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In this application note, you will learn how to synchronize multiple oscilloscopes.
■ Things to consider when synchronizing an oscilloscope
■ Causes of timing errors between oscilloscopes
■ Recommended synchronization method
■ Multi-oscilloscope software settings
This application note uses the Tektronix 4, 5, and 6 Series MSO as examples to explain the procedures and principles of multi-oscilloscope synchronization . The 4, 5, and 6 Series MSO support synchronization between any model of oscilloscopes, thereby achieving a synchronized acquisition system with more channels.
Why is the number of channels required to be more than 4?
The 4 Series MSO oscilloscope is the first 6-channel model in the series to meet a variety of test application scenarios. It can be used to capture complex particle physics experiments, measure multiple power rails, analyze three-phase power converters, and more. Measurements can include power crosstalk in serial buses, analyze RF interference, and synchronously observe the transmission of input/output signals.
Users can also measure more channels by synchronizing multiple oscilloscopes. In multi-channel applications or measurement scenarios, it is very important to maintain precise synchronization between channels in order to accurately analyze the timing relationship of the entire system under test.
Multi-scope measurement considerations
software
For multi-oscilloscope measurement systems, software can play a key role. At the most basic level, the software needs to integrate the data from multiple instruments and perform the triggering and acquisition settings of the instruments. The software can also provide display and analysis capabilities for the combined waveform.
In addition, the software can help complete the phase difference correction. Although users can complete these tasks by writing custom software, compared with the tedious program development process, TekScope PC analysis software directly provides these functions, which can complete complex settings more quickly and efficiently, allowing users to focus more on the test itself. In this application guide, TekScope PC software will be used for multi-oscilloscope control and acquisition, and the following chapters introduce how to use the software.
System Configuration
When considering a test system synchronization approach, it is important to understand the various synchronization strategies and the amount of timing error that can be tolerated between channels. Different cabling, triggering, and delay compensation methods can have a significant impact on timing errors. Differences in channel delays inside and outside the oscilloscope (i.e., cables and probes) can cause timing errors or "skew" between channels. When deciding on a synchronization strategy, there are a few key questions to answer: How much skew can be tolerated between the test system input channels? Do all input channels need to meet a strict skew tolerance, or only some? For measurements in electromechanical or human-machine applications, for example, a few tenths of a millisecond may be tolerated. However, measurements on high-speed electronic systems require a higher degree of synchronization.
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To better understand the sources of timing errors, they can be divided into four types:
1. Trigger jitter
Trigger jitter is the acquisition-to-acquisition variation in timing error. This can be seen when setting the oscilloscope to infinite persistence and observing a signal synchronized with the trigger. This is shown in the difference between Figure 1a and Figure 1c . Using an external trigger source or probing the 4, 5, and 6 Series MSO input channels, the jitter will be less than 10 ps. Using the auxiliary trigger input adds over 200 ps of jitter.
2. Phase difference between oscilloscope channels
The 4, 5, and 6 Series MSO data sheets state that when using probes, the delay between analog channels will be less than 100ps.
3. Phase difference caused by the cable propagation delay of each oscilloscope's external trigger or probe
When using an external trigger and power divider, any difference in cable length will result in a phase difference of about 70ps/cm. If the same analog probe is used as the trigger source on each oscilloscope, the phase difference should be less than 100ps. See Figure 1b .
Figure 2: Different trigger setups result in different phase differences or delays. The setup on the left feeds the same trigger signal to the trigger channels of two instruments using phase-matched cables. The setup on the right shows the effects of "daisy chaining" where the auxiliary output of one oscilloscope feeds the auxiliary trigger input channel of the next oscilloscope, resulting in a noticeable delay.
4. The phase difference between the trigger event and the auxiliary trigger output signal
When the auxiliary output port of the triggered oscilloscope is designated as the trigger output signal, there is an inherent phase difference of 1s. If it is not corrected, this amount of phase difference may be too large for most application scenarios. If the record length is long enough, it can be corrected by using pre-trigger delay. This is shown on the right side of Figure 2.
Low phase difference synchronization method using external source
The most accurate synchronization technique is to use a single trigger source, split the trigger signal through a power splitter (BNC or SMA), and feed the same signal to multiple oscilloscopes, as shown in Figure 3. The same length of cable (preferably phase-matched) should be used to connect the splitter to all instruments to minimize phase differences caused by different propagation delays.
Figure 3: In this system, the auxiliary trigger input and timebase reference are both fed by splitters and matched 50Ω cables. This setup provides the best phase difference results without sacrificing each oscilloscope channel. Using the input channel instead of the auxiliary trigger input will reduce the number of measurement channels, but the trigger jitter will be reduced by about 200ps.
