Technical Insights > RF + Microwave

Consider the Source Part 2: Direct Digital Synthesis

2021-01-14  |  4 min read 

In our last blog, we explored traditional vector network analyzer architecture. Now, let’s look at how network analyzers based on direct digital synthesis (DDS) provide an extra performance boost for an edge in the latest 5G component test. 

How DDS Signal Sources work

DDS sources create signals digitally using a numerically controlled oscillator. On each digital clock cycle, the phase accumulator receives a delta phase value (ΔΦ). The accumulator then generates phase values based on each ΔΦ value for the phase-to-amplitude converter. The phase-to-amplitude converter uses a lookup table to match each phase value to the corresponding sinusoidal amplitude, creating a digital sine wave. The digital to analog converter (DAC) converts the sine wave to analog, a filter removes harmonics, and then the DDS outputs the signal.

direct digital synthesizer (DDS) block diagram
DDS block diagram

The number of bits in each ΔΦ value passed on to the phase-to-amplitude converter determines the resolution of the signal. Since the sinusoidal lookup table contains a finite amount of values, some of the ΔΦ bits get truncated. The truncation can result in phase errors and spurious signals, but Keysight’s proprietary DDS sources account for these errors to provide low phase noise and low spurious signals across their operating range. 

numerically controlled oscillator block diagram
Detail of the numerically controlled oscillator

A DDS can change the frequency of the output signal in one clock cycle. The fast frequency changes are phase continuous and phase repeatable. Therefore, there are no jumps in phase as you change frequencies, and signals at a given frequency will be in phase with each other. Consistent phase from the source reduces uncertainty about whether phase errors are coming from your instrument or the DUT. This is especially useful when testing devices that require multiple sources, such as differential devices.

DDS Sources in Vector Network Analyzers

A network analyzer using DDS sources experiences much lower phase noise than with PLL sources because of the continuous and repeatable phase of the DDS sources. The low phase noise lowers the noise floor for modulation distortion measurements such as error vector magnitude (EVM), giving a clearer view of the DUT’s performance without interference from the test setup.

spurious output on PLL source vs dds source
Spurious output on a PLL vector network analyzer (left) and a DDS vector network analyzer (right)

The clean DDS signal also enables the network analyzer to use a wider intermediate frequency bandwidth without compromising accuracy.  When you need to make many measurements on wideband devices, faster measurement speeds significantly reduce overall test time.

The NExt-Generation PNA and PNA-X Vector NEtwork Analyzers

The PNA and PNA-X network analyzers now contain DDS sources to provide the lowest phase noise available on a network analyzer. This technology enhances applications such as modulation distortion, nonlinear vector network analysis, converter measurements with phase, differential mixer measurements, and I/Q converter measurements.

The flexibility and performance of the PNA and PNA-X allow you to simplify your test setup by integrating clean signal generation and high-performance spectrum analysis into your network analyzer. Learn more about what the enhanced hardware can do for you by visiting the link below.