Digital communications
with
BT is the 3 dB bandwidth-symbol time product, a dimensionless factor similar to
α
in raised cosine
fi
lters that controls the bandwidth of the
fi
lter. The following
graph shows the impulse response of the Gaussian
fi
lter for BT = 1.0, BT = 0.5,
and BT = 0.3. Intersymbol interference is limited to the nearest neighbor symbols
which simpli
fi
es receiver design.
Error vector magnitude
As discussed previously, digital communication protocols often stipulate both a
symbol constellation and a pulse shaping
fi
lter. Given a constellation, a pulse
shaping
fi
lter, and a set of symbols to transmit, you can map out the expected
trajectory of the modulated RF carrier as a function of time as each symbol is
transmitted. The trajectory can be characterized by a vector quantity which
identi
fi
es the amplitude and phase of the RF at a given moment in time. You can
then evaluate the quality of a digital transmission by comparing the received
trajectory with the expected reference trajectory. The deviation between the two is
a vector quantity indicating the error of the received signal at a given moment in
time. The magnitude of the error is called the error vector magnitude (EVM).
The following
fi
gure diagrams the relationship on the IQ plane. The measured
signal is compared to the reference signal and the difference is given by the error
vector. The length of the error vector is the error vector magnitude. The error
vector magnitude is often reported as a percentage relative to some standard
signal, such as the magnitude of a constellation point.
44
TSG4100A Series RF Signal Generators User Manual
