3.15. Parameter Sweeping
Note
Parameter sweeping is a beta feature and is subject to change.
The M81-SSM provides the ability to configure a high speed parameter sweep on either DC source amplitude or AC frequency. The source must be set to the appropriate shape for the configured parameter sweep. All source modules configured to sweep will do so synchronously. To set up a source module to sweep, the source mode must be changed from fixed to sweep. Any attached source module can be changed between fixed and sweep mode via the SOURce#:VOLTage:MODE, SOURce#:CURRent:MODE, or SOURce#:FREQuency:MODE commands.
Sweep Set Up
To set up a parameter sweep the span, duration, and number of points must be defined.
Figure 3.36 Sweeping settings
Span
The span is used when referring to the difference between the start and stop values. Span is configured by setting both the start and stop voltage or the start and stop current of the sweep. When setting the span, it is possible to set the stop value lower than the start value. This will result in a sweep that decreases in either voltage or current, depending on whether the module is sourcing voltage or current. The term span may be used elsewhere for the sake of explaining calculations.
Duration
Duration is used when referring to the length of the parameter sweep. The duration of the parameter sweep can be set in two seperate ways. Setting the dwell time will directly affect the overall run time of the sweep by setting the duration of each step in the sweep. Presently, dwell times are restricted to multiples of 200 µs per step, and must be a minimum of 200 µs. When a dwell time is set to a value that is invalid the instrument will adjust to the nearest valid value. For example, 0.0006 seconds is a valid dwell time, however, 0.0005 seconds is invalid. In this case, the instrument will apply the dwell time of 0.0006 seconds, which can be seen with the relevant query.
It is also possible to set the overall duration of the parameter sweep, allowing the instrument to calculate the closest dwell time to achieve the desired duration. The overall time is also bound by the same rules as dwell time.
Dwell time is calculated using the following formula: \(DWELl=TIME/POINts\)
Points
The number of points defined in the stepped sweep. A higher value will create higher granularity in the span of the sweep, but will increase the total duration of the parameter sweep. Setting the number of points also directly affects the step size.
Alternatively, setting the step size of the sweep will set the number of points according to the following formula: \(STEP=SPAN/(POINts-1)\)
It is important to remember that the number of points specified should include the ending value. For example, if the desired sweep is 0 Volts to 10 Volts with a step size of 1 Volt, the number of points should be 11.
Note
Step cannot be set when using logarithmic spacing.
Direction
The direction of the sweep controls whether the instrument sweeps up or down. While sweeping up, the sweep will begin at the start value and end at the stop value. Alternatively, when sweeping down, the sweep will begin at the stop value and end at the start value.
When round trip sweeping is enabled the instrument will begin and end at the same value. The instrument will repeat the middle value and double the number of points. For example a sweep with a start value of 1, a stop value of 5, and 5 points would have output values like so:
Direction: UP |
Direction: DOWN |
|---|---|
1 |
5 |
2 |
4 |
3 |
3 |
4 |
2 |
5 |
1 |
5 |
1 |
4 |
2 |
3 |
3 |
2 |
4 |
1 |
5 |
Linear and logarithmic sweeps
The sweep steps may be linearly or logarithmically spaced. For example a sweep with a start value of 1, a stop value of 10, and 10 points would have output values like so:
Linear spacing |
Logarithmic spacing |
|---|---|
1 |
1 |
2 |
1.291549665 |
3 |
1.668100537 |
4 |
2.15443469 |
5 |
2.782559402 |
6 |
3.593813664 |
7 |
4.641588834 |
8 |
5.994842503 |
9 |
7.742636827 |
10 |
10 |
Logarithmic sweeps are useful when sweeping values over multiple orders of magnitude. It is important to note that logarithmic sweeps cannot include zero.
Initiating the Sweep
By default, data sweeping is closely tied to data stream to allow for easy collection of data during parameter sweeps. To disable this behavior the SWEep:INITiate:TRACe[:STATe] command can be used. After all of the sweep parameters have been configured, a data stream should be configured to capture relevant data. Once the data stream has been configured, calling TRACe:STARt will start data streaming collection and simultaneously initiate any configured parameter sweeps.
To initiate a sweep without a data stream, use the SWEep:INITiate command. Any configured sweeps will be initiated immediately. This command initiates the sweep regardless of the state set by SWEep:INITiate:TRACe[:STATe]. Initiating a sweep without a data stream is useful when ramping the output from one value to another to minimize discontinuities.
Parameter Sweep Status
To determine whether a parameter sweep is in progress use the following query: SOURce#:SWEep[:STATus]?
Aborting a Sweep
To abort all sweeps in progress use the command SWEep:ABORt. Aborting leaves the output enabled and at the last sweep point applied. The following actions will also cause a sweep in progress to abort:
Changing any setting on a source module while it is sweeping.
Resetting a data stream in progress
Example Parameter Sweep and Data Stream
The following example illustrates how to configure a current parameter sweep on the first source channel and read the resulting measured voltage on the first measure channel.
Set the source module to a linear sweep.
SOURce1:SWEep:SPACing LINear
SOURce1:CURRent:MODE SWEep
Set the span of the sweep to start at 25 mA and end at 100 mA.
SOURce1:CURRent:STARt .025
SOURce1:CURRent:STOP .100
Set the dwell time to 5 ms per step.
SOURce1:SWEep:DWELl 0.005
Set the number of points to 100, which is equivalent to step size of 75 µA.
SOURce1:SWEep:POINts 100
Configure the data stream and start it to begin the sweep and collect the data. Set the data stream to provide information in a comma separated format. Set the data stream to provide the source channel 1 amplitude, measure channel 1 DC measurement, and measure channel 1 settling status. Set the data stream rate to 200 points per second to match the 5 ms dwell time.
TRACe:RESet
TRACe:FORMat:ENCOding CSV
TRACe:FORMat:ELEMents SAMP,1,MDC,1,MSET,1
TRACe:RATE 200
TRACe:STARt 100