EricHsu

How Digital Down Conversion (DDC) Benefits Your RF Measurement

Blog Post created by EricHsu Employee on Mar 9, 2018

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.

Outcomes