Advanced Arbitary Waveform control
In many cases it is fine to have arbitrary waveforms that are free running.(i.e. I&Q input signals, etc.) In some other cases the ARBs may need to be synchronized with another instrument, such as in BER or perform their function once, and measure a DUT response, such as to measure a transmitters response to a change in IQ Bias level. These functions are available with an RI74202B and newer ARB.
For these cases the ARB can be started and stopped using the standard test plan conventions of pre and post measure. In this note we will address basic ARB function with the Run and Stop Functions. PM start PM stop and Trigger are part of the BER Option and used in this Note: Setting up Bit Error Rate (BER) Measurements on the RI7100A Test System.
A few basic concepts should be understood
1.) The waveform needs to be stopped before it is started
2.) When a waveform is stopped it is at it's final value
3.) In order to execute a single cycle of a waveform a Run is followed immediately by a Stop in a premeasure
4.) There is a small amount of overhead that needs to be accounted for in precise timing.
Several examples will be shown to assist it providing a better understanding of these basic concepts for Advanced Arbritary Waveform control.
As a first example lets look at the hypothetical case where we want to:
1.) Send a pulse to a device (This is our initial time marker)
2.) Initiate a ramp from 0 to 400 mV over 300 uSec.
3.) Then digitize 1 mSec worth of data with the ramp fixed at 400 mV.
So the challenge is to put out 0 V, run a single cycle of a Ramp, stop it at it's end and then make a measurement while it is at its final value.
Note : Creating a user defined ARB waveform is described in another document, see: Creating Custom Waveforms
The waveform created for this example is a linear ramp comprised of 30 points. The set sample rate is 10 uSec. Resulting in a 300 uSec ramp of the output Voltage from 0 to 400 mV.
Here is a sample panel used to generate the waveform described previously.
( 300 uSec ramp of the output Voltage from 0 to 400 mV.)
1.) In the Panel the Waveform is defined as none and Stopped. This sets the initial value at 0 V.
2.) The file selected (but not loaded) is the custom file 300ramp.wav/ At a 10 uSec sample rate.
3.) In the premeasure the new waveform is selected (but is still in stop mode from the panel)
4.) The DB is turned on and off to start the sequence
5.) An optional sequence delay was added to vary the time between the pulse and the ramp
6.) Waveform Mode >Run ; Waveform Mode > Stop: This triggers one cycle of the waveform: It is important to note that the Stop button occurs in time immediately with the run.
7.) The sequence delay occurs while the ARB pattern is running. (see #6 above).
8.) The measurement of V vs T occurs for 1 mSec (51 points at 50 kHz)
9.) In the post measure the Waveform is set back to None (0 V)
Example 1 output
The scope trace below is of the waveform output of the 300 uSec ramp of the output Voltage from 0 to 400 mV from the Test Panel.
1.) Approximately 30 uSec exists between the trigger and the ramp (due to system overhead)
2.) The waveform remains for 1 mSec at the 400 mV level (this is the duration of the digitization measurement)
Waveform Stop Location
In the previous example, the Waveform Mode Stop button was immediately after the Waveform Mode Start button, Resulting in only 1 cycle.
This next example has the Waveform Mode Stop button moved from the premeasure to the post measure.
Now a repetative waveform of multiple cycles is generated, since the arb contiues to run during the duration of the digitization measurement.
Example 2 output
In looking at the resultant waveform we see:
1.) We get multiple waveforms from the initial "Run" until the measurement is Finished (at the 1 mSec cursor)
2.) The immediate stop button executes stopping the waveform mid wave and then sets the voltage at its final stop value: 400 mV.
3.) There is about 20 uSec of overhead (plateau @ 400 mV) as the ARB is changed from the "Mode>Stopped" "Type>file" to "Type>None" and the output returns to 0 V.
Change Waveform Without Stopping:
This next example has the Waveform Mode Stop button again moved from the premeasure to the post measure, but the order of post measure sequence is reversed. The Waveform Type >None button before the Waveform Mode> Stop button.
Again a repetative waveform of multiple cycles is generated, since the arb contiues to run during the duration of the digitization measurement, but the waveform is reset to 0V before the Stop would set it to the final waveforms voltage.
Example 3 output
The resultant waveform shows:
1.) We again get multiple waveforms from the initial "Run" until the measurement is Finished (at the 1 mSec cursor)
2.) The ARB changes type at the end of the measurement (with the same 25 uSec overhead) stopping the waveform mid wave but this time sets the voltage at its current value. Then goes to the Type>None (0 V)
Type>File Defined in the Panel
In this case, starting with example 2, the change from Type>None to Type>File was removed from the premeasure. What this means is that the panel is entered with the waveform file ( 300ramp.wav in this case ) already loaded, but stopped.
When a file is loaded but stopped it goes to its final value.
Example 4 output
In looking at the resultant waveform we see:
1.) The resultant waveform is normally set at a fixed 400 mV.
2.) Then again we get multiple waveforms from the initial "Run" until the measurement is Finished (at the 1 mSec cursor)
3.) The immediate stop button executes stopping the waveform mid wave and then sets the voltage at its final stop value: 400 mV.
4.) There is about 20 uSec of overhead (plateau @ 400 mV) as the ARB is changed from the "Mode>Stopped" "Type>file" to "Type>None" and the output returns to 0 V.