Digitally Modulated Signal Error Vector Measurements
RI Measurement Approach
Intrinsic Errors measured directly
I/Q DC Offsets and Mag & Phase errors for Mod/Demod
AM to AM and AM to PM for PA's
CW Signal Generator
Arbitrary Waveform Generator
Digital Modulation Signal Generator
EVM can be measured multiple ways divided into two main methods.
- 1) Model based EVM. This is used for circuits that have known relationships between parametric performance and EVM. (ie AM/AM and AM/PM for an amplifier). The precise measurement of mag and phase distortion can calculate EVM. The time required for this is the time required to measure the model (ie 10-20 msec for something like an amplifier). But one would then have the data required to calculate EVM for multiple modulation types.
2.) Functional EVM. This is the typical EVM. It does require a modulated source and wide-band receiver. We typically embed the EVM calculation into the logic of the receiver, so the calculation is instantaneous. The time required is only for the capture.
The RF measurement hardware provides high performance Intrinsic signal measurements very quickly (less than 35 msec) which is the root cause of EVM.
For I/Q Modulators and Demodulators the DC offsets as well as the Magnitude and Phase errors of the devices are easily measured and characterized with our efficient swept measurement methodology. 7 Different measurements, 4 different RF carrier measurements at different DC offsets and 3 RF image measurements at different magnitude and phase balances, solves for the DC and AC intrinsic errors. This method minimizes base band stimulus error, eliminates the test system phase noise errors, minimizes the waveform capture errors, is very fast <100msec and improves standard deviations.
For PA's the AM to AM and AM to PM contributions are measured with our RF measurement system producing an RMS EVM. This has several advantages, as with the mod/demod devices, No EVM source error, no EVM capture error in the receiver, no frequency error and no dynamic capture errors. Measurement times are approaching >30 msec and repeatability is improved to less than 0.1 %
See published papers on this subject at :
"Fast EVM Measurements with FPGAs on ATE" - Published 2010 ITC by ROOS
"The Error Vector and Amplifier Distortion" - Published 1997 at the Wireless Communications Conference - Lucent / Bell Labs - Heutmaker
"WCDMA Transmit IC : Application case study" - Wireless Workshop - Published at the 2002 International Test Conference ( ITC) - Jointly by IBM and RI
" Edge PA EVM" - to be Published at the 2004 International Test Conference ( ITC) -
What is EVM?
The Ideal Waveform
EVM is Magnitude of the errors that are present in a digitally modulated signal from it's ideal constellation. Above is an HPSK 3GPP digitally modulated signals ideal constellation pattern. There are 8 vectors that can be drawn from the origin to each of the constellations phase points.
This is EVM?
The Real World
EVM is more clearly shown now as the Magnitude of the errors that are present at each ideal vector in a digitally modulated signal. Above is the HPSK 3GPP digitally modulated signals ideal constellation pattern and the measurement through a device. Notice each of the 8 vectors has unique errors and so we typically report an RMS value of all 8 errors together as the devices total EVM.