PicoScope Technical Tips
Oversampling is a technique used with oscilloscopes and spectrum analyzers to reduce noise and effectively increase resolution and dynamic range.
To look at an 1 KHz sine wave on an oscilloscope, you might record 1000 samples at 10 KS/s. If however you take 4000 samples at 40 KS/s then average together blocks of 4 samples you will have reduced any random noise pick up and effectively increased the resolution of your sampled data by 1 bit.
The PicoScope 3206 can sample at 200 MS/s and collect up to 1 million samples. This performance is far more than is required for some applications such as when looking at audio signals. By oversampling the data by 256 times, you can effectively increase the resolution of the oscilloscope from 8 bits to 12 bits.
The main disadvantage of oversampling is that it can slow down the screen update rate as more samples have to be transferred back to the PC, with the fast USB 2.0 interface of the PicoScope 3000 series this effect is minimal. The other potential disadvantage is that oversampling acts like a low pass filter so has the effect of reducing or removing high frequency signals. This is usually not an issue for the spectrum analyzer where you have chosen the frequency range of interest, but can be an issue when using the oscilloscope as it may suppress spikes and other signals you wish to observe.
With the PicoScope 3000 series it is possible to set different oversample settings for the oscilloscope and spectrum analyzer. To do this open psw.ini (usually in c:\windows) in a text editor such as notepad.
Next find the section that starts with [PicoScope for Windows] and add two additional lines as follows:
MaxScopeOversample=1
MaxSpectrumOversample=64
In the above example, we have set the scope oversample to 1 (no oversample) this ensures any high frequency spikes will be displayed properly. If you wish to filter an oscilloscope trace to remove random noise, try using the Current | Filtered display mode instead.
By setting the spectrum oversample to 64 we effectively gain 3 extra bits of resolution, so increase the dynamic range of the spectrum analyzer. The max oversamples can be set between 1 and 256, but please note that it is not always possible to oversample - for example if you set the spectrum analyzer frequency to 100 MHz then the hardware is already sampling at 200 MS/s so no oversampling is possible.
The spectrum analyzer built into the PicoScope software has a number of powerful options that can be accessed by selecting 'options' from the 'settings' menu.
These include:
No of spectrum bands- this defines how many "bins" the frequency range will be divided into. It can be set between 256 and 16384 (product dependant). Lower numbers give a faster update time, higher numbers give more resolution.
Window Type - this gives seven options on how the edges of the block of sampled data are filtered to reduce distortion. For most users the default window (blackman) is adequate, the PicoScope help file gives more details on which window should be used for which application.
Display Mode - this offers three options. "Normal" displays the spectrum for the current cycle. "Average" gives a moving average of successive cycles which is useful for removing random noise. "Peak" displays the maximum of all cycles (useful for bandwidth testing of filters and amplifiers.
X & Y Scales - these give options for the vertical scale to be in dB or Volts and the horizontal frequency scale either to be in linear or logarithmic form.
PicoScope has over thirty automated measurements. Oscilloscope measurements include frequency, rise time and pulse width. Spectrum measurements include THD, SFDR and SNR.
For each measurement, upper and lower alarm limits can be set. This makes it possible for example to set an alarm if a pulse width goes out of specification. To add an alarm, first add the Measurement you wish to use using
Settings | Measurements | Add
From the resulting dialog box enter minimum and maximum values to be used then click OK. From
Measurements | Options
you can select the action to occur on alarm, the choices are to either stop the scope (so capturing the error on the screen) or to sound an audible alarm.
To save a trace in PicoScope as a JPEG (.jpg) file:
File | Save as |
In the dropdown box labeled 'Save file as type' select the jpg extension. Now enter the name of the file in the 'File Name' box, not forgetting to include the .jpg extension and removing the astrix (*). Finally click OK or select the directory where the file is to be saved to and then click OK.
For many measurement applications, noise is a big concern. A typical example in the automotive field is measuring injector current using a current clamp. The signals from the current clamp are only a few mV in amplitude, yet nearby are ignition signals of many kV.
The best way to minimize noise is to try to eliminate it at source, in the above example keep the current clamp as far away from ignition components as practical and ensure all connecting leads are screened.
Physical precautions as described above will help minimize noise pickup, simple software filtering can often be used to further reduce its effects.
PicoScope oscilloscope software has several display modes that can be used to reduce noise pickup, the differences between these modes are described below and can be selected from the settings | options menu:
Current (Unfiltered)
This is the default display mode, it takes all the data captured by the oscilloscope and maps it onto the PC monitor using a min/max algorithm. This ensures any fast transients are displayed, but will also act to highlight any high frequency noise.
Current (Filtered)
This mode applies a simple low pass filter to remove high frequency noise from the oscilloscope display. This mode would be ideal for the automotive application described.
Average
This mode build up an average trace based on all cycles captured. This is useful for removing random noise from repetitive signals.
Min, Max and Average
This displays this displays a shaded area showing the minimum and maximum of all cycles captured. In addition it displays the average trace as well. As with the average mode, noise is removed from repetitive signals, in addition the shaded area gives an indication of how noisy the signal is.
PicoScope, our oscilloscope software has the useful ability to save a waveform to disk (complete with a time / date stamp) each time a trigger event occurs. This can prove invaluable when tracking down intermittent faults, especially those that happen infrequently over a period of hours or days.
The trigger mode can be enabled by selecting Settings | Trigger. By default a maximum of 100 waveforms will be recorded to disk (to avoid filling you hard drive). This default can be overridden by editing the settings in the WIN.INI file as shown below:
Using a text editor such as notepad, open WIN.INI (usually in c:\windows)
Find the section called [PicoScope for Windows]
Add a line MaxSOTFiles=200 (where 200 is the maximum number of files you wish to save)
Save WIN.INI and restart PicoScope
Thick trace lines in PicoScope
If you are using PicoScope for a demonstration to a group of people, or are visually impaired, you may find that the trace is difficult to see. It is possible to thicken PicoScope's line thickness to increase visibility
Choose File | Setup | Colors
Adjust the line thickness
Choose OK
Loading reference waveforms in PicoScope
A common requirement with oscilloscopes is to compare 'live' waveforms with stored 'reference' waveforms.
This can easily be done in PicoScope as follows:
Open the reference waveform
From the View menu select 'Freeze/thaw one'
Press the new scope icon to open a second oscilloscope window
Display the 'live' waveform in the second oscilloscope window
If the two scope windows have exactly the same settings (timebase, trigger, voltage etc) then the waveforms can be overlaid by choosing 'New Composite' from the View menu. More details on this can be found in the PicoScope help file manual under 'Composite View'.
PicoScope general hints and tips
Below are some 'cool features' within PicoScope software that you may not be aware of:
+Bar graph option in meter view. As well as the normal numeric view, there is also an option to turn on a bar graph view. To enable this, just select Settings | Parameters and click on the bar graph tick box.
+Trace thickness. If you wish to view scope or spectrum traces from a distance, or if you have a large number of people viewing the screen, you may find it helpful to use a thicker line to display the trace. This can be done by selecting File | Setup | Colors, which gives you the option of thin, medium and thick trace lines. You can also change the colors of the traces and backgrounds using this menu option.
Exporting data to spreadsheets
Data can be exported to a spreadsheet using EnviroMon for Windows or PicoLog by copying a pasting from their spreadsheet views. Data can be exported from PicoScope by choosing Edit | CopyAsText and pasting (select Paste Special) into the spreadsheet.
For more information on DDE see the 'Dynamic Data Exchange' section of the 'Technical Information' Chapter in the PicoLog, PicoScope help files.
