PC Oscilloscope Basics
A PC Oscilloscope is a measuring instrument that consists of a hardware scope device and an oscilloscope program running on a PC. Oscilloscopes were originally stand-alone instruments with no signal processing or measuring abilities, and with storage only available as an expensive extra. Later oscilloscopes began to use new digital technology to introduce more functions, but they remained highly specialized and expensive instruments. PC Oscilloscopes are the latest step in the evolution of oscilloscopes, combining the measuring power of Pico Technology's scope devices with the convenience of the PC that's already on your desk.
Connections for the PicoScope 5243A and 4227
•Standard oscilloscope connectors
•PicoScope PC Oscilloscopes have BNC oscilloscope connectors. The inputs have an impedance of 1 M ohms, so they are compatible with all standard scope probes.
PicoScope 5000 A Series Oscilloscope Connections
A.Input channel A
B.Input channel B
1.Probe compensation output
2.LED: red when scope is connected but not operating. Flashes green when the oscilloscope is capturing data.
3.External trigger input
4.Signal generator output
5.DC power socket: for use with the AC adapter supplied.
6.USB 2.0 port: connects to your PC using the Hi-Speed USB cable supplied.
7.Earth terminal: Some laptop power supplies can produce electrical noise which may interfere with your measurements. If this occurs, the earth terminal can be connected to an external ground point (for example, on the system you are testing) to provide a ground reference for the scope. This can help to avoid external noise interfering with your measurements. Alternatively you can run the laptop using battery power.
The MxSuite PicoScope Transform is used to set up the PicoScope module with the required parameters (timebase, voltage range, trigger voltage, and sampling mode) and capture the waveforms to display in an MxVDev TestCase. The command to set up PicoScope is a message type input Signal from the Transform and the captured waveform is a continuous output Signal. The discrete Signal CaptureControl is used to start and stop the waveform capture.
Double-click on the plot line for ScopeSetupCmd to create a transition and configure its values. If you don't change the parameters, the default values are set for the Signal.
The ScopeSetupCmd message (the transition) must be transmitted before the CaptureControl Signal goes high.
Timebase: This defines the time gap between the two samples. Depending on the requirements, different timebase values can be selected.
The timebase can be varied and is set by default to seconds per division. The value goes up in multiples of 1, 2 and 5, for example 1 s/div, 2 s/div, 5 s/div. The minimum timebase varies from product to product depending on the sampling rate of the device, while the maximum timebase for all products is 5000 s/div.
Minimum sample interval achieved in PicoScope 4227 is 4ns i.e., at timebase 10ns/div with only one channel enabled. With both channels enabled, the minimum sample interval achieved is 8ns i.e. 20ns/div even if the user selects 10ns/div.
VtgRangeChA & B: Voltage scaling
The voltage ranges are selectable in increments of 1, 2 and 5. For example, ±100 mV, ±200 mV, ±500 mV, ±1 V.....
In each of these ranges the device maintains its full resolution, so a 12 bit device will use 12 bits in each of those ranges. Choosing the most appropriate voltage range will get the best detail out of a signal. If you use too large a range and zoom in, you will lose detail.
Select appropriate voltage range. Disabling both channels stops the test with a notification.
Trigger Source: This is the channel PicoScope monitors for the trigger condition.
Rising Edge: Click to trigger on the rising edge of the waveform.
Falling Edge: Click to trigger on the falling edge of the waveform.
Streaming Mode: Streaming mode is a sampling mode in which the oscilloscope samples data and returns it to the computer in an unbroken stream. This mode allows the capture of data sets whose size is not limited by the size of the scope’s memory buffer, at sampling rates up to a few million samples per second.
Block Mode: In this mode, the scope stores data in internal RAM and then transfers it to the PC. When the data has been collected it is possible to examine the data, with an optional downsampling factor.
Select one of the sampling modes to run the test. Streaming mode is the default.
DownsampleRatio: Used for downsampling or subsampling of a signal. This is usually done to reduce the data rate or the size of the data.
Select a downsample ratio value between 0-100, specifying how much to reduce the data.
DownsampleMode: There are some situations in which we might want to reduce the number of samples after capturing them in streaming mode. This can occur in PicoScope 6 when the scope has captured more samples than the number of pixels across the display. It can also occur with the PicoScope API, for reasons that will be unique to each application. All of these cases call for some method of data reduction.
Downsample Modes used are:
1.Average : Reduces every block of n values to just two values: a minimum and a maximum
2.Aggregate : Reduces every block of n values to a single value representing the average (arithmetic mean) of all the values.
3.Decimate : Reduces every block of n values to just the first value in the block, discarding all the other values.
Trigger Voltage: A trigger is a threshold voltage level that, when a signal passes through it, signals the oscilloscope to capture or lock onto the waveform.
Pre trigger: This is a very useful trigger adjustment which allows you to see what happened before the triggering. The images below show an injector voltage on a 500 µs/div timebase giving 5 ms (for 10 division) across the screen.
The first image shows a 22% pre-trigger with 1.1 ms of data before the trigger event. However, looking at the signal you cannot see what happened before this. The second image shows a 50% pre-trigger showing 2.5 ms before the trigger event, which fully shows what the signal does.
Select a Pre trigger value less than 100, as the value is a percentage. Exceeding 100% causes the test to stop with a notification.
