Measuring DVB-T Signals - Digital Video and Audio Broadcasting Technology - page 403

The process of noise-like crosstalk can be described easily by means of
simple trigonometric operations which can be derived from the vector dia-
gram.

a1 a2 = a1 (1AI)

Noise N

N = a1a2;

a2 = a1(1AI)

a1

Signal S

S = a1 + a2;

S/N = (a1+a2)/(a1a2) = (a1+a1(1AI)/(a1a1(1AI) = (2AI)/AI;

S/N[dB] = 20lg((2AI[%]/100)/(AI[%]/100));

Fig. 21.21. Determining the S/N with amplitude imbalance

In the case of amplitude imbalance, the opposing vectors no longer can-
cel completely (Fig. 21.21.), resulting in a noise vector causing crosstalk
from the upper DVB-T band to the lower band and vice versa. The actual
useful signal amplitude decreases by the same amount by which the
crosstalk increases.

a

Noise N

a

x=90/2 x N

N = 2 a cos(90/2);

Signal S

a

S = 2 a sin(90/2);

S/N = (2a)/(2a) (sin(90/2)/cos(90/2)) = tan(90/2);

S/N[dB] = 20lg(tan(90/2));

Fig. 21.22. Determining the S/N in the presence of an I/Q phase error

A phase error will result in a noise vector the length of which can be de-
termined from the vector parallelogram. The useful signal amplitude also