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In a data sheet for a wireless transceiver I read

100 dB blocking immunity

what does this actually mean?

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This is likely a statement of a measurement similar to blocking dynamic range, which is a measure of how far above the noise floor a strong signal falling outside a receiver's filters can be, without degrading performance for desired signals.

However, this is a measurement that can vary widely for a given receiver, depending on how exactly it is defined/taken. Halfway decent receivers will have great numbers for a signal which falls outside of all of the filters (and avoids any frequencies where the the design is susceptible to imaging, mixing with spurs, component limitations etc). But the same receiver's ability to block a signal which gets through some filters - for example, passing through the analog filtering and being rejected only at a post-ADC digitial filter - will typically be notably lower.

Without knowing how the measurement is taken, you can't really interpret the given number.

Similarly, as precise definitions of blocking dynamic range differ from source to source, I am specifically choosing not to endorse any of them with links.

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Interesting question. I found a page on Wikipedia describing RF Immunity

Integrated circuits tend to demodulate high-frequency carrier signals commonly found in regular environment due to presence of cell phones. These ICs demodulate the high frequency cell phone carrier (e.g., GSM850 and GSM1900, GSM900 and GSM1800) and produce low-frequency (e.g., 217 Hz) demodulated signals. This demodulation manifests itself into unwanted audible buzz in audio appliances such as microphone amplifier, speaker amplifier, car radio, telephones etc. Adding on-board EMI filters or special layout techniques help in bypassing EMI or improving RF immunity.

I hope this answers the question.

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    \$\begingroup\$ What you're talking about would not be a specification that applies to the transceiver itself. I'm not going to downvote, but this answer doesn't address the actual question. \$\endgroup\$ – Dave Tweed Apr 23 '13 at 15:13

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