Measuring Digital TV Signals in the Broadband Cable - Digital Video and Audio Broadcasting Technology - page 294

• Channel bandwidth: 6, 7, 8 MHz

• QAM level: 16QAM, 32QAM, 64QAM, 128QAM, 256QAM

• Symbol rate: approx. 2 to 7 MS/s

• SAW filter: ON with adjacent channels occupied

• Input attenuation control: to AUTO if provided

If there is simply no signal in the selected RF channel, the constellation
analyzer of the test receiver will display a completely noisy constellation
diagram (Fig. 18.3.) which exhibits no regular features whatever. It ap-
pears like a giant constellation point in the center of the display, but with-
out sharp contours.
If accidentally an analog channel has been selected instead of, e.g. a
DVB-C channel, constellation diagrams like Lissajou figures are produced
which change continuously depending on the current content of the analog
TV channel. If, however, there is a QAM signal in the selected channel but
some of the receiver parameters have been selected wrongly (RF not ex-
actly right, maybe the wrong symbol rate, wrong QAM level etc), a giant
constellation point with much sharper contours appears.

Fig. 18.4. Constellation diagram with the wrong carrier frequency and wrong
symbol Rate selected (completely unsynchronized)

If all parameters have been selected correctly and only the carrier fre-
quency is still divergent, the constellation diagram will rotate. It is then
possible to see concentric circles.
An ideal, completely undistorted constellation diagram would show
only a single constellation point per decision field in the exact center of the