Power, Linearity, and Distortion Compression and Intercept Points Informational

How do I calculate the spurious free dynamic range of a receiver chain?

Spurious-free dynamic range (SFDR) is the range of signal levels from the noise floor to the level where the largest spurious signal (typically the third-order intermodulation product, IM3) equals the noise floor. It defines the usable dynamic range of a receiver: (1) Formula: SFDR (dB) = (2/3) × (IIP3 - N_floor). Where IIP3 = input-referred third-order intercept point (dBm). N_floor = noise floor of the receiver (dBm) = -174 + NF + 10×log10(B). B = noise bandwidth (Hz), NF = noise figure (dB). (2) Example: receiver with NF = 6 dB, IIP3 = -10 dBm, bandwidth = 10 MHz. Noise floor: N = -174 + 6 + 10×log10(10e6) = -174 + 6 + 70 = -98 dBm. SFDR = (2/3) × (-10 - (-98)) = (2/3) × 88 = 58.7 dB. This means: the receiver can detect signals from -98 dBm (noise floor) down to -39.3 dBm (where IM3 products would equal the noise floor). The SFDR is 58.7 dB. (3) Why 2/3 factor: the IM3 product level rises 3 dB for every 1 dB increase in input signal (third-order product grows at 3× the input rate). So the IM3 level catches up to the noise floor from below at a rate of 2 dB per 1 dB of input signal increase. The point where IM3 = noise floor is 2/3 of the way from noise floor to IP3. (4) SFDR vs bandwidth: SFDR depends on the noise bandwidth. Narrower bandwidths have lower noise floors, increasing the SFDR. SFDR for 1 Hz bandwidth: multiply the formula by the bandwidth factor. Normalized SFDR (per Hz): SFDR_1Hz = (2/3) × (IIP3 + 174 - NF). This is bandwidth-independent and useful for comparing receivers.

Category: Power, Linearity, and Distortion

Updated: April 2026

Product Tie-In: Amplifiers, Mixers, Attenuators

SFDR Calculation

SFDR is the primary dynamic range metric for receivers that must operate in environments with multiple strong signals (cellular, radar, EW).

Cascaded SFDR

In a multi-stage receiver: the SFDR is determined by the cascade IIP3 and the cascade noise figure. The cascade IIP3 is typically dominated by the last stage (the stage with the highest gain before it). The cascade NF is dominated by the first stage (per the Friis formula). Optimization: the first stage (LNA) should have low NF (to minimize the noise floor) and moderate IIP3. The later stages should have high IIP3 (to handle the amplified signals without distortion). The overall SFDR is maximized when the gain distribution is optimized to balance NF and IIP3 contributions.

SFDR Equations

SFDR = ⅔(IIP3 - N_floor)
N_floor = -174 + NF + 10log₁₀(B)
NF=6, IIP3=-10, B=10MHz: SFDR=58.7dB
IM3 rises 3dB per 1dB input increase
SFDR_1Hz = ⅔(IIP3 + 174 - NF)

Common Questions

Frequently Asked Questions

What is a good SFDR?

Depends on the application: consumer Wi-Fi receiver: SFDR > 50 dB (moderate dynamic range requirements). Cellular base station: SFDR > 70 dB (must handle both strong and weak users). Military ECM/ELINT receiver: SFDR > 80 dB (extremely demanding). Test equipment (spectrum analyzer): SFDR > 90 dB.

SFDR vs blocking dynamic range?

SFDR: measures the range from noise floor to IM3=noise floor. Blocking dynamic range: measures the range from noise floor to the 1 dB compression point. BDR = P1dB_input - N_floor. For a typical receiver: P1dB is approximately 10 dB below IIP3. BDR > SFDR (blocking range is larger because P1dB > IM3 intercept at the SFDR signal level).

How does ADC SFDR relate to receiver SFDR?

The ADC SFDR (determined by quantization and nonlinearity) is an additional limitation on the receiver SFDR. The overall receiver SFDR cannot exceed the ADC SFDR. For a 12-bit ADC: SFDR ≈ 72 dB (ideal). For a 14-bit ADC: SFDR ≈ 84 dB. The RF front end and the ADC SFDR must both meet the system requirement.

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