back and forth between time scales. 50 Silicon Chip Australia's electronics magazine We hooked up the QC1938 to the SPI lines on an LCD BackPack to test the protocol decoding. We used the probe on the clock line for CH1 (since this is the faster signal) and one of the BNC-alligator leads for data on CH2. Initially, we could not see any decoded data no matter what we tried. We contacted Jaycar, and they determined that this was due to a firmware bug that was fixed in a later version. The oscilloscope we received originally had version 3204 of the firmware (the current version is listed under the UTILITY menu). The update to version 3205 only took a couple of minutes. Jaycar has told us that this updated firmware file will be available for download on their website. So if you receive a QC1938 oscilloscope with version 3204 of the firmware, you should update it. After that, we had no trouble getting the protocol decoding working with our SPI data. By the way, we also experienced a couple of ‘freezes’ with the older firmware, where the oscilloscope randomly stopped responding to user input. We didn’t experience that anymore after the firmware update. You can see in Scope 1 that the clock line is cleaner than the data line (because we’re using the proper probe). The decoded value is shown below. We know this data is correct as the 0x2A (2a) command is regularly used by the BackPack software. The 12MHz signal shown is as fast as we could successfully decode using the BNC-alligator leads, with faster signals returning corrupted data. So you will need a second probe to work with faster SPI signals. Note that decoding of SPI data is pretty limited on any two-channel oscilloscope; it would be preferable to have extra channels available for connecting other data channels and slave select lines. If you frequently probe SPI buses, a four-channel oscilloscope or dedicated logic analyser might be a better choice. Scope 2 shows UART data being decoded at 460400 baud – this is the fastest UART baud rate that the QC1938 can decode. We had no siliconchip.com.au problems with UART decoding, even after trying several baud rates. Protocol decoding also requires you to set a matching trigger setting; these seem to duplicate the decoding settings, but mostly need to be changed separately. It is also necessary to set the threshold when multiple channels are involved in the decoding, such as I2C and SPI. The oscilloscope has handy features like being able to match on specific data values, including exact matches, mismatches and comparisons. This makes it much easier to sift through large amounts of data. The sample depth appears to be fixed at 4kSa when decoding is active, so the full 8MSa is not available. We think the protocol decoding would be better if it could use the entire sample memory, as it would allow longer sequences to be decoded. Zoom view The zoom feature is a simple and effective way of inspecting a waveform closely. It’s activated by pressing down on the timebase knob; there are markings on the oscilloscope controls to this effect. When zoom is active, the screen is split; the full waveform is shown in the top half, and a zoomed version is below. A window over the full waveform shows the portion shown in the zoomed version. You can see what this looks like in Scope 3. The timebase and horizontal position controls are then used to change the extent and position of the zoomed window. It all works intuitively, with the lower window showing the zoomed graticule spacing and a time offset. We thought it was handy to view a longer waveform and also be able to inspect it closely. Our only complaint is that the zoom and time offset settings are reset when you leave the zoomed view. USB interfaces and software The included software is called HantekDSO2000; the QC1938 appears very similar to the Hantek model DSO2D10. From what we could see, only Windows software is provided; we tested it on a Windows 11 machine. The provided software includes a DigitalScope program, which is the virtual oscilloscope program that interfaces to the QC1938 and a Wave­ Editor program. WaveEditor is used to generate files for the arbitrary waveform generator on the oscilloscope. The included USB-A to USB-B cable allows a host computer to be connected to the USB-B socket at the rear of the oscilloscope. Oddly, we could not establish communication through any of the USB 3.0 ports on our computer. Interposing a USB 2.0 hub fixed this problem. The USB port in the back of the oscilloscope is not recessed like the power socket, so using the USB socket will eat into the space on your bench. Screen 1 shows the software with an active oscilloscope view. We found that the software mostly echoed the features and controls on the oscilloscope itself. The trace takes up most of the window, with the controls compressed to a small region above. When the software is controlling the oscilloscope, its controls are disabled, so we mostly preferred to use the oscilloscope in standalone mode without the USB connection to a computer. Screen 2 shows WaveEditor. It has various settings for generating