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This limit of acceptable wanted signal degradation is assumed to be 3 dB, based on suggestion in ITU-R SM.332-4 and the blocking measurement procedure described in ETSI EN 300 113. The latter procedure is briefly summarised as follows (see Figure 394396):

• The wanted signal (Generator A) is switched on, tuned at receiver frequency and its level is adjusted to the level of maximum useable sensitivety of the receiver, which corresponds to BER=10^-2 at the receiver output. The level of wanted signal is then further increased by 3 dB;
• The unwanted signal is added by switching on Generator B, with given frequency shift from receiver frequency, and its output level is first increased until a BER=10^-1 or worse is obtained; then decreased again until the BER=10^-2 is re-attained at the receiver output;
• The corresponding level of unwanted signal is noted. The “blocking” ratio is then expressed as ratio, in dB, of the level of unwanted signal to the level of wanted signal;
• Test is repeated for other frequency shifts Df.