SFDR
Understanding SFDR
SFDR is the most complete single-number specification of receiver dynamic range. It answers: over what range of input signal levels can the receiver operate without either losing the signal in noise (too weak) or generating detectable spurious products (too strong)?
SFDR Calculation
SFDR (dB) = 2/3 x (OIP3 - N_floor) for third-order SFDR, where OIP3 is the output IP3 and N_floor is the noise floor. This relationship comes from the fact that IM3 products grow at 3x the rate of the fundamental relative to the IP3 point.
SFDR vs Bandwidth
SFDR depends on noise floor, which depends on bandwidth. Narrower bandwidth lowers the noise floor, increasing SFDR. SFDR is often specified as 'SFDR in 1 Hz bandwidth' to normalize the comparison across different receiver bandwidths.
SFDR = (2/3) x (IIP3 - NF_floor) dB
Noise floor: NF_floor = -174 + 10log(BW) + NF
Example:
IIP3 = -5 dBm, NF = 3 dB, BW = 1 MHz
NF_floor = -174 + 60 + 3 = -111 dBm
SFDR = (2/3)((-5) - (-111)) = (2/3)(106) = 70.7 dB
Frequently Asked Questions
What is SFDR?
SFDR is the input signal range between the noise floor and the level where the largest spurious product equals the noise floor. It represents the usable dynamic range within which no distortion products are detectable. Wider SFDR means better receiver performance.
How is SFDR calculated?
Third-order SFDR = (2/3) x (IIP3 - noise floor). The noise floor depends on bandwidth and noise figure. SFDR improves with higher IP3, lower noise figure, and narrower bandwidth.
What is a good SFDR?
For narrowband communications receivers, SFDR of 80-100 dB (in 1 Hz BW) is typical. For wideband surveillance receivers, 60-80 dB. For audio equipment, 90-120 dB. SFDR requirements depend on the signal environment and the system application.