What are blocking, selectivity and reciprocal mixing?

Q: What is desensitization?

Loss of sensitivity due to a strong off-channel signal.

Q: Can filtering fix reciprocal mixing?

Only filtering before the mixer that attenuates the blocker; otherwise LO phase noise must improve.

Q: What is an intermodulation test?

Two interferers whose IM3 falls on the wanted channel, at specified levels.

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Selectivity is a receiver’s ability to reject unwanted signals at other frequencies. Strong unwanted signals degrade a receiver by blocking (gain compression or desensitization), intermodulation (IM products on the wanted channel), reciprocal mixing (blocker mixing with LO phase noise) and spurious responses. Standards test each with a wanted signal slightly above sensitivity.^[1]

How does reciprocal mixing set a phase noise requirement?

A blocker of power P_B at offset Δf mixes with LO phase noise ℒ(Δf) to produce in-channel noise of about P_B + ℒ(Δf) + 10·log10(B). For this to stay below the allowed noise or interference level N_allow: ℒ(Δf) < N_allow − P_B − 10·log10(B).^[1]

What is adjacent channel selectivity?

The ratio of adjacent-channel interferer power to wanted signal power at which performance just meets the criterion. It depends on channel filter rejection, reciprocal mixing and ADC dynamic range.

How is blocking performance improved?

Front-end filtering (SAW/BAW, cavity) before the LNA.
Higher P1dB and IIP3 in early stages.
Low LO phase noise at the blocker offsets.
Careful frequency planning to avoid spurious responses.

Frequently asked questions

What is desensitization?

Loss of sensitivity due to a strong off-channel signal.

Can filtering fix reciprocal mixing?

Only filtering before the mixer that attenuates the blocker; otherwise LO phase noise must improve.

What is an intermodulation test?

Two interferers whose IM3 falls on the wanted channel, at specified levels.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.

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