What causes receiver desensitization?

A strong interferer enters the receiver and uses up the LNA or mixer's linear dynamic range. The desired signal and the interferer share the same amplifier. If the interferer is strong enough to push the amplifier toward compression, the gain for both signals decreases. The desired signal, being much weaker, drops below the detection threshold. Example: LNA with IP1dB = -10 dBm at input. A -15 dBm interferer at the antenna (only 5 dB below compression) reduces the LNA gain by approximately 0.5 dB. A -20 dBm interferer causes approximately 0.1 dB gain reduction. The effect is worse for narrowband desired signals because they cannot be distinguished from the interferer by the wideband front-end.

How do you calculate the blocking dynamic range?

Blocking Dynamic Range (BDR) = IP1dB(input) - MDS, where MDS is the minimum detectable signal. For a receiver with: NF = 5 dB, BW = 10 kHz: MDS = -174 + NF + 10*log(BW) = -174 + 5 + 40 = -129 dBm. IP1dB(input) = -25 dBm. BDR = -25 - (-129) = 104 dB. This means the receiver can handle interferers up to 104 dB above the noise floor before 1 dB of sensitivity degradation occurs. Standards like 3GPP specify blocking tests: interferer at -15 dBm at antenna, desired signal at reference sensitivity + 3 dB. The receiver must still demodulate the desired signal.

How do you prevent desensitization?

(1) Preselector filter: bandpass filter before the LNA that rejects out-of-band interferers. A cavity filter with 50 dB rejection at the interferer frequency prevents it from reaching the LNA. (2) Higher IP1dB LNA: use an LNA with higher compression point. GaN LNAs can have IP1dB > 0 dBm (vs. -20 dBm for GaAs), dramatically improving blocking performance. (3) AGC (automatic gain control): reduce LNA gain when strong signals are detected. Trades sensitivity for dynamic range. (4) Switched attenuation: insert a pad before the LNA in high-interference environments. (5) Receiver architecture: use a high-IP3 passive mixer front-end that handles large signals without compression.

Receiver Performance

Desensitization

/dee-sen-sih-tih-zay-shun/ (desense)

Desensitization occurs when a strong interferer compresses the receiver's front-end gain, reducing sensitivity for the weak desired signal. The 1 dB desensitization level is the interferer power causing 1 dB sensitivity degradation. Blocking Dynamic Range (BDR) = IP1dB(input) - MDS. 3GPP tests: -15 dBm blocker at antenna, desired at reference sensitivity + 3 dB.

Category: Receiver Performance

BDR: IP1dB - MDS

3GPP Blocker: -15 dBm

Understanding Desensitization

Desensitization is the practical limit on receiver performance in real-world environments. In a lab, a receiver achieves its specified sensitivity. In the field, strong signals from nearby transmitters, co-located radios, or out-of-band emitters compress the front-end, reducing the gain available for the desired signal. Designing for desense immunity requires careful selection of preselector filtering, LNA linearity (IP1dB), and receiver architecture to maintain sensitivity in the presence of strong interferers.

Desensitization Analysis

Desense level:
ΔNF = 10log(1 + P_blocker/P_IIP3) dB

Blocking dynamic range:
BDR = IIP3 − MDS − 3 dB

1 dB desense:
P_block,1dB = IIP3 − 20log(√((10^0.1−1)·3))
≈ IIP3 − 17.3 dB

Desensitization Mitigation Comparison

Method	Improvement	Tradeoff	Application
Preselector filter	40-70 dB	Insertion loss, size	Base station, military
GaN LNA	20-25 dB	Higher NF (0.5-1 dB)	Co-site, urban
AGC	Dynamic	Sensitivity trade	Commercial Rx
Switched attenuation	6-30 dB	Reduced sensitivity	Military, co-site
High-IP3 passive mixer	20+ dB	Higher NF	SDR, wideband Rx

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Source	Mechanism	Typical level	Mitigation	Impact
TX leakage	LNA compression	−20 to 0 dBm	Duplexer ISO	1–5 dB NF rise
Adjacent ch	IM3 products	−40 to −20 dBm	Filter selectivity	BER degradation
Co-site	Wideband sat	−10 to +20 dBm	Cavity filter	Receiver blocking
Jammer	Intentional	0 to +40 dBm	Limiter + filter	Total blocking
Self-interference	Full-duplex	−10 to +20 dBm	SIC canceller	30–100 dB cancel

Common Questions

Frequently Asked Questions

What causes it?

Strong interferer uses LNA/mixer dynamic range. Amplifier compresses; gain for desired signal drops. Worse for wideband front-ends that cannot distinguish interferer from desired signal before amplification.

BDR calculation?

BDR = IP1dB(input) - MDS. MDS = -174 + NF + 10*log(BW). NF=5 dB, BW=10 kHz: MDS=-129 dBm. IP1dB=-25 dBm: BDR=104 dB. 3GPP: -15 dBm blocker at antenna, must still demodulate.

Prevention?

Preselector filter (40-70 dB rejection). GaN LNA (IP1dB +0 to +5 dBm vs. GaAs -20 dBm). AGC (trade sensitivity for dynamic range). Switched attenuation. High-IP3 passive mixer front-end.

Related Terms

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