Keysight Technologies
How to Take Fast,
Simultaneous Measurements
of Two or More Signals
Using BenchVue Software
Application Note
02 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
When you want to capture state changes in a device under test (DUT), you need to
simultaneously measure different signals as fast as possible. This presents several
problems:
–– How do you determine the sample rate of all the instruments?
–– How do you achieve precise sample timing?
–– How do you get the results into readable form?
BenchVue software from Keysight Technologies, Inc. provides a way to address these
problems enabling you to be productive quickly without having to write a computer
program.
Choosing the Right Instruments
Once you know how fast the signal to capture is changing, you can choose the right
instrument for the job. An oscilloscope can capture a voltage waveform at virtually any
frequency, but with only 8-bits of resolution, this is good for only about 2½ digits. If you
need finer voltage resolution, a scope cannot be used. If you need to measure current,
you will need to use a sense resistor in series with one lead, and use the scope to measure the voltage across it, or use a differential measurement using two scope channels.
You could also use a current probe, but they can be expensive and hard to find.
A data acquisition (DAQ) instrument, such as the Keysight 34970A or 34972A, can also
be used since it has a digital multimeter (DMM) with a choice of plug-in switch cards,
which allows you to take different types of readings on each channel. It can also be reconfigured quickly and easily. However, the top speed including channel switching is only
60-240 channels per second depending on the switch card. If the signal you are trying to
digitize is faster, a DAQ device won’t work.
Digital multimeters are useful and relatively inexpensive instruments. One DMM can be
set up to measure voltage, another one current, and they can be set up to take readings at exactly the same time. Measurement accuracy of up to 6½ digits is common, and
DMMs can read AC and DC voltage, current, resistance, frequency and more. As floating
instruments, they can do this without having to have one lead at ground potential. They
can sample very quickly without having to be reconfigured, called “digitizing”, but the
resolution is reduced accordingly and may be as low as 3½ digits.
03 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
Let’s take a simple example. You want to measure both voltage and current of a signal
driving a load. Figure 1 shows how you would typically set this up. Your DUT generates a
stimulus signal that drives a load. One DMM is inserted in the path to measure current,
and the other is connected across the load to measure voltage. Let’s suppose that the
DUT is generating a repetitive exponential rise with a 125 ms period.
DMM1: voltmeter
DMM2: ammeter
Device
under
test
Load
Figure 1. Measuring both voltage and current of a signal driving a load.
Measurement Speed
The fastest measurement period (“aperture”) of the 34461A DMM is 0.02 power line
cycles (PLC), which is 400 µs, however the DMM limits the sample rate to a much longer
time. In fact it is about 1 ms, or 1000 readings per second. Some other Keysight DMMs can
measure much faster. The 34465A, for example, has an optional aperture time as low as
.001 PLC, or 20 µs, with no additional overhead, which allows a top reading speed of
50,000 readings per second, which is equivalent to a 50 KHz sampling rate. Without the
digitizer option, the 34465A can take up to 5,000 readings per second. To avoid having the
interface bus slow down the reading rate, measurements are often stored in the DMM’s
memory, then read back to the PC later. Some DMMs can take readings and pass them to the
interface at the same time without affecting the sample rate.
There are other measurement options that can slow down readings: autorange, autozero,
and in Ohms mode, offset compensation. All of these cause extra readings to be taken
to improve the accuracy of the measurement. When digitizing a signal, these are not
generally needed and should be turned off.
Measurement Tip
For number of power line cycles (NPLCs) of 1, 10 and 100,
the sample time is actually
based on internally measured
power line frequency. In countries with 60 Hz power, one
power line cycle is 16.67 ms,
and in countries with 50 Hz
power, one power line cycle
is 20 ms. These apertures are
used to cancel out line-related
noise. When higher speeds
(and resulting lower accuracy)
are required, a PLC less than
1 is specified. For these, aperture time, though specified in
NPLC, is actually an approximation, not a precise time.
04 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
Precise Timing
Triggering
DMMs start taking measurements when “triggered.” A trigger can be generated internally, externally, or via a software command over the chosen interface (USB, LAN or
GPIB). The software command is called a bus trigger, and there will be processing delays
depending on when the command is received by the instrument and how long it takes
its internal processor to decode and act on the command. Internal (also called immediate)
triggers let the instrument free-run at its own speed, so two free-running DMMs could
be off by as much as one measurement cycle after triggered. The only way to achieve
truly precise timing is to use an external trigger, usually in the form of a TTL-compatible
pulse applied to a BNC input connector on the back panel (typically labeled “Ext Trig”).
Once set up, a pulse with a width of at least 1 µs starts the reading cycle. The trigger
system lets you specify the rising or falling edge of this signal. Multiple instruments can
be started by connecting all external trigger inputs together and tying them to a common trigger source, such as a function generator set to output pulses. The DMMs can be
set up to take more than one sample per trigger if desired.
VM Complete
When a DMM finishes a reading, it outputs a pulse to a BNC connector on the back panel
called voltmeter complete (typically labeled “VM Comp”). If no external trigger source is
available, the VM Comp signal from one instrument can be used as an external trigger
for any others; this will result in a timing delay for the other instruments of exactly one
measurement cycle. In many cases, this may be sufficient to satisfy the definition of “at
the same time”.
