const_evm()
Takes the results of a Circuit Envelope simulation and generates data for the ideal and distorted constellation and trajectory diagrams, as well as the error vector magnitude, in percent, and a plot of the error vector magnitude versus time.
Syntax
data = const_evm(vfund_ideal, vfund_dist, symbol_rate, sampling_delay, rotation, transient_duration, path_delay)
Arguments
| Name | Description | Range | Type | Default | Required |
|---|---|---|---|---|---|
| vfund_ideal | single complex voltage spectral component (for example the fundamental) that is ideal (undistorted). This could be constructed from two baseband signals instead, by using the function cmplx(). | (-∞, ∞) | complex | yes | |
| vfund_dist | single complex voltage spectral component (for example, the fundamental) that has been distorted by the network being simulated. This could be constructed from two baseband signals instead, by using the function cmplx() | (-∞, ∞) | complex | yes | |
| symbol_rate | symbol rate of the modulation signal | [0, ∞) | integer, real | yes | |
| sampling_delay | sampling delay † | [0, ∞) | integer, real | 0 | no |
| rotation | parameter that rotates the constellations by that many radian ‡ | [0, ∞) | integer, real | 0 | no |
| transient_duration | time in seconds that causes this time duration of symbols to be eliminated from the error-vector-magnitude calculation ‡ † | [0, ∞) | integer, real | 0 | no |
| path_delay | time in seconds of the sum of all delays in the signal path ‡ ‡ | [0, ∞) | integer, real | 0 | no |
| † sampling_delay - (if nonzero) throws away the first delay = N seconds of data from the constellation and trajectory plots. It is also used to interpolate between simulation time points, which is necessary if the optimal symbol-sampling instant is not exactly at a simulation time point. Usually this parameter must be nonzero to generate a constellation diagram with the smallest grouping of sample points ‡ rotation does not need to be entered, and it will not affect the error-vector-magnitude calculation, because both the ideal and distorted constellations will be rotated by the same amount. ‡ † Usually the filters in the simulation have transient responses, and the error-vector-magnitude calculation should not start until these transient responses have finished. ‡ ‡ If the delay is 0, this parameter may be omitted. If it is non-zero, enter the delay value. This can be calculated by using the function delay_path(). | |||||
Examples
rotation = -0.21
sampling_delay = 1/sym_rate[0, 0] - 0.5 * tstep[0, 0]
vfund_ideal = vOut_ideal[1]
vfund_dist = vOut_dist[1]
symbol_rate = sym_rate[0, 0]
data = const_evm(vfund_ideal, vfund_dist, symbol_rate, sampling_delay, rotation, 1.5ms, path_delay)
where the parameter sampling_delay can be a numeric value, or in this case an equation using sym_rate, the symbol rate of the modulated signal, and tstep, the time step of the simulation. If these equations are to be used in a Data Display window, sym_rate and tstep must be defined by means of a variable (VAR) component, and they must be passed into the dataset as follows: Make the parameter Other visible on the Envelope simulation component, and edit the parameter so that:
Other = OutVar = sym_rate OutVar = tstep
In some cases, it may be necessary to experiment with the delay value to get the constellation diagrams with the tightest points.
Note that const_evm() returns a list of data. So in the example above,
data[0]= ideal constellation
data[1]= ideal trajectory
data[2]= distorted constellation
data[3]= distorted trajectory
data[4]= error vector magnitude versus time
data[5]= percent error vector magnitude
Refer to the example file to see how these data are plotted.
Defined in
$HPEESOF_DIR/expressions/ael/digital_wireless_fun.ael
See Also
constellation(), delay_path(), sample_delay_pi4dqpsk(), sample_delay_qpsk()
Notes/Equations
Used in constellation and trajectory diagram generation and error-vector-magnitude calculation.
The user must supply a single complex voltage spectral component (for example, the fundamental) that is ideal (undistorted), as well as a single complex voltage spectral component (for example, the fundamental) that has been distorted by the network being simulated. These ideal and distorted complex voltage waveforms could be generated from baseband I and Q data. The user must also supply the symbol rate, a delay parameter, a rotation factor, and a parameter to eliminate any turn-on transient from the error-vector-magnitude calculation are optional parameters.
The error vector magnitude is computed after correcting for the average phase difference and RMS amplitude difference between the ideal and distorted constellations.
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