Measuring Digital TV Signals in the Broadband Cable - Digital Video and Audio Broadcasting Technology - page 293

If the correct DVB-C or J83A, B, C signal is present at the test receiver
and all settings at the receiver have been selected so that it can correctly
lock to the QAM signal, a constellation diagram with constellation points
of varying size (Fig. 18.2.) and the appearance of noise clouds is obtained.
The size of the constellation points depends on the magnitude of the inter-
ference effects. The smaller the constellation points, the better the signal
quality.

Fig. 18.2. Correctly locked 64QAM constellation diagram with noise

Fig. 18.3. No QAM signal in the selected channel, only noise

For the receiver to lock up in DVB-C and J83A, B, C, the following ad-
justment parameters of the test receiver must be selected correctly:

• Channel frequency: channel band center, approx. 47 to 862 MHz

• Standard: DVB-C/J83A, J83B or J83C