- Created on Sunday, 24 January 2010 10:42
- Last Updated on Saturday, 10 March 2012 21:04
- Published on Saturday, 03 March 2012 10:48
Laboatory research is about verifying theoretical results by acquiring information through electrical measurements, typically by using data acquisition instrumentation. The instruments used for this purpose usually fall under the general category of oscilloscopes. For a hobbyist design engineer this can be quite an expensive item, especially if you consider items at the top end of the market.
If like me you have been eyeing the USB PC-Oscilloscopes offered on Ebay and similar outlets at quite modest prices, then as an electronic design hobbyist, every time you see one you must feel like a lion placed in a game reserve fenced off from a load of free running antelope.
However, unlike the lion that can do nothing about his situation and who probably consoles himself by thinking the antelope wouldn't taste very nice anyway, I have been fortunate enough to get my hands on one of these prized hobbyist possessions: The ever versatile USB PC Oscilloscope! Yum!
The USB PC oscilloscopes on offer come, as you would expect, in a variety of forms and sizes and probably colours too. However, the main selection criteria, when choosing an oscilloscope, are the:
- Bandwidth and the
- Real Time Sampling Rate.
In my case the oscilloscope that I finally have in my possession is manufactured by a company called Hantek (http://www.hantek.com.cn/english/) and provides a bandwidth of 60 MHz and a real time sampling rate of 150 Million Samples per Second (MS/s).
The model I purchased is one of those that interface to two USB ports on your computer, one of the ports seems to be used solely for power whilst the other is used primarily for data. Which sounds quite reasonable until when, however, you consider using it, like I do, with a laptop that has only four USB ports. Then you might need to consider adding a USB hub to the list of accessories that could be needed to accompany the practical use of the oscilloscope.
Why? Well, if you consider my case where I have a USB mouse and a USB JTAG configuration cable semi-permanently attached to my laptop, then the USB oscilloscope would occupy my two remaining USB slots. This should not be considered to be a show stopper however, merely a mild inconvenience.
That apart, there is no doubt that once you have made your purchase and a few days later you hear the postman's knock on your door, you eagerly want to tear open the packaging and open the box. If however you have purchased the item directly from Hong Kong as I did, to save a few bucks, then you probably have to wait for a few weeks and when it does arrive look for a decent pair of scissors to cut through the endless layers of packaging! When you finally remove the packaging your thought is probably : so what's in the box?
Figure 1 : The DSO-2150 USB PC-Oscilloscope with the accompanying CD is "officially" compatible with most versions of Windows and even, reportedly ,Windows 7.
Well, the Hantek DSO-2150 can be seen in Figure 1. It is housed in an instrumentation grey case with three 50 Ohm BNC inputs on the front labelled CH1, CH2 and EXT. Each of the inputs is capable of accepting a voltage of up to a maximum value of 35V. At the rear end is a Type A USB socket along with a 2Vpp, 1KHz calibration signal hook.
Figure 2 : Included in the packaging are two oscilloscope clip probes. Each one is worth about 6% of the total cost of the USB PC Oscilloscope package. Oscilloscope clip probes are fairly cheap these days. I remember once upon a time when you would have paid an arm and a leg and probably thrown a shoe at the sales assistant on your way out of the shop for probes like these ones!
Included are two oscilloscope clip probes as can be seen in Figure 2. I can't really determine the quality of the probes as I am not an oscilloscope aficionado. However, as can be seen in the image(s) acquired and displayed later on this article they did not seem to impact negatively on the data acquisition process.
Figure 3 : The accompanying CD consists of the data acquisition software, the DSO-2150 device drivers and the SDK for Lab View, Visual Studio and Visual Basic.
Once the excitement of seeing the box and its contents has subsided most users would decide that the next thing to do is to install the software (v220.127.116.11) and I am no exception. This is an incredibly painless affair on computers using the supported versions of the Windows operating systems, as you only need to point and click and follow the on-screen instructions. After the software installation all you need to do is plug in the oscilloscope install the software drivers and away you go. Gone are the days of plug and pray on the Windows platofrm!
Although Windows 7 is not one of the listed supported operating systems on this model, a search for users of similar items on the Internet would suggest that the software should work all the same. Although I have not tested this personally. What about LINUX users then?
Well, although there is no official support for LINUX users from Hantek as far as I know, "Googling" threw up a pleasant surprise of a solo sourceforce effort called "Hantek DSO-2150 Software". At the time of this writing it is in version 0.5.3 (HantekDSO-0.5.3.tar.gz). You can read more about it at http://sourceforge.net/projects/hantekdso/. When I get to try it myself I will blog my findings.
Also on the CD are Software Development Kits (SDKs) for Labview, Visual Basic and Visual C. On discovering this I was quite disappointed as I thought that a Borland C++ SDK was supported too. I will have to investigate a bit further to see whether Borland C++ is still supported as it would have been my favoured software development route. I will need to do some investigation and report back!
The DSO-2150 is a very much self-contained USB PC oscilloscope and by that I mean it is usable straight out of the box, once the software has been installed, without any additional expense necessary. However, if you have designed (or intend buying!) a prototype driver board similar to the one designed in Series 1 then you could consider purchasing a few BNC to BNC cables (1) as seen in Figure 4.
Figure 4: Although not required, purchasing a few BNC to BNC cables (1) and a SMA to BNC connectors (2) could prove to be very handy indeed!
