Tektronix SPECMON3 Manual page 324

GP Digital Modulation
The measurement filter is applied before the demodulation bit is detected and the ideal reference is
calculated.
The reference filter is applied to the internally generated ideal reference signal before the EVM is
calculated.
How to Select Filters
In a signal transmitter/receiver system, the baseband signal might be filtered for bandwidth limiting or for
another kind of necessary shaping that needs to be applied. Normally, a filter in the transmitter (Ft) and a
filter in the receiver (Fr) are applied.
The Measurement Filter setting in the analyzer corresponds to the baseband filter in the receiver (Fr): This
setting tells the analyzer what filter your receiver uses. When the analyzer is set to the same filter used by
the receiver, the analyzer sees the signal as your receiver would. The Measurement Filter setting should be
the same as the filter used in the receiver under normal operation (as opposed to testing).
The Reference Filter setting in the analyzer corresponds to the baseband filter in the transmitter-receiver
combination (Fr * Ft). The baseband filter for the transmitter-receiver combination is often referred to as
the System Filter. This filter is called the reference filter because it is used to recreate a reference signal
that is compared to the received signal. This recreated reference signal is the ideal signal with Fr * Ft
applied; differences between this ideal signal and the received signal enables the determination of signal
quality, such as EVM measurements.
The following is an example of a hypothetical signal that is transmitted into a vector signal analyzer
for analysis:
Assume that a signal is transmitted using a baseband filter (Ft). It then travels through a transmission
medium (air/cable/etc) where it may affected by the communication channel (Fc). The signal is received
and filtered by the receiver's filter (Fr). At this point, the signal has passed through Ft and Fr, and in
addition, the communication channel might have affected it (so: Ft * Fr * Fc). This double-filtered
signal is demodulated as it was received to determine the symbols/bits in it. The obtained bits are used
to regenerate a baseband ideal signal that can be compared against the received signal to determine
signal quality. However, to determine the effect of the environment on the signal quality, the ideal
signal must be filtered by the REFERENCE FILTER (Ft * Fr), so that the ideal signal and the filtered
signal differ only by the effect of the environment. So, the received signal is the ideal signal filtered by
Ft * Fr * Fc
by the effect of Fc, the comparison will show the effect of the communication channel on the signal.
The communication channel can also include the hardware path the signal follows after (Tx) or before
(Rx) digitizing; this would account for Tx/Rx hardware linear and non-linear distortion.
Common examples of how these filters are used are shown below:
For Transmit Filter = Root Raised Cosine (RRC), Measurement Filter = RRC, the Reference Filter
= RRC ^2 = Raised Cosine
For Transmit Filter = Raised Cosine (RC), Measurement Filter = None, the Reference Filter = Raised
Cosine (When the Measurement Filter = None, the Reference Filter = Transmit Filter)
For Transmit Filter = Gaussian, Measurement Filter = None, the Reference Filter = Gaussian
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and the reference signal is the ideal signal filtered by
Modulation Params Tab
, since they only differ
Ft * Fr
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