1 kHz sine wave.
For someone who already owns a computer but not an oscilloscope this could be a good entry level instrument. Its price is right at about 75 dollars. The bandwidth of 20 MHz might be a deal breaker but many hobbyists I know only build and experiment with audio amplifiers so that should not be a limitation. Although I own a 200 MHz scope I keep the 20 MHz filter turned on most of the time. Let's take a closer look.What Does It Offer.
Main Features.
- Bandwidth: 20 MHz.
- Channels: 2
- Vertical Ranges Without Probe: 20mV/div to 5 V/div in 1, 2, 5 sequence.
- Vertical Ranges With X10 Probe: 200mV/div to 50V/div.
- Vertical Ranges With X100 Probe: 2 V/div to 500 V/div.
- Input Coupling: DC only.
- Invert: on/off, each channel.
- Horizontal Sweep Ranges: 5000 seconds/div to 1 ns/div set in a 1,2,5, sequence.
- Trigger Mode: Edge only.
- Trigger Sweep: Auto, Normal, Single.
- Trigger Source: Channel 1, Channel 2.
- Trigger Slope: + (positive), - (negative).
- Capability to save and reload data in several text and one image format.
- Capability to save and reload settings.
- Cursors: Vertical, Horizontal, and both, selected by icons on the toolbar or from the menu.
- Math Functions: A+B, A-B, A*B, A/B, and FFT.
Although shown in the menu and on the toolbar the interpolation function apparently has not been enabled on this model.
Similar to other USB scopes I have used it seems that the designers couldn't get the power consumption low enough to be powered from a single USB port. The box connects to the scope via a two headed snake which needs two open sockets on the host computer to operate. It only takes up one port in the operating system.
The calibration is a little off but I have never met a USB scope that was correct right out of the box. There is a calibrate function in the menu.
The first thing I did was to look at sine waves of different frequencies. I used the X10 probe and connected to the function generator via a short BNC to pigtail adapter, about 1 inch long. At 1 kHz it looks pretty good. Except for the missing trigger time marker.
It seems likely that persons reading this page may have never operated a DSO (digital storage oscilloscope). You may have an old analog scope but want to move into the digital age. Here is what a sine wave looks like on a high quality DSO.
1 kHz sine wave.
Unlike an analog scope the timing of the trigger point can be set along with the voltage level. In the figure above the yellow triangle at the right of the screen marks the voltage level. Its color tells the user which channel is the source and the brightly illuminated half says which slope has been set for triggering. The blue triangle at the top center of the screen tells where in time the triggering takes place. Draw a line down from the blue triangle and another to the left from the yellow triangle on the right and where they cross is where the sweep is triggered. The trigger time is adjustable as well as the trigger level. The trigger time control takes the place of the horizontal position control on traditional analog scopes.
Below is the same frequency wave probably from the same function generator as above. The trigger level is clear enough. On the computer screen it is set using the mouse by the procedure known as "drag and drop". The 100 kHz image shows the entire control panel. Note that there is no other way to adjust the trigger level. The trigger time should also be a "drag and drop" but it seems to have been overlooked by the designers. There is no marker to tell us where the trigger time is. It might be near the center of the screen but that is pure conjecture. On the 10 kHz image the trigger point might be at -2.75 (2.75 divisions to the left of the center) or + 2.3 divisions. (Note: That's how trigger time is represented in the digital world.) Without a marker there is no way to tell and nothing to drag and drop.
1 kHz sine wave.
The 1 kHz wave above and 10 kHz wave below look as good as they can on this scope.
10 kHz sine wave.
But at 100 kHz all hell breaks loose. Still pictures don't tell the whole story. I tried taking a movie with my digital camera. I could transfer it to my computer and view it but when I tried to include it in a web page it wouldn't show its first frame and wouldn't show when clicked or double clicked. The stop point of the wave changes more than 10 times a second. At higher frequencies, 1 and 10 MHz the part of the wave on the left half of the screen begins to jitter as the part on the right is randomly chopped off.
It was my original intention to evaluate the rise time and high frequency cutoff but at the extreme edge of performance there was so much jitter as to make any measurements impossible.
100 kHz sine wave.
1 MHz sine wave.
10 MHz sine wave.
This scope is of no use beyond the audible frequency range. That's a shame too because it will give digital scopes a bad name among the remaining analog diehards. However, it is not a total loss.
It is completely usable over the frequency range commonly given as audible to the human ear. That is 20 to 20000 Hz. And because it is a DSO it is usable far below 20 Hz. No analog scope can make that statement. (There were storage scopes made by Tek and HP but they never worked very well even when new. I'm sure as the special CRTs aged their performance dropped off so much as to make them useless.)
The slowest sweep rate is 5000 seconds / div. That's 1 hour 23 minutes and 20 seconds just to move one division. It's ten times that for an entire trip across the screen. That's an extreme case but the low frequency capability is there. Many homemade tube amplifiers exhibit a response peak in the range of 1 to 5 Hz. This is usually unsuspected by their owners because they don't have any instruments capable of generating or detecting this low frequency peak. Transistor amplifiers can have a response peak at frequencies of less than 1 hz. Only a DSO teamed with a digital function generator can detect and troubleshoot these peaks.
The one outstanding feature is the FFT (fast Fourier transform). Here is what it shows on a 1 kHz square wave.
FFT on 1 kHz square wave.
Conclusion.
I can only give this little number 2 stars with a "don't buy unless it's all you can afford" recommendation. I would recommend you spend a little more money and buy one with AC and DC X input coupling, External trigger, trigger coupling of AC, DC, LF Reject, and HF Reject, and wave form averaging.I have no problems with USB scopes as long as the software is well designed and updated regularly. Check with current users' of the one you want to see how well the makers have done with fixing bugs. If people have stories about indifference to bug reports, don't buy that one.