Learning Oscilloscope Basics with the RS PRO 2205A-20Follow article
Setting up the software, getting started.
If you have read any of my other blog posts you will know I have done a fair few audio-based projects. I have always tested them by plugging in an audio source and then simply listening to the output. This is obviously less than ideal if I want a more accurate picture of how any circuits I build are affecting the audio signal. As I have some more fairly ambitious projects planned, it was time for me to get to grips with the oscilloscope.
Being of a certain age the word oscilloscope immediately conjures up images of a green cathode ray tube, but as I discovered, things have moved on somewhat. Digital versions are now common, including PC based oscilloscopes that connect to your computer via USB, with the PC providing the display and the control interface using installed software. This has the advantage of reducing cost, increasing portability and making it easy to capture traces.
The RS 2205A-20 PC Based Oscilloscope
I decided on an RS 2205A-20 modelwhich is a small practically pocket-sized black box, about 9cm x 12cm x 2cm. It has 2 channels and a signal generator so is ideal for testing audio equipment. It is powered over USB and comes complete with a USB cable and 2 probes.
An extra probe with interchangeable tipsalso proved a good investment, as it allows me to easily connect the output of the signal generator and provided some alternative probing options for the future.
The PicoScope Software
The RS 2205A-20 uses the PicoScope software which is freely downloadable from the PicoScope web site and is available for Mac, Windows, and Linux, including a Raspberry Pi version which means you could easily build a relatively inexpensive standalone oscilloscope.
Once downloaded it is a good idea to install the software before plugging the oscilloscope into your PC if you are running Windows. This way it detects the hardware and installs the USB drivers automatically. Installing on my Windows PC presented no problems.
There is a user guide in the Help menu that offers a good introduction to the software and there are loads of YouTube Tutorials mainly aimed at the automotive industry, but I found one on using the oscilloscope to measure the output level and gain of an audio amplifier, which was exactly what I wanted. But as usual, I was getting a bit ahead of myself and I first needed to check the compensation of the probes following the instructions in the accompanying leaflet.
It is simply a matter of generating a square wave and then connecting the output to the probe.
Then set the switch on the probe to the 10x position and use the supplied tool to adjust the trimmer until you can see a flat top on the square wave in the display.
Measuring the output level of an audio amplifier
I thought I would start with something relatively simple as an introduction to the oscilloscope and the PicoScope software, and found a YouTube tutorial on measuring the output level of an audio amplifier. I have the Nutube headphone amp that I had built a while ago and decided to use that. I plugged two 3.5 mm Stereo Jack Plugs into the amp without their covers, so that I could connect the probes, with an input sine wave and output to the oscilloscope.
As instructed in the tutorial, I set the probes to 10x on the probes themselves and in the probe settings in PicoScope.
I generated a sine wave by clicking the Signal generator button in PicoScope and set it to 1kHz and 100mV. As the signal generator output and probe A were both connected to the headphone amplifier input I could immediately see the sine wave. It is shown in blue in the picture above.
When I turned the amplifier on I could also see the sine wave on Probe B connected to the amplifier output in red on the picture.
As I adjusted the volume I could see the amplitude of the output sine wave increase. I also discovered setting the Trigger to Auto stabilised the display.
So far I had been using the overlay display but PicoScope also allows you to have displays running in separate windows side-by-side. To do this simply click Add View in the Views menu and a duplicate window will be created below the existing one. By right-clicking on a window you can select which channel it displays and rename it, so in this example, I have Input in blue in the upper window and Output in red in the other.
Not just sine waves
To see the oscilloscope displaying some different waveforms I first tried using the Bela Effects unit I had built for the Red Tin. I connected it in exactly the same way I had the headphone amplifier and could see that the output was not just amplifying the sine wave input but distorting it somewhat too.
I have been looking at ways of generating random numbers for a forthcoming project and have built the Noise Generator Kit that was produced for the Open Source Hardware Camp in 2016. It generates random numbers by sampling white noise produced when billions of electrons smash into silicon atoms. When assembling this, I deliberately left the pins of the speaker protruding from the back of the board so that I could easily hook an oscilloscope probe on and sample the white noise, which can be seen in the picture above.
I can see that the oscilloscope is going to be an invaluable tool for testing the audio projects I have in the pipeline!