About Power Divider
To maintain the integrity of the trigger signal, a high-quality power splitter is used. This splitter acts as a balanced voltage divider, connecting the 50Ω trigger source to the 50Ω cable that connects to the 50Ω input of the oscilloscope. The power splitter ( shown in Figure 4) distributes the voltage to the four branches so that a 5V peak trigger provides 1.25V to each branch. Please pay attention to the specifications of the power splitter and the trigger signal requirements. The signal level driving the auxiliary trigger input of the 4, 5, and 6 Series MSOs should preferably be greater than 500mV. The larger the trigger signal provided, the better the oscilloscope's trigger system will respond, the more stable it will be, and the better the phase difference results will be.
Figure 4: An SMA power splitter connected to four matched cables and a trigger source
Figures 3 and 4 show the Tektronix recommended synchronization accessories: SMA high bandwidth 4-way power splitter (Tektronix part number: 174-6214-00) and 4 matched SMA cables (Tektronix part number: 174-6212-00). The cables shown are matched within ps to control the phase difference.
Synchronous reference clock
It is also very important to lock the oscilloscope's sampler to a high-fidelity 10MHz reference clock. This eliminates the effects of long-term drift between time bases, minimizing timing accuracy errors in channel-to-channel measurements with larger spans (>2ms).
There are two ways to synchronize the reference clock:
1. The best approach is to use a high stability external clock and use a power divider to feed each reference clock input. This is similar to the approach used to separate the trigger signal, as shown in Figures 3 and 4 .
2. Another method is to use the internal reference clock of one oscilloscope and feed it to the next oscilloscope, as shown in Figure 5. The auxiliary output of this oscilloscope can feed the reference input of the next oscilloscope in series, and so on. This method is suitable for situations where the accuracy of the internal reference time base meets the requirements.
In either case, for instruments receiving a 10MHz reference clock, the reference clock source should be set to External. This setting can be found by double-clicking the “Acquisition” badge on the 4, 5, and 6 Series MSOs, as shown on the left side of Figure 6. Once the transmit and receive oscilloscopes are configured and synchronized, the timebase reference source should show a green “Locked” indication.
On instruments that output a reference clock, you must designate the reference clock as an auxiliary output by going into the Utility menu → Aux Out and selecting the reference clock, as shown on the right side of Figure 6.
Figure 6: The 4, 5 and 6 Series MSO menus for setting the reference clock and locking the timebase reference. On the left is the reference clock setting on the receiving oscilloscope. On the right is the output reference clock setting on the transmitting oscilloscope.
Figure 7: TekScope PC supports connecting up to four oscilloscopes to a single computer and can display the signal from any active channel on a single monitor
TekScope PC Analysis Software is an application from Tektronix that is ideal for multi-oscilloscope configurations. The software operates in the same way as the 4/5/6 Series MSO user interface, but runs remotely on a Windows PC. With TekScope, you can connect multiple oscilloscopes and display all waveforms on a single interface, just as if you were running on a single oscilloscope. The software can also save all data from all connected oscilloscopes in a single file.
Configuring TekScope PC for Multi-Oscilloscope Applications
Connecting a 4, 5 or 6 Series MSO Oscilloscope is very simple. Click on the "Add New Scope" icon and a new oscilloscope will be added.
Double-click the oscilloscope logo, enter the IP address, and connect, as shown in Figure 8.
Figure 10: The trigger signal is split off and connected to the auxiliary input port, and the calibration signal is split off and connected to channel 1 on each oscilloscope.
A clock signal other than the trigger signal needs to be connected to the two channels to be skewed, as shown in Figure 10. The signal should have a fast rise time (e.g. 50ps). Use TekScope PC to connect two oscilloscopes at once. Select one channel as the reference, as shown in Figure 11.
Figure 11: The same signal is connected to the two channels to be corrected for phase difference
The next step is to overlay the two channels, as shown in Figure 12. Then, zoom in on the leading edge of the signal so that the cursors can be used to measure the time difference, as shown in Figure 13.
Figure 13: Measuring the time difference between two channels on different oscilloscopes
Now you need to eliminate the phase difference between the channels. Double-click the vertical menu of the channel. Enter the measured time difference in the "Deskew" setting. See Figure 14. The above steps must be repeated for all channels.
Figure 14: The deskew correction between the two channels can be done by entering the measured time difference in the “Deskew” setting on the channel vertical badges.
Summarize
This technical brief describes how to synchronize a multi-oscilloscope measurement system using the 4, 5, and 6 Series MSO oscilloscopes and TekScope PC analysis software. The 4, 5, and 6 Series MSOs support synchronization between any oscilloscope model, allowing for a more synchronized acquisition system with more channels.
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