| Glossary |
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AC/DC switch
The AC/DC switches for ADC-200 and PicoScope 2000 and 3000 series oscilloscopes operate under software control, rather than manually. When you are using an ADC-200 or PicoScope 2000 or 3000 series oscilloscope, one, two or four AC/DC combo boxes appear on the instrument toolbar.
Note: Changing the AC/DC switch settings in one instrument window will also change the settings in all the other instrument windows that are running.
Aliasing
When you take a set of samples from an input signal, any frequency in the input signal which is above the Nyquist frequency (half the sampling rate) appears to be at a frequency below the Nyquist frequency. This phenomenon is called aliasing.
For example, with a frequency of 20,000 samples per second, the Nyquist frequency is 10,000 Hz. If the input signal contains a component at 11,000 Hz, it will appear at 9,000 Hz.
It is easy to check for aliasing by changing the sampling frequency: real signals will stay at the same frequency, whereas aliased signals will move.
It is possible to reduce the effects of aliasing by taking several readings for each sample (oversampling), then applying a digital filter to attenuate signals above the Nyquist frequency. Note: This is only possible when the sampling rate is well below the maximum sampling rate at which the ADC can operate.
Alternate mode
If the event you are observing is repeated, you can use alternate mode. This enables PicoScope to take blocks of readings alternately from one channel to the other, at full speed.
Channel
This is for specifying from which channel to measure data.
Channel A/B
This is for specifying an input voltage range. If you select Auto, the ADC will automatically adjust the voltage range to suit the input signal. Selecting Off will turn a channel off.
Chop mode
The computer takes a reading from one channel and then a reading from the other channel. This is useful for events that occur only once and you wish to see the same event in both channels. The disadvantage of chop mode is that the sampling rate on two channels is half the sampling rate with one channel.
dB
Decibels. The decibel is a unit of power ratio. The decibel value can be expressed as:
| 10 x log10 (power / power_zero_db)
|
If you work this out when power = power_zero_db, you will find that the dB value is zero.
As power is proportional to the square of the voltage, this formula can also be expressed as:
| 20 x log10 (voltage / voltage_zero_db)
|
There are a number of possible values for the zero dB voltage. We have chosen to use 1 volt peak (0.707 V RMS).
Another measure is with respect to 0.776 V RMS (1 mW into 600 ohms). This is sometimes called db(m).
See Initialisation settings for information on how to change the zero-dB level.
ETS
Equivalent Time Sampling. This facility offers a higher effective sampling rate when used with repetitive signals, which some products support. Note: ETS should not be used for one-shot or non-repetitive signals.
Note: ETS is used only in oscilloscope windows. When you access this function from the trigger sample bar, some faster timebases are added to the instrument toolbar.
See under ETS on the Trigger Setup topic for more details.
FFT
Fast Fourier Transform. This is a process that converts a set of values equating to time into a set of values equating to frequency. PicoScope uses this technique to generate the spectrum analyser window.
Function
This specifies which function to use. The options are as follows:
DC volts. The DC component, or average input voltage.
AC volts. The RMS AC voltage. This excludes any DC component.
dB. AC volts converted to dB, with respect to 0.776 V. Equivalent to dB(m).
Frequency. Frequency of the AC component, in hertz.
Maximum frequency
This is for controlling the sampling rate of the data collected for spectrum analysis. It determines the highest frequency that the spectrum analyser can display.
Note: The lower the maximum frequency, the longer it takes to collect enough data to display a spectrum. At very low maximum frequencies, this may affect the response time of your computer to keyboard input.
Range
This allows you to specify an input voltage range, or select Auto so that the oscilloscope will automatically adjust the voltage range to suit the input signal.
If you have defined any custom ranges, these will appear in the list of options after the voltages.
Sampling status
This window indicates whether PicoScope is collecting data.
You can click on the Stop/Go button or press the space bar to start or stop collecting. If the computer detects that no ADC is connected, a message will appear here. When you plug in the ADC and turn on power (if required) the status will change to Stopped.
Stop/Go button
The Stop/Go button to the left of the sample bar has a dual purpose. When PicoScope is collecting data, the button displays the word 'Stop.' If you click on it, data collection is stopped. When PicoScope has stopped collecting data - because the Stop button or some trigger event has been activated - the button displays the word 'Go.' If you click on it, PicoScope resumes data collection.
Timebase
The timebase control allows you to adjust the value of the time interval across the oscilloscope display.
If you select Scope timebase is time per division in the Scope Advanced Options dialog box, the display works like a normal oscilloscope. There are ten divisions across the screen, so the total time interval is ten times the timebase.
Trigger channel
This is for selecting the channel to be used as the trigger input.
Trigger delay
This allows you to control the time delay between the trigger event and the start of data collection. There is a number and a time units field. You can type in a new value for the number, or adjust its current value using the up and down arrows.
Trigger direction
Use this to select the direction in which the signal crosses the trigger event threshold. The trigger directions are as follows:
Rising. Triggers when the trigger channel signal rises past the trigger threshold
Falling. Triggers when the signal falls past the threshold
Trigger mode
You use this to select the trigger mode. The trigger modes are as follows:
None. No trigger event, so it collects and displays data continuously
Auto. Starts collecting when a trigger event occurs or after the auto-trigger time, whichever is sooner
Repeat. Starts collecting only when a trigger event occurs. After displaying the data it waits for the next trigger event
Single. Starts collecting only when a trigger event occurs and stops after displaying the data
ETS. This enables equivalent time sampling (not available with all products)
If you select None, the remaining trigger options are shaded.
Note: If you select Repeat or Single when using an oscilloscope not belonging to the ADC-200 or PicoScope 2000 or 3000 series, PicoScope locks the computer until a trigger event occurs. If PicoScope does become stuck, click on the Stop button in the sample bar, then press F9 (16-bit applications) or F10 (32-bit applications) to cancel the trigger.
Trigger threshold
This shows the voltage, in mV, that the trigger channel must cross in order for a trigger event to occur. You can type in a new value for the trigger threshold, or adjust the current value using the up and down arrows.
Note: The trigger threshold is acted upon immediately. The minus sign on its own is not valid: if you want a negative trigger delay, type in a number followed by the minus sign.
X multiplier
When the X multiplier is set at 1, the computer displays the collected data across the full width of the window. If you change the value of the X multiplier to x2, the computer magnifies the data to double its original size, and displays a scroll bar across the bottom of the window. You use this to scroll through different portions of the data.
Note 1: You can change the Y multiplier to expand the voltage range.
Note 2: If you set the X multiplier to a value of x10 or above, it may be worthwhile increasing the max samples per scope trace value.
Note 3: In the XY Scope display, the X multiplier acts on the input voltage selected for the X channel, not on the timebase.
Y multiplier
When the Y multiplier is set at 1, the computer displays the full voltage range within the boundaries of the instrument window.
If you change the Y multiplier to x1, the computer initially displays the full voltage range, but also displays a scroll bar. You can use this scroll bar to move the trace up or down, away from other traces.
If you change the Y multiplier to x2, the computer displays only half of the voltage range, but at double magnification. The scroll bar now controls which portion of the voltage range is displayed.
Note: You can change the X multiplier to expand the time axis (or, in the XY oscilloscope window, the horizontal voltage axis).