VNA Simplifies Wideband Demodulation

2022-07-01 | 5 min read

Demand for high data throughput and low latency is pushing device specifications and operational standards toward wideband modulation. Wideband performance benefits come with a heavy cost: significantly increased noise. This noise introduces additional test complexity and measurement uncertainty which is further exacerbated by industry’s desire to pack as many modulated signals into the available bandwidth as possible. This not only constrains error vector magnitude (EVM) requirements further, but also degrades system signal-to-noise ratio (SNR). So how can you simplify the performance verification process while ensuring component's standard compliance?

Simplified Measurement Set-up

In part two of webinar series, VNA Active Device Characterization Essentials, Keysight PNA Product Manager, Nizar Messaoudi, discusses how to measure an amplifier’s EVM using only a vector network analyzer (VNA). Demonstrating digital demodulation on a sub-6 GHz amplifier, Nizar explains why doing complex modulation scheme EVM measurements on a VNA increases testing pace by reducing setup complexity. Using the modulation distortion analysis application to measure the amplifier’s distortion, Nizar also walks-through how to use the VNA’s signal analysis application to capture the amplifier’s EVM to a given constellation and demodulation standard.

Simplified, single-connection VNA solution setup doing active device characterization under modulated stimulus — Figure 1: Simplified, single-connection VNA setup conducting active device characterization under modulated stimulus

Improved Measurement Quality

While, in the past, these measurements were conducted on a signal analyzer (SA), as Nizar explains, the VNA’s advanced feature set offers various wideband measurement upgrades for component testing. Beyond test setup simplification, the VNA equips designers with more than just harmonic testing and spur search. When using VNAs, the stimulus signal is always known. Nizar details how taking advantage of this functionality improves digital image rejection. Additionally, the VNA uses coherent averaging to reduce problematic wideband noise and enable other complex tests like noise power ratio (NPR) or even full waveform demodulation.

PNA-X single analysis measurements with multi-receiver architecture diagram — Figure 2: PNA-X multi-receiver architecture allows simultaneous measurement of input, output, and reflected modulated signals while built-in source calibration algorithm reduces source residual EVM

Upgraded Error Isolation

Nizar addresses why a designer would want to demodulate waveforms as well. He explains that EVM is the foremost specification that component vendors must provide for their product. As standard stringency increases with wideband modulation implementation, vendors want to provide standard specific EVM to their customers. To do so requires demodulation. The VNA use inverse fast Fourier transforms (IFFT) to produce statistically representative compact signals equivalent to signals of the desired standard (5G NR, Wi-Fi, DBSX2, etc.) and then measures fully demodulated EVM in the time domain. Additionally, access to the VNA’s multi-receiver architecture allows for the measurement of a fully calibrated input signal into the input reference plane. Without isolation of the system error, SAs erroneously assume the measured EVM is that of the DUT. Alternatively, VNAs measure output signal given the know input signal and perform signal correction on the input reference plan. In doing so, VNAs drive down the residual EVM of the source and deliver the DUT’s distortion EVM isolated from the system error contributions.

Summary

Wideband modulation schemes offer many benefits, but to successfully create compatible technologies requires designers to deal with wideband noise. Modern VNA signal analysis's simplified setup, improved test quality, and upgraded calibration and error isolation uniquely enable realistic and repeatable wideband measurements.