What is SFDR and why does it use a 2/3 factor?

Spurious-free dynamic range (SFDR) is the input power range over which the receiver produces no intermodulation product above the noise floor. The upper limit occurs when the IMD3 product level equals the noise floor. Since IMD3 power grows 3 dB for every 1 dB increase in input power, the gap between the fundamental and IMD3 shrinks at 2 dB per dB. The SFDR is 2/3 of the distance between IIP3 and the noise floor: SFDR = (2/3) times (IIP3 minus NF_floor) in dB. For a receiver with IIP3 = +10 dBm and noise floor at minus 104 dBm (10 MHz BW, 2 dB NF): SFDR = (2/3) times 114 = 76 dB.

How does NF trade off against IP3?

NF sets the noise floor (lower bound of dynamic range). IP3 sets the distortion ceiling (upper bound). To maximize SFDR, you want both low NF and high IP3, but they often conflict: adding gain early improves NF (Friis) but reduces the system IP3 because the high-gain stage amplifies strong interferers into compression. The solution is to carefully distribute gain: enough LNA gain to suppress subsequent noise, but not so much that the mixer or ADC is driven into nonlinearity. A passive preselector filter before the LNA also helps by attenuating out-of-band interferers before they reach the active stages.

What dynamic range does a typical receiver achieve?

A high-end military or spectrum-analyzer receiver with a GaN LNA front-end achieves SFDR of 90 to 110 dB in a 1 Hz bandwidth (instantaneous). A cellular base station achieves 70 to 85 dB SFDR in its channel bandwidth. A consumer WLAN receiver achieves 50 to 65 dB. An automotive radar receiver achieves 60 to 80 dB. The dynamic range in a wider measurement bandwidth is always less than in a narrow bandwidth because the noise floor rises with bandwidth: every doubling of bandwidth raises the noise floor by 3 dB and reduces SFDR by 2 dB.

System Design

Dynamic Range

A radar receiver must detect a −110 dBm echo from a target 200 km away while a −20 dBm return from a nearby building arrives at the same instant. The receiver needs 90 dB of dynamic range just to process both signals without the strong one generating spurious products that mask the weak one. Dynamic range is the power window between the noise floor (below which signals are undetectable) and the distortion ceiling (above which signals corrupt each other). Expanding this window requires simultaneously lowering the noise floor (better NF) and raising the distortion ceiling (higher IP3), two goals that often pull the gain distribution in opposite directions.

Category: System Design

Types: SFDR, BDR, CDR

Unit: dB (in stated BW)

Three Definitions, One Concept

Spurious-Free Dynamic Range (SFDR):
SFDR = (2/3) × (IIP3 − N_floor) dB
N_floor = −174 + NF + 10·log(BW) dBm

Compression Dynamic Range (CDR):
CDR = P1dB_in − MDS dB

Blocking Dynamic Range (BDR):
BDR = P1dB_in − (MDS + SNR_min) dB

Example: cellular base station RX (10 MHz BW, NF = 2 dB, IIP3 = −5 dBm):
N_floor = −174 + 2 + 70 = −102 dBm
SFDR = (2/3) × (−5 − (−102)) = (2/3) × 97 = 64.7 dB
MDS = −102 + 10 (SNR for 64QAM) = −92 dBm
CDR = −15 − (−92) = 77 dB

Dynamic Range by Receiver Class

Receiver Type	NF	IIP3	SFDR (1 Hz)	SFDR (Channel BW)
Military (GaN front-end)	2 dB	+25 dBm	109 dB	95 to 105 dB
Cellular base station	2 dB	−5 dBm	89 dB	60 to 70 dB
Software-defined radio	5 dB	+10 dBm	90 dB	65 to 75 dB
Consumer WLAN	5 dB	−10 dBm	79 dB	50 to 60 dB
Automotive radar	8 dB	+5 dBm	79 dB	60 to 75 dB

Common Questions

Frequently Asked Questions

Why the 2/3 factor in SFDR?

IMD3 grows 3 dB per 1 dB input increase, so the gap shrinks at 2 dB/dB. SFDR = (2/3) × (IIP3 − noise floor). For IIP3 = +10 dBm and noise floor = −104 dBm: SFDR = 76 dB.

NF vs. IP3 trade-off?

More LNA gain improves NF (Friis) but reduces system IP3 (strong signals drive downstream stages into compression). Solution: enough gain to suppress noise, not so much that mixers are overdriven. Preselector filters help by attenuating out-of-band blockers.

Typical receiver dynamic range?

Military: 95 to 110 dB SFDR. Cellular BS: 60 to 70 dB. Consumer WLAN: 50 to 60 dB. Wider BW always reduces DR: every 3 dB more BW raises the noise floor and costs 2 dB of SFDR.

Related Terms

← Duty Cycle Effective Area →

System Analysis

Dynamic Range Calculator

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