simple waveforms such as sine, square and triangular waves. Waveforms can also be drawn freehand or imported from and exported to CSV (comma-separated variable) format files, allowing manipulation by spreadsheet programs. With that said, the inbuilt wave generator can deliver several different waveforms at an adjustable frequency and amplitude and can AM or FM modulate the output to produce complex waveforms without the hassle of delving into WaveEditor. The USB-A socket on the front of the oscilloscope is for connecting a USB flash drive. You can’t use it at the same Screen 1: the supplied “DigitalScope” PC program can control the oscilloscope via a USB cable. The controls on the oscilloscope are disabled when the USB interface is running, so we didn’t use this software much. It might be convenient if you take a lot of screen grabs as they can be stored directly on the host computer. siliconchip.com.au Australia's electronics magazine December 2022  51 time as the other USB connection since the controls are disabled. As you might expect, it can be used for exporting screen grabs. These are transferred as uncompressed 24-bit bitmaps (BMP) files. We prefer PNG files as they are compressed (losslessly) and thus take less time and space to move around, but storage is cheap enough that it is not a big problem. Saving a screen grab is as simple as pressing the “SAVE TO USB” button on the oscilloscope’s front panel. Unfortunately, the oscilloscope does not have a real-time clock, so it can’t timestamp the grabs. Instead, the files are stamped based on the time since the oscilloscope was turned on, so you can at least distinguish the order within a given session. Display configurations (real and calculated trace scales and positions, measured parameters) can also be saved and recalled. There are internal memory slots for this purpose, too, so we would use this feature mainly for different testing schemes. Waveforms for the arbitrary waveform generator can also be transferred to the oscilloscope as files via a USB flash drive. As noted, these can be created in the WaveEditor program. And as we found out earlier, firmware upgrades can be applied by copying the necessary upgrade file onto a USB drive and starting the upgrade process from the UTILITY menu. Like many other oscilloscopes, the most use we made of the USB feature was to create screen grabs by saving them to a USB flash drive, although we occasionally uploaded waveforms from the WaveEditor program. The display and controls on the oscilloscope itself are pretty good, so we found little use for the host USB interface, especially as the cable sticks out the back and gets in the way. Our evaluation The main features of the QC1938 are laid out in a standard manner, so they should be familiar even if you are only familiar with the most basic oscilloscopes. There are a couple of missing features that would have been nice to have, such as a real-time clock and the ability to export PNG screen grabs. While the former would require a small amount of extra hardware, the latter could possibly occur in the future with a firmware upgrade. The 100MHz bandwidth is ample for almost anything we do, although you need another oscilloscope probe to use the full bandwidth on both channels. Some of the values that are dialled in using the V0 knob have a very broad span and can take a while to select. It’s a pity that it does not have a fine/coarse adjustment option to speed that up. The MEASURE and MATH displays are very useful for gleaning extra information about a waveform. There is some noticeable warmth above the vent at the left-hand end of the case, which is presumably where the power supply is located adjacent to the incoming mains. But we never noticed it getting too hot to touch. Summary While we have some minor feature requests that we’ve seen before on other similar oscilloscopes, overall, the QC1938 is an oscilloscope that is easy to use and will do practically everything the average user needs. The 100MHz bandwidth is fairly standard and covers many use cases, although we definitely recommend purchasing a second proper oscilloscope probe. We also recommend performing the firmware update straight away if your oscilloscope is on firmware version 3204 or earlier. This is certainly a good choice of oscilloscope for reasonably advanced users but also for anyone getting started with oscilloscopes; it has the features to make it useful for years to come. The standard inclusion of serial protocol decoders, a waveform generator, a decent memory depth and zoom feature makes it good value at the price. The QC1938 DSO is available from Jaycar stores and online (www.jaycar. com.au/p/QC1938) for $549, including GST, at the time of writing. Adding the second QC1902 oscilloscope probe SC brings the total to $588.95. Screen 2: “WaveEditor” makes it very easy to create all sorts of arbitrary waveforms. They can be saved as files to be copied over to the oscilloscope. As well as some basic presets, waveforms can be drawn or imported from CSV files. 52 Silicon Chip Australia's electronics magazine siliconchip.com.au