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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
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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
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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.
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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
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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.
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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.
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