05 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
Test Setup
Let’s see how to set this measurement up in BenchVue. Accessing the digitizer functionality in BenchVue requires the “Pro” license. Also, this is an advanced measurement, so
some front panel interaction is also needed. For example, when you select External Trigger, BenchVue enables the DMM to capture the specified sample count at its maximum
rate once a single trigger arrives. So we need one and only one pulse from the function
generator. If you use two identical DMMs, such as two 34461As, the sample times at this
maximum rate will be virtually identical between the instruments. But if one DMM, for
example, a 34465A, which has a specified maximum standard reading rate that is five
times faster than the 34461A, the readings will not occur simultaneously. Therefore, if
you want to pace the readings from an external source to keep them in sync, the DMMs
cannot be started from BenchVue. Also, the Scope App has no way of setting “Normal”
trigger mode, which is essential if you want to capture the waveforms as they happen.
But there is a way to work around these limitations.
First let’s connect the cables. See Figure 2. A function generator is set up to externally
trigger both DMMs. Although any interface can be used, USB 2.0 was chosen. (USB 3.0
connections, marked “SS” next to the USB connector, have been shown to be unreliable
for instrument I/O depending on motherboard, O/S, USB driver and cabling.) A scope
is connected to capture the various signals just for purposes of this application note to
show how the trigger information is captured.
Oscilloscope:
monitors signals
Function generator:
external trigger
source
DMM1: voltmeter
PC connections: USB
DMM2: ammeter
Device
under
test
Load
For DMMs, enable USB SCPI & file access
Figure 2. Instrument connections. A function generator generates external triggers to the two DMMs, one set up
to measure voltage and the other to measure current. The scope is only used to monitor signals to show how the
timing works.
06 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
Now let’s start BenchVue 2 and launch the oscilloscope, function generator, and two
DMMs. See Figure 3. First select the 4-pane view.
Figure 3a. Selecting 4-pane view.
Figure 3b. All four instruments launched in BenchVue. The position of instruments can be changed as desired by
dragging and dropping the instrument icons along the instrument bar at the bottom of the screen.
Figure 3c. To see all the setup information,
we can click the maximize icon within any app.
Set up the two DMMs first, one for voltage and one for current. To maximize reading
speed, turn off Auto Zero, select a fixed range, set the Trigger Delay to 0, and pick the
fastest aperture of .02 NPLC. Enter 1000 for the Sample Count, and select External
Trigger. See Figure 4.
Figure 4. DMM setups. Both DMMs are set up as digitizers with 1000 samples, fastest aperture (.02 NPLC), fixed
range and Auto Zero off.
07 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
Next select the Function Generator App and set it to generate one TTL pulse after a
2 second delay. See Figure 5.
Figure 5a. Function generator setup for use as an external trigger to DMMs. This setup outputs one TTL-compatible pulse after a 2 second delay when started.
Figure 5b. Manual trigger mode generates a “bus” trigger to this instrument when either BenchVue’s Start button
is clicked or the instrument’s front panel Trigger button is pushed.
08 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
Synchronizing the readings to the signal
It may be advantageous to start the measurements in response to a signal from the DUT.
To do this, you could set the function generator up with its own external trigger. This
could be as simple as turning on a 5 V power supply, or perhaps you have access to a
timing signal that occurs when you need to take measurements. In this example, it was
started manually from BenchVue.
Start the measurement cycle
A 2 second trigger delay is chosen so that the function generator does not start at the
same time as the DMMs or the oscilloscope. This gives the DMMs and the scope plenty
of time to initialize. When Start All is clicked, both DMMs enter the Wait for Trigger state
and the scope goes to SINGLE mode. After the 2 second trigger delay, the function generator outputs the trigger pulse. Once the readings are done, BenchVue reads in the data
and displays the results graphically. Figure 6 shows the DMM readings, and Figure 7 the
triggering results.
Figure 6. DMM readings returned to BenchVue.
Figure 7a. Triggering results, showing that one external trigger started the operation. The two DMMs, because
they are the same type of instrument, measured two parameters at the same time.
Figure 7b. The data can now be exported to Excel by selecting Export All at the top right of the screen.
09 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
However, the x-axis (time) values consist of calculated times based on starting and
ending timestamps,, not the actual 900 µs sample interval that was achieved. There is
no way to know this exact timing without measuring it and then manually placing the
correct time intervals into the Excel file.
Let’s now run the same experiment using one 34461A and one 34465A. As shown in
Figure 8, the slower DMM samples at 900 µs intervals, while the faster one samples five
times as fast. The faster DMM will therefore finish gathering 1000 readings in 1/5 the
time of the slower one. But notice that the representation of the waveform is much more
accurate using the faster digitization. So, in this case, two 34465A DMMs would give us
a much more accurate picture of what’s going on.
Figure 8. Two DMMs sampling at different rates. The signal being digitized is shown in yellow. The 34461A’s VM
Comp output is shown in green, the 34465A’s in blue. A single external trigger is shown in pink.
Conclusion
This application note has described how to measure two signals at the same time using
BenchVue. By extension, you can see that many more signals could also be measured
and captured, all without having to write any software. In fact, although only four panes
are visible at any one time in BenchVue’s main window, they can be “popped out” (moved
into their own separate windows), allowing many instruments to be controlled at once.
BenchVue streamlines the process of using instruments, making you more productive.
BenchVue Software
Download BenchVue software today for free. If you’re interested
in the “Pro” apps, which extend datalogging time beyond the 1
hour available in the free version and add the digitizing capabilities
described in this application note, contact your sales representative or order online through an authorized Keysight distributor.
www.keysight.com/find/BenchVue
10 | Keysight How to Take Fast, Simultaneous Measurements of Two or More Signals Using BenchVue Software - Application Note
From Hewlett-Packard through Agilent to Keysight
For more than 75 years, we‘ve been helping you unlock measurement insights.
Our unique combination of hardware, software and people can help you reach
your next breakthrough. Unlocking measurement insights since 1939.
1939
THE FUTURE
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This information is subject to change without notice.
© Keysight Technologies, 2015
Published in USA, July 30, 2015
5992-0555EN
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