Tektronix 071-0590-00 user manual Attenuation dB

BERT Technical Articles

BER vs. Attenuation appears as a straight line because the BER scale is again distorted. The line here is for the case SNR0 = 18 dB. For other values of SNR0, the plot would fall parallel to the line in Figure 5. Remember that we are talking about electrical attenuation here; 1 dB of optical attenuation is worth 2 dB of electrical attenuation.

10 2
10 3
BER
10 4
10 5
10 6
10 7
10 8
10 9
10 10
10 12
10 14
10	16	1	2	3	4	5	6	7	8	9	10	11
	0
					Attenuation (dB)

Figure 5. BER is plotted as function of electrical-signal attenuation for the case of a system with BER=10-6for no attenuation. That is, the SNR0 for no attenuation is 18 dB. Then, SNR with attenuation is 18 dB minus attenuation, and BER follows from Figure 4.

It isn't practical to plot a curve such as that in Figure 5; it would take too long to plot the points for low BER. But if the noise is known to be Gaussian and the BER is found to be 3.5×10−5 for 6 dB of attenuation, then the BER can be inferred to be 10–16for no attenuation. It is actually better to plot two points for two attenuations and extrapolate to zero attenuation. An example will show how this extrapolation is done.

GB1400 User Manual B-37