Why we care about Residual EVM of a Signal Analyzer (802.11ac) Monday, January 9th, 2012
Error Vector Magnitude (EVM) is a measure used to quantify the quality or performance of a modulated signal from a transmitter or receiver. In simple terms, if we consider a constellation diagram the EVM is the magnitude of the difference between the measured vector and the ideal (reference) vector. This can be visualized as below.
It can be seen from this simple diagram that EVM is influenced by a number of parameters such as below:
- Phase Error
- Frequency Error
- Magnitude Error
- Noise that contributes to all of the above
Each of these areas are contributed to not only by the signal being measured, but also by the test instrument itself which has an effect on how well it can capture the signal, but also how it is able to generate an “ideal” reference signal to use for the calculation. If we take a look at this block diagram which shows a model of transmitter EVM contributions such errors also exist during the demodulation process. Therefor the limit of EVM demodulation performance can only be as good as the error contributions added during the demodulation process in the signal analyzer.
Many of the effects we are able to correct for in DSP as part of signal synchronization etc. Although phase noise is not so easy to correct for and has a direct impact on performance.
Below are some graphics showing a visual representation of the effects of the distortions described in the model on the constellation diagram.
EVM Performance when measuring the new 802.11ac 80MHz standard 256QAM
Below are two graphics showing screenshots taken from an R&S Vector Signal Analyzer on an 802.11ac signal with a specific EVM of the transmit signal. The good signal has 33dB of EVM and the bad signal only 30dB of EVM performance. It is clear to see on the bad signal that some symbols (constellation points) are much further away from the ideal point which would result in more error and a poorer EVM result.
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Constellation with 30dB of EVM Performance |
Constellation with 33dB of EVM Performance |
Instruments with a worse EVM performance will contribute directly to this error and on signals over a wide bandwidth and such high order modulation schemes (256QAM / 1024QAM etc) EVM performance and phase noise performance of the test instrument becomes critical to measuring such parameters.
Graphics courtesy Rohde & Schwarz FSV-K70 user manual and some very helpful colleagues relating to the EVM plots above.
Post Tags: 802.11ac » Error Vector Magnitude » EVM » FSV » FSW » Spectrum Analysis » Vector Signal Analyzer