In a rock band, the drummer keeps the beat steady and the other musicians follow the rhythm. The drummer keeps the entire band in synch. The same concept is true when you integrate multiple instruments into a test system. The individual instruments need to be synchronized, especially when you are making multi-channel RF measurements. Like a drummer, a trigger and a reference clock communicate the “beat” to synchronize the instruments so they can make precise, time-aligned measurements. Let’s take a closer look at multi-channel measurements and how to achieve an accurate multi-channel test setup.
Multi-antenna RF techniques
Most modern wireless systems, whether in commercial applications or aerospace and defense, have adopted some kind of multi-antenna technique, such as MIMO (multiple input, multiple output), beamforming or phased-array radar. These techniques improve:
- Spectral efficiency (bit/sec/Hz)
- Signal quality
- Signal coverage
For example, MIMO increases data rates by using two or more streams of data transmitted with multiple antennas. The antennas transmit the data on the same frequency and at the same time without interfering with one another, as shown in Figure 1. Spectral efficiency is improved using the same bandwidth.
Figure 1. A simplified 2x2 MIMO system with two transmitters and two receivers.
Keys to synchronize multiple instruments
While MIMO and other technologies deliver increased data rates, they also increase the number of antennas in a device. And, as the number of antennas increases, test complexity increases significantly. For example, the latest IEEE WLAN technology, 802.11ax, use up to 8x8 MIMO. That means your test setup must have eight transmit channels and eight receive channels! And, it’s crucial that they are synchronized.
To synchronize your test system, there are three key elements: the trigger, the sampling clock, and the event-signal effects.
An easy method to synchronize multiple instruments is to use a trigger. A trigger is a coordination signal that is sent to each instrument in a test setup. When the trigger signal is detected, each instrument performs its task. Using a trigger signal ensures all your instruments are in synch. However, there are two sources of error that must be addressed:
- Sampling clock: Even when all the instruments being triggered are identical, for example your signal generators, the initial phase of each instrument’s sampling clock is random. To align the sampling clock of each instrument, use the same reference frequency for all the instruments.
- Event-signal effects: Cabling and external devices can affect how long it takes your trigger signal to reach each instrument. This is called trigger delay. These event-signal effects need to be accounted for so that your instruments still transmit or receive at the same time. Using a channel skew control on your master instrument allows for precise time synchronization between all channels.
Figure 2 illustrates two arbitrary waveform generators (AWGs) that are in time alignment. Here’s a quick review of the setup:
- First, use a common frequency reference to synchronize the timing clocks for all instruments.
- Second, connect the master's "trigger out" to the slave's "trigger in" connector. The AWG will start generating the signal after a trigger event is detected.
- Finally, remove the effects of master-to-slave trigger delay to align the two waveforms. The trigger delay can be measured with an oscilloscope or a digitizer. Then, add the delay time to the master AWG.
This process also applies to analyzers. You can use one splitter to distribute signals to a multi-channel analyzer and measure the time differences among the channels. The relative delays of each channel can be compensated by application software. Having the timing synchronized between the instruments allows you to build a multi-channel RF test system.
Figure 2. These two AWGs (master and slave) are configured to generate time-aligned signals. To remove the effects of master-to-slave delay, it is necessary to delay the signal generated by the master.
Modular instruments can make implementation easier
While the number of synchronized channels increases, the cabling between the instruments becomes much more complicated and achieving proper time-synchronization can take a significant amount of time. Modular instruments are based on standard instrumentation buses such as PXI, AXIe, and VXI. These instruments can share clocks and trigger signals through a backplane bus. This makes it easier to implement synchronization and makes the trigger events more repeatable because the test environment is controlled with minimal cabling.
For example, a PXI trigger bus consists of eight trigger lines spanning the backplane connectors. The trigger lines are divided into three trigger bus segments, slot numbers 1-6, 7-12 and 13-18. Figure 3 shows an easy PXI trigger setup with Keysight IO Library software.
Figure 3. PXI trigger setup using Keysight IO Library software. In this example there are eight trigger lines (0-7) and three bus segments. The trigger routing direction between the segment of each trigger line can also be configured.
Figure 4 below shows two PXI chassis being used as a WLAN 802.11ax test solution that fully supports 8x8 MIMO. The PXI backplane bus routes trigger signals to target modules for eight-channel signal generation and acquisition. This system takes advantage of the PXI standards that minimize a chassis’ slot-to-slot trigger time and clock skew to hundreds of pico seconds. This results in very accurate timing synchronization so you don't need to make any adjustment for MIMO transmitter and receiver testing.
Figure 4. WLAN 802.11ax test solution that fully supports 8x8 MIMO configuration in two PXI chassis.
Trigger and Time Synchronization Lead to Better Testing
To effectively test today’s multi-channel devices, you must perform tightly synchronized, multi-channel signal generation and analysis. With accurate triggering among the instruments, you ensure that all measurements start at precisely the right time. (If you require carrier phase coherency, you will also need to use a common local oscillator (LO) reference.) To simplify your test synchronization, consider a modular test system that allows easier integration of multiple instruments into a multi-channel test system.
If you’d like to know more about instrument interactions, refer to the following application note: Solutions for Design and Test of LTE/LTE-A Higher Order MIMO and Beamforming.
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