Likewise, if you have a development kit similar to the Xilinx FPGA Spartan 3A Starter Kit then you could also consider purchasing a few SMA to BNC connectors (2) too. I find these two items particularly useful especially if you require connecting the SMA connector to the oscilloscope's EXT input for use as an external clocking source. A typical example of when you could be required to do this is when you are debugging a serial data stream generated in VHDL from the starter kit and you need a reference signal to clock the serial data against.
Well, before you purchase one of these items it could be a good idea to determine whether a particular model meets your particular requirements or not. A way to do so is to study the hardware specification parameters (for the DSO-2150) shown in Figures 5 and 6 and found on most websites that sale similar units.
Figure 5: some Hardware specification parameters......
- Channel : The DSO-2150 like most oscilloscopes has two channels for acquiring data.
- Impedance: The input impedance consists of a 1MOhm resistor and a 25pF capacitor in parallel.
- Coupling : AC, DC and GND: The method by which the oscilloscope probe inputs are connected or "coupled" to the internal acquisition circuitry.
- Vertical Resolution: An 8-bit data acquisition process is used. Offers 256 quantisation levels of the sampled input data. Not bad but could be better especially by today's standards.
- Gain Range: 10mV-5V, 9Steps:
- DC Accuracy: An error tolerance of +-3% is to be expected.
- Timebase Range:4ns - 1h, 38 Steps. The available sampling periods.
- Vertical Adjustable:Yes : ?
- Input Protection: Diode, clamping.: The method used to conduct unwanted currents to ground.
- X-Y:Yes : Can perform x-y plots useful for graphing I-V curves as well as plotting Lissajous Figures.
- Autoset : yes 30Hz to 60Mhz.
- EXT Input : Yes : Can trigger of of an external clock input.
- Trigger Mode : Auto, Normal and Single: The different methods of acquiring data.
- Trigger Slope: +/- : ?
- Trigger Level Adjustable : Yes: Good.
- Trigger Type : Rising Falling, Falling Edge: Good.
- Trigger Source: CH1, CH2, EXT. You can set the trigger off of any of the three inputs.
- Pre/Post Trigger : 0 - 100% : The percentage of the trigger level value to trigger against?
- Buffer Size: 10K - 32KB/Channel . You can set the oscilloscope to capture between 10,000 and 32,000 samples per channel. Which can be analysed in a package like excel or injected into a Testbench!
Figure 6 : .... and some more.
- Shot Bandwidth : DC to 60MHz : Only signals measured between these frequencies are guaranteed to provide reasonable results. Although in reality I suspect that you can probably measure signals up to between frequencies of 70MHz to 80MHz if the real-time sampling rate is 150 MS/s. Remember Nyquist!
- Max Sampling Rate : 150Ms/s :
- Sampling Selection :Yes : Must be good, Surely?
- Waveform Display : DC to 60MHz : Port/Line, waveform average, persistence and intensity. Good
- Network : Open/Close ?
- Vertical Mode: CH1, CH2 Dual, Add:
- Cursor Measurement : Yes
- Spectrum Analyser : Can also function as a spectrum analyser with the following functions:
- Channel : 2 Channels
- Math : Can Perform the maths functions of: FFT, addition, subtraction, multiplication and division.
- Bandwidth : 60 MHz
- Cursor : Frequency, Voltage : An on-screen cursor is available.
- Data Samples : 10K - 32 / Channel.
- Accessories: S/W CD. Probes, Manual, USB Coord.
Application to the LQ043T3DX02 Driver Board
For those of you following Series 1 : A LQ043T3DX02 Driver Board you would think that the first thing that I would like to do on receiving this product is to hook it up to the prototype development board and see what the accompanying output looks like. Well you are right!
The VHDL code that generates the colour bars image seen in the Colour Bars Milestone picture and again in Figure 7 below, has the horizontal and vertical synchronisation signals, Hsync and Vsync respectively, not only connected to the display, but also pulled out to the two BNC test connectors as can be seen in Figure 7.
Figure 7 : Developing prototype development boards with BNC connectors could be very handy when FPGA code needs debugging as they offer a hands free experience! There is no need to mess around with hidden test points here, neither do you need to ask for assistance from family or friends assuming you have any left!
When you actually take measurements with the oscilloscope you might be as surprised as I was too hear a clicking sound when the gain setting is changed. After "Googling" I have been led to believe that the clicking is the sound of the internal relays being switched on and off as the respective voltage ranges are selected.
Amongst other reasons, relays are used as gain range switchers as they introduce very little distortion to the signals being measured. This is a feature of top-end oscilloscopes and should be considered to be a good thing! I now regard the clicking sound as a reassuring sound of the quality of the oscilloscope or so I am led to believe!.
Figure 8 :Finally, an image capture of the Hsync (green) and Vsync (yellow) signals of the prototype driver board in action, using the DSO-2150 In this particular screen shot there are 10 Hsync pulses between the negative pulse of the Vsync sigl as expected.
Am I a happy bunny? Well, considering the tone of this article you can probably gather that I am. The software is embarrassingly easy to use once it is installed and although it appears that LINUX support is still in its infancy, all in all I would say that this is a well put together piece of equipment. It took me quite a while to take the plunge and make this purchase as I had been sitting on the wrong side of the "game park" fence for too long but now that I have, I am fairly pleased.
Now that I have an oscilloscope to support the blog I can not only verify and relay the results to readers but I can also refine the system analysis and component selection process in some areas where the results are not quite as expected!
Is this the digital USB PC oscilloscope for you? Well, I would suggest that you perform a search on the Internet and try and gather as much information as possible, after all this is only a single opinion! However, for what I need it for, I would suggest that this is a thoroughly good purchase! for any hobbyist engineer.