Oscilloscope vs VNA Load Pull for PA Design

Introduction :

Both of these techniques viz. oscilloscope based load pull and traditional VNA(Vector Network Analyzer) are used in power amplifier design. Let us explore them and summarize major differences between them.

To design highly efficient and linear power amplifier, it is essential to test it under various load conditions. This type of test is known as load-pull testing. This test case helps in finding optimal impedance matching for parameters such as output power, gain and power added efficiency.

Earlier days, this test was performed using frequency domain instruments such as VNAs. However, modern wireless standards such as 4G LTE, 5G NR rely on complex digital modulated signals. These new standards use time domain approaches using high speed oscilloscopes.

1. Traditional VNA systems

These systems use continuous wave (CW) or simple multi-tone signals to simulate DUT (Device Under Test). In addition to VNAs, traditional load-pull setups rely on Large Signal Network Analyzers (LSNAs) or Non-linear VNAs (NVNAs).

• Advantages : Highly Accurate

• Drawbacks :

  • continuous wave does not accurately represent the behavior of real world digitally modulated signals. PAs behave differently when subjected to the fluctuating peak to average power ratios of modern telecom signals. Hence CW based measurements often fail to accurately predict crucial linearity metrics like the Adjacent Channel Power Ratio (ACPR).
  • Traditional frequency domain setups require complex calibration routines.
  • They often need a prior characterization of the impedance tuner and rely on external phase references, such as comb generators, to align the phase of the measured signals.

  2. Oscilloscope based systems

  An oscilloscope based load pull system shifts the paradigm from the frequency domain to the time domain. This method uses high speed digital oscilloscope paired with directional couplers. It helps to directly capture the time domain waveforms of the incident and reflected voltages at the input and output ports of the PA (Power Amplifier).

  • Advantages :
    • It natively supports digitally modulated signals (like a 20 MHz LTE signal).
    • As the system captures the actual modulated waveforms in real time, engineers can extract highly accurate, real world load pull contours for output power, PAE and ACPR.
    • The calibration process is vastly simplified. The oscilloscope method does not require a comb generator for phase referencing, nor does it require the tuner to be characterized beforehand. Instead, it relies on standard open short load (OSL) calibration and basic power loss accounting between the oscilloscope and the DUT (Device Under Test).
  • Drawbacks or Limitations :
    • As oscilloscopes are limited by their vertical resolution (typically 8 to 12 bits), their dynamic range (around 48 to 72 dB) is noticeably lower than that of a high-end VNA or spectrum analyzer. This makes measurement of very low power adjacent channel signals very challenging without statistical averaging techniques.

Key differences

Summary: While traditional VNA systems remain the gold standard for high dynamic range, frequency domain measurements, oscilloscope based load pull systems represent the practical future of real world PA testing. By directly capturing time domain waveforms, oscilloscopes offer a highly efficient, simplified method for optimizing PA efficiency and linearity under actual digitally modulated signal conditions.