Tektronix MTM400 - Manual (Page 169)

Interface Configuration and Interpretation
MTM400 MPEG Transport Stream Monitor User Manual 3-125
The following graphs are available when the COFDM interface card is installed:
RF constellation (all data carriers)
When the receiver is in RF lock, the constellation points are
displayed. Note that according to ETR290, only the data pilots
are used for MER and EVM calculation. The samples collected
are not real time, so there is some dwell (delay) before a
complete constellation is displayed. The persistence of the
display can be varied using the Persist slider control at the
bottom of the graph.
MER and EVM calculations are performed on the constellation
data according to the formulae given in ETR290-101. Note that
this calculation specifically excludes the TPS carriers.
Overall SNR (all continuous pilots)
A time-line graph of measured Signal-to-Noise Ratio is
displayed, over a period of the preceding 255 seconds.
Overall MER (all data carriers)
A time-line graph of measured Modulation Error Ratio is
displayed, over the preceding 255 seconds.
Linear Amplitude response (selected carriers)
A linear amplitude response is shown for the carriers selected
using the carrier START and STOP controls. Allows you to
view and zoom in on frequency-dependent amplitude
distortions.
Impulse response
A channel response graph is shown, allowing time-related
distortions, particularly echoes, to be displayed relative to the
main signal. Echoes occurring before and after the main signal
are displayed. It is derived form the linear amplitude response,
transformed into the time domain.