Noise Power Ratio
Understanding NPR
NPR provides a more realistic assessment of amplifier linearity than two-tone IP3 testing because it uses a noise-like stimulus that closely resembles the statistics of multi-carrier and OFDM signals. The notch-filling directly shows how much distortion power will appear in any sub-channel.
NPR Measurement
- Generate band-limited white noise covering the channel bandwidth.
- Create a narrow notch (typically 1-5% of the bandwidth) in the noise spectrum.
- Apply the noise-with-notch to the DUT (amplifier, system).
- Measure the noise level outside the notch and the distortion level inside the notch.
- NPR = outside level - inside level (dB).
NPR vs IP3
- IP3: Two-tone test. Simple. Good for narrowband CW signals. Does not capture memory effects or multi-signal behavior.
- NPR: Multi-carrier-like test. More realistic for OFDM/multi-carrier. Captures memory effects and all intermodulation products.
Frequently Asked Questions
What is NPR?
NPR measures linearity by applying noise with a notch to the DUT and measuring distortion that fills the notch. It simulates multi-carrier loading and is more realistic than two-tone IP3 for OFDM and satellite multi-carrier signals.
What NPR is needed?
For satellite transponders: NPR > 15-20 dB. For 5G base station PAs: NPR > 25 dB. Higher NPR means less distortion filling adjacent subcarriers. The required NPR depends on the modulation order and EVM requirements.
How does NPR relate to IP3?
NPR and IP3 both characterize linearity but in different ways. For a memoryless amplifier, NPR can be approximately calculated from IP3 and the number of carriers. For amplifiers with memory effects, NPR provides information that IP3 alone cannot.