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All Places > Keysight Blogs > Better Measurements: The RF Test Blog > Blog > Authors EricHsu

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.

 

Simplified 2x2 MIMO system

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:

  1. 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.
  2. 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.

 

Two AWGs configured to generate time-aligned signals.

 

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.

 

PXI triegger setup using 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.

 

WLAN 802.11ax test solution for 8x8 MIMIO.

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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Helping You Achieve Greater Performance and Fast Measurement Speed

At an exhibition demo booth, an engineer complained to me about the measurement speed of a PXI oscilloscope. To make a measurement, he programmed the data acquisition and post-analysis himself. The test took him over a minute to get each result. I told him that he didn’t have to do all of that; all he needed to do was setup the measurement on the oscilloscope and fetch the measurement item result directly. The process should only take a couple of microseconds. An on-board ASIC helps minimize data transfer volumes and speed-up analysis!

 

Like an oscilloscope has on-board digital signal processing, RF signal analysis tools also have on-board processing to accelerate measurement speed.

 

RF Measurement Challenges

For RF signal analysis, it's common to frequency-shift the RF signal to an intermediate frequency (IF) so that you can use a high-resolution digitizer for a high dynamic range signal acquisition. This then gets sent to a PC for data analysis. However, the complexity of this analysis increases with today's wireless communication systems, such as 5G technologies, 802.11ax standard and so on. Measuring these systems can include complex modulation schemes (e.g., orthogonal frequency-division multiplexing, OFDM), carrier aggregation, or MIMO (multi-input multi-output) signals.

 

These complications require significant signal processing, which in turn slows the measurement speed. This is a challenge as measurement throughput is critical in most applications, especially in high volume production testing.

In most signal analyzers, a digitizer is an indispensable component. For wider bandwidth analysis, you need a high-speed digitizer to capture signals. At the heart of a high-speed digitizer is a powerful FPGA or ASIC that processes data in real-time. This allows data reduction and storage to be carried out at the digital level, minimizing data transfer volumes and speeding-up analysis.

 

A key feature often available on digitizers is real-time digital down conversion (DDC). In frequency domain applications, DDC allows engineers to focus on a specific part of the signal using a higher resolution, and transfer only the data of interest to the controller/PC. It works directly on ADC data providing frequency translation and decimation sometimes called "tune" and "zoom." The block diagram shown in Figure 1 illustrates this basic concept of DDC.

 

Digital down-converter block diagram

Figure 1. Digital down-converter block diagram

 

How DDC Works

The frequency translation (tune) stage of the DDC generates complex samples by multiplying the digitized stream of samples from the ADC with a digitized cosine (in-phase channel) and a digitized sine (quadrature channel).

The in-phase and quadrature signals can then be filtered to remove unwanted frequency components. Then, you can zoom in on the signal of interest and reduce the sampling rate (decimation).

 

Finally, the on-board processor sends only the data you care about (I/Q data) to the on-board memory for further analysis. Most of Keysight's digitizers and signal analyzers have implemented DDC to accelerate measurement speed and for demodulation acceleration.

 

In addition, you can perform FFT with I/Q data in parallel for spectral analysis.  Some signal analyzers can do real-time FFT processing (nearly 300,000 times/second) and use comprehensive spectrum displays (density and spectrum) so that you won't lose any agile signals on the screen, shown in Figure 2.

 

Real-time spectrum analysis at 2.4 GHz ISM bandFigure 2. Real-time spectrum analysis at 2.4 GHz ISM (industrial, scientific and medical) band

 

Benefits and Limitations of a High-Speed Digitizer with DDC

Using a high-speed digitizer with DDC for your RF testing can be significantly more efficient:

  1. The frequency translation (tune) reduces both on-board memory and data transfer requirements. The resulting data is in complex form (I+jQ), which is usable for demodulation analysis directly and accelerates measurement speed.
  2. Digital filtering and decimation (zoom) reduce the wideband integrated noise and improve overall SNR.

 

However, there are some limitations with DDC implementation:

  1. The ADC's sampling rate is limited. It's not possible to digitize the high-frequency carrier directly. A common workaround is to use an analog circuit to bring the carrier to an IF so the digitizer can acquire the signal.
  2. The ADC's dynamic range is also limited. In wireless communication systems, you may need to capture both large and small signals at the same time.

 

New generations of high-speed and high-resolution ADC technologies provide excellent resolution and dynamic range into the tens of GHz, which allows you to capture high-resolution wideband waveforms. DDC accelerates measurement speed and increases processing gain to improve performance.

 

Furthermore, the I/Q data can be processed further for advanced real-time signal analysis or sent to a customized FPGA for user-defined signal processing algorithms. These provide you better RF measurement fidelity, signal integrity and higher measurement throughput.

 

If you’d like to learn more about wideband signal acquisitions, you can refer to the following white paper Understanding the Differences Between Oscilloscopes and Digitizers for Wideband Signal Acquisitions to understand what you should be using for your application.