RoadTest: Picoscope 2205A Oscilloscope
Author: gpolder
Creation date:
Evaluation Type: Independent Products
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: stand-alone oscilloscope from Rigol, Tektronix, Owon, Siglent, and others.
What were the biggest problems encountered?: Works fine with VirtualBox on MacOS X, but native Mac software would be preferred.
Detailed Review:
First of all thanks to Element 14 for giving me the opportunity to review the Picoscope.
I start with some unboxing pictures. This is the box I got.
And here the opened box.
Here a picture of everything whats in the box. From the ruler on the bottom you have an impression of the size. Units are in cm! The Picoscope itself is a very neat and little box, with three BNC's on one side and an USB on the other side.
Bottom side of the Picoscope, showing the most important specs.
One of the probes.
On the bottom of the box is a poster with some basic information on using the instrument.
Since I don't have a plain PC available I needed virtualization software to run PicoScope on my Mac.
Native Mac OS X software would be preferred, but unfortunately this is not available.
A Linux version is available although it is not fully supported by picotech. Hopefully there will be a Mac version in the future.
But for the time being it only runs in a virtual machine, like Oracles VirtualBox, Parallels desktop or vmware fusion.
For this test Oracles VirtualBox was used.
Instead of using the supplied CD, I downloaded the most recent version of picoscope from the pico tech website and installed it on a Windows 7 virtual machine.
Although PicoScope properly started, it didn't find the connected device in first instance.
Further investigations of the windows device manager showed that the driver was installed, but it could not be started.
The problem was that USB ports on VirtualBox standard only implement USB 1.1. It is possible to enable 2.0, but this can only be done after installing an extension pack.
See https://forums.virtualbox.org/viewtopic.php?f=24&t=36752 for more information.
After these adaptations PicoScope runs just fine, as can be seen in the image below:
Next step was to test whether PicoScope can run from CrossOver, which is a more integrated Mac environment than VirtualBox.
CrossOver is a polished version of Wine provided by CodeWeavers.
Unfortunately it didn't work. I tried to install PicoScope in a WinXP 'bottle' and in a Windows 7 'bottle' but both didn't work as you can see in the error message below:
(a 'bottle' is the term Wine uses for virtual machine)
So ideas to get this working are very welcome.
Now the scope is unpacked and the software installed, we are ready for the real thing.
For the evaluation I let me be guided by the excellent articles on reviewing oscilloscopes by Colin O'Flynn of circuit cellar.
Colin gives a nice consideration for choosing a pc-based or stand-alone instrument. Personal preference determines your choice.
It is clear that the PicoScope 2205A is a PC-Based instrument. I have some experience with old analog low bandwidth scopes and
recently with an Siglent digital scope with a bandwidth of 70 MHz. The Siglent is equipped with an usb port and can also be operated from a PC.
I have to say that I never used this option, I prefer to use a scope standalone. On the other hand the picoscope is rather small and can easily be taken with you in your laptop bag.
So for portable usage the picoscope has its advantages. Another advantage is the large screen of your laptop where you can display several signals at once in high resolution.
A big disadvantage of the picoscope is that there are no knobs. When peeking and poking in my electronic designs to find the fault, or try to tune the circuit to its best performance, I like to press buttons or tune knobs, in stead of choosing menu items and fill in input boxes. But as already said this is a strict personal preference.
Where is the ground is an important question for all scopes, but you have to be careful when your measurement ground is equal to the USB ground, You need to ensure there isn’t a voltage difference between the ground of your device under test and the computer.
looking at the specs, the capabilities of the picoscope 2205a are more or less standard. Two inputs with a max rating of ± 20V pk. So with the probe at 10:1 the input range is ± 200 V.
Minimum range is ± 50 mV. The input input impedance is 1Mohm and 14 pF. 50 ohm input is not available. The 25MHz bandwidth is a little bit low compared to low level desktop scopes from Rigol or Siglent.
The scope comes with two probes (part number MI007). The probes are rated at 15 MHz for 1:1 attenuation and 60 MHz for 10:1 attenuation.
The quality looks reasonably, it doesn't have a spring-loaded tip, but the size of the tip is pretty small. The switch for attenuation and the compensation trimmer are on the probe head.
There are even plastic guard add-ons, which fit standard surface mount device (SMD) sizes (e.g., 1.27 mm, 1 mm, 0.8 mm, 0.5 mm) to probe TQFP/SOIC/TSSOP packages.
I measured the analog bandwidth by applying a 10 MHz signal, note the amplitude and then increase the frequency until the amplitude is 0.7 times the original value.
You can see the result in the pictures below. As signal source a DDS-60 direct digital synthesis signal generator is used. The probe at 10:1 is rated at 60 MHz.
BTW, a nice option of the picoscope software is that you can display measurement values below the signal, as you can see in the pictures above where the AC RMS value and the frequency are displayed.
As you can see the -3 dB point is at 29 MHz, which fits well within the specifications of 25 MHz.
I did the same measurements using the probe at 1:1, see result below:
As you can see the -3 dB point is 22 MHz which is well beyond the probe rating of 15 MHz.
The sample rate of the 2205A is 200 MS/s, which means 8 samples per period for a 25 MHz signal when using single channel, or 4 samples when using the scope in dual channel mode.
There is an option in the software to reduce the samplerate, which I tried, to see the effect of the number of samples on the signal, but unfortunately this didn't work.