Sampling Mode (Block Mode or Stream Mode)
You can choose whether to use block mode or streaming mode. If the amount of data that you need to capture fits in the scope’s buffer memory, then you should use block mode as this gives you the widest choice of sampling rates. If record length is more important to you than the sampling rate, choose streaming mode.
If you choose streaming mode, be careful with the Timebase as it doesn’t work for values less than 50us/div. (Sample interval at 50us/div is 192ns). In Block Mode, you can use any timebase value.
Note: There must be a gap of at least 4 to 5 seconds between PicoScope initialization and any Signal transition as it takes 4 seconds to open the PicoScope.
The CaptureControl signal is used to start and stop the acquisition of data. You can select the time to start capture and stop capture by setting the transition time.
When CaptureControl transitions to 1, it means Start Capture.
When CaptureControl transitions to 0, it means Stop Capture.
Note: ScopeSetupCmd must be configured before using the CaptureControl Signal, otherwise the TestCase stops and a notification appears.
AutoTune offset: Whenever this signal goes high , PicoScope will measure and compensate the both probe output.
ScopedataChannel1 & ScopedataChannel2: These ports will contain the data coming from Channel A and Channel B respectively.
Capture State: This will show the actual transition of acquisition of data.
ScopedataSize: ScopedataSize represents the number of data samples captured by the PicoScope.
TriggeratIndex: It represents the index value at which PicoScope triggered.
TransitionTime : It represents the sample Interval (in nanoseconds) used during the capture.
ScopeSetupCompleted: This signal represents the end of PicoScope initialization by going high.
AvgScopeAoutput & AvgScopeAoutput: These represents the average of the total data collected on respective channel.
Note: If you doesn’t want to use function Generator functionality, avoid exporting FunctionGenControl, Amplitude, Frequency and Offset signals.
Function Generator / Arbitrary Waveform Generator (AWG)
A function generator is usually used to generate different types of waveforms over a wide range of frequencies. Some of the most common waveforms produced by the function generator are the sine, square, triangular shapes.
The PicoScope Transform is used to set up the PicoScope module for the function generation with the required parameters (Offset, Amplitude, Wavetype, Frequency, sweep mode, and trigger) and capture the waveforms for display in an MxVDev TestCase. The command to set up PicoScope for function generation can be either a message type input Signal or discrete signals like Amplitude, Frequency and Offset from the Transform. The discrete Signal FunctionGenControl is used to start and stop the function generator.
To access the function generator from the Scope, add the ScopeSetupCmd signal to the test case and double click on the transition editor to open it. Select AWGControl from the “Select Scope Command” list box. Select the appropriate parameter values as described below.
The parameters for AWG Control are as follows:
Offset Voltage : the voltage offset, in millivolts, to be applied to the waveform. Range for the offset voltage is -20V to 20V.
Amplitude : the peak-to-peak voltage, in millivolts, of the waveform signal. Range for the Amplitude is -20V to 20V.
The sum of Offset Voltage and Amplitude summation can’t be more than 20V as these are the input to the microscope as per x10 mode and the maximum allowable voltage is 20V. If the Signal voltages described by the combination of Offset Voltage and pkToPk extend outside the voltage range of the signal generator, the output waveform will be clipped.
Wavetype : the type of waveform to be generated. For example, Sine, Square, Triangle, or DC voltage.
Start Frequency : the frequency that the signal generator will initially produce. Maximum and minimum allowable frequencies are 100 KHz and 30 mHz.
The Signal can be swept based on Stop Frequency, Frequency Increment, Dwell Time, and Sweep Type.
Stop Frequency : the frequency at which the sweep reverses direction or returns to the initial frequency. Maximum and minimum allowable frequencies are 100 KHz and 30 mHz.
Frequency Increment : the amount of frequency increase or decrease in sweep mode. Maximum and minimum allowable frequencies are 100 KHz and 30 mHz.
Dwell Time : the time the sweep stays at each frequency, in seconds.
Sweep Type: whether the frequency sweeps from Start Frequency to Stop Frequency, or in the opposite direction.
Operation: For B models only.
0: sweep the frequency as specified by sweeps
1...MAX_SWEEPS_SHOTS: the number of cycles of the waveform to be produced after a trigger event. Sweeps must be zero.
0: produce number of cycles specified by shots
1.. MAX_SWEEPS_SHOTS: the number of times to sweep the frequency after a trigger event, according to Sweep Type. Shots must be zero.
Trigger Type : the type of trigger applied to the signal generator.
Trigger Source : the source that triggers the signal generator. If a trigger source other than P5000A_SIGGEN_NONE is specified, then either shots or sweeps, but not both, must be non-zero.
ExtInThreshold : used to set trigger level for external trigger.
A transition (low to high) on FunctionGenControl signal is required to start the function generator. A high to low transition stops the function generator.
Amplitude, frequency, and offset are discrete Signals used as input to the function generator. Use these Signals to change these three parameters only. In these cases, you don’t have to use the ScopeSetupCmd message Signal to configure the function generator.
As shown in above diagram, the sequence for using function generator : A -> B -> C -> D
To capture the data, initialize the microscope by selecting Scope Command in the ScopeSetupCmd Signal as setup and then set the parameters accordingly. See the sequence shown in the image above.