In all cases the samplerate sticked to 200 MS/s. See figure below:
With 8 or 4 samples per period you can't give a good visual representation of your signal, luckily this scope does have equivalent time sampling (ETS) mode, which makes it possible to show more samples when measuring periodic signals. The caveat is that this high sample rate is achieved by doing careful phase shifts of the A/D sampling clock to sample “in between” the regular intervals. This requires your input waveform be periodic and very stable, since the waveform will actually be “built up” over a longer time interval. Read the articles of Colin O'Flynn for a detailed description of this mode.
As with all scopes in this price class the adc resolution is 8 bit, resulting in 2^8 = 256 different quantization levels. For visual evaluation of your signal on the screen this is fine, but when you do some calculations, for instance FFT to show the frequency the result is hampered by quantization noise. The picoscope software has an option to enhance the quantization resolution by applying a moving average filter. But as always this comes at a price. For the filter you need more samples, so the bandwidth and thus the maximum frequency will be reduced. This option is wel described in the picoscope manual at page 114.
Below you can see the effect of the resolution setting. The first image is a sine captured at 8 bit, zoomed in on the y axis.
The same signal captured at 12 bit is shown below, it more resembles the original sine.
The memory depth of the this scope is rather small, just 16 kS, which means that it can store only 80 us of data at 200 Ms/s. When you need to store over a longer period of time, it would be needed to lower the sample rate, but as already said in topic 2.2 this didn't work for me. The manual has a bit vague statement on page 126 that 'The actual number of samples captured may be different from the number requested, depending on which timebase is selected and which scope device is in use'.
Since the FFT is done on the PC it outperforms desktop oscilloscopes. Where a desktop oscilloscope only has 4096 or 2048 bins for the FFT, on the PC you can set a much larger buffer. Below you see an example of a FFT of a sine wave at 1 kHz, on a Siglent desktop scope, and on the picoscope application using a buffer of 65 kS. The downside is that it takes several seconds to calculate the FFT.
Depending on the settings you have chosen, PicoScope may store more than one waveform in its waveform buffer. When you click the Start button or change a capture setting, PicoScope clears the buffer and then adds a new waveform to it each time the scope device captures data. This continues until the buffer is full or you click the Stop button. You can limit the number of waveforms in the buffer to a number between 1 and 10,000 using the General preferences page. Using this option you can set the scope to trigger on a event, and then it wil record a number of wavelengths. You now can search for abnormalities in the saved buffers.
This means that the scope can be controlled from the PC, using dedicated software, but also using general software like Matlab, Python, C, C++, .net etc. Where most modern desktop scopes have this option, for the picoscope this is a key concept. Besides the full-function picoscope software, there is also a SDK (software development kit) available to use the picoscope from your own applications.
Decoding of serial protocols is another useful feature, which is available in picoscope. Although for the 2205A with only 2 analog inputs it has less value than for other models with more digital inputs. I did not test it further, so for some experiences I like to point to the review of honx (http://www.element14.com/community/roadTestReviews/1718).
As already said, picoscope version 6 offers a lot of software options, this is really an advantage of this oscilloscope above bench top models.
Also the user interface and the screen layout are very intuitively.
There are a eight different triggering modes, like simple pulse, advanced, window, interval, etc. When selecting a trigger method the software gives a brief explanation with a picture of the signal. Compared to a bench top scope this is very helpful, but I have the impression that the total functionality is somewhat limited. This has of course to do with the fact that triggering is a hardware issue, and all possibilities needs to be implemented in this small blue box. The picoscope manual show some more additional options, but these are for more expensive picoscope hardware.
Most oscilloscopes also have an “external trigger input.”, the 2205A has not! This is a shortcoming of this device. Of course, you can use the B input as external trigger and A for the signal, but this will cost you bandwidth, since your trigger channel does count against the “ADC channels.” So no full sample rate on one channel and trigger on another, or two channels with external trigger with this scope.
The 2205A is equipped with a so called arbitrary waveform generator (AWG). It is the third BNC connector on the front. It does have quite some possibilities, as it can generate sines, square, triangle, ramp, Gaussian signals, sin(x)/x, half sinus, and arbitrary waveforms, which means that you can draw any waveform you want. It is also possible to sweep the output signal from one frequency to another, with a certain time step. This feature can be very useful for testing filter responses. The maximum frequency of 100 kHz is a little bit limited.
Clock synchronization can be very useful, as described by Colin O'Flynn of circuit cellar, but this feature is not available on the 2205A.
So to finish up this review, in conclusion, the picoscope software is great, the 2205A hardware is a little bit limited, with its bandwidth of 25 MHz and just 2 signal inputs. Compared to Chinese scopes from Rigol, Owon etc, the price is high. For a little more you will get a bench top scope with much higher bandwidth, (50 or 70 MHz). Although you don't get a signal generator with those scopes.
For on site servicing, you can put the 2205A with a laptop in your bag, which is a big benefit over carrying another bag with your desktop scope.
I really don't like that there is no native Mac OS X software. With the growing popularity of Apple product PicoTech should invest in this.
Since summer 2014 a (beta) Mac OS X version is available. (https://www.picotech.com/support/topic14197.html)
They maybe also have to think about support for the iPad and iPhone, but then it should be interfaced via WiFi or bluetooth in stead of USB.
Thanks for reading, and if there are any questions left, please post them below this article.
