RF Utilities
SFDR and intercept-point calculator for RF systems integration. Compute the third and second-order spurious-free dynamic range from IIP3, noise figure, and bandwidth, read the intermod at a real drive level, cascade a receiver front end, and check a receiver against a site environment.
Free to use, no sign-up needed.
Walkthrough
SFDR mode with the noise floor, minimum detectable signal, third-order SFDR, and the top of the window.
Overview
Spurious-free dynamic range is the window a receiver actually has to work in: strong enough at the bottom to be above the noise floor, but not so strong at the top that the receiver makes its own intermodulation products and mistakes them for signals. The bottom is the noise floor, the thermal noise in the bandwidth plus the noise figure. The top is set by the third-order input intercept, because third-order products rise three times as fast as the wanted signal and end the spurious-free range when they reach the noise. Written out, SFDR equals two thirds of the intercept above the noise floor, less the SNR the signal needs.
The noIM₃ Spurious-Free Dynamic Range Calculator is the intercept and dynamic-range utility for that work, written for a systems integrator rather than a circuit designer. The intercept points and noise figure are datasheet inputs, not synthesised, and the output is framed as the numbers that decide a receiver. The SFDR mode reports the noise floor, the minimum detectable signal, the third-order SFDR and the input level at the top of the window, and a second-order SFDR from the input IP2 that often governs in wideband and zero-IF front ends where second-order products fall directly in band.
Beyond the headline figure the tool does the three jobs that surround it. Spur Level ties the SFDR to a real drive level: the third-order intermod product, its rejection below the carrier, and whether it sits above the noise floor. Cascade resolves a receiver chain into its system noise figure by Friis, its system input IIP3 by the intercept cascade, and its system SFDR, showing where a high-gain first stage trades noise figure against intercept. Requirements turns a strong-blocker site into the IIP3 a receiver must have and checks whether a candidate passes.
Compute the third-order spurious-free dynamic range as SFDR = ⅔·(IIP3 − N) − SNR, with the noise floor, the minimum detectable signal, and the input level at the top of the window all reported. The two-thirds factor comes from the three-to-one slope of the third-order product.
Enter the input second-order intercept IP2 and the tool reports the second-order SFDR of ½·(IP2 − N) − SNR. Second-order products matter in wideband and direct-conversion front ends, where the sum and difference of two strong signals fall directly in band and the IP2 window often governs.
At a two-tone drive level the tool reports the third-order product (3·P − 2·IIP3), its rejection below the carrier (2·(IIP3 − P)), and whether the product sits above the noise floor, colour coded from under the noise to dominant. The second-order product and the output intercept are alongside.
Build a chain of stages, each with a gain, a noise figure, and an input IIP3, and the tool computes the system noise figure by Friis, the system input IIP3 by the intercept cascade, and the system SFDR, with a per-stage build-up that shows where the dynamic range is won and lost.
A high-gain first stage improves the noise figure but pushes the system intercept down, and the two pull in opposite directions. The cascade build-up shows the system IIP3 and SFDR through each stage, so the trade between sensitivity and dynamic range is explicit rather than assumed.
Given a weak wanted signal and a strong two-tone blocker whose intermod lands on the wanted channel, the tool computes the IIP3 a receiver must have to keep that product a protection ratio below the wanted signal, checks a candidate receiver, and reports the margin with a clear pass or fail.
The intercept model is a small-signal two-tone approximation, valid well below the 1 dB compression point. A prominent confidence indicator carries the assumptions on every screen, so the point where the model stops applying is never hidden.
Runs entirely in your browser. No intercept points, noise figures, or signal environments are submitted to a server. Useful for commercially confidential work, classified projects, or any environment where information security policy prohibits sending engineering data to third party services.
Inputs and outputs
Inputs 8
Outputs 8
Use cases
It is the range of input signal over which a receiver is useful: strong enough to be above the noise floor at the bottom, but not so strong that the receiver own third-order intermodulation products rise out of the noise at the top. The bottom is the noise floor, kTB plus the noise figure. The top is where the third-order products reach the noise floor. The result is SFDR = two thirds of the intercept above the noise floor, less the SNR the wanted signal needs.
Take the input third-order intercept, subtract the noise floor, multiply by two thirds, and subtract the minimum SNR. SFDR3 = ⅔·(IIP3 − N) − SNR_min, where N = −174 dBm/Hz + 10·log₁₀(B) + NF. For example a receiver with a 0 dBm IIP3, a 3 dB noise figure, and a 1 MHz bandwidth has a noise floor of −111 dBm and an SFDR of about 74 dB. The calculator does this live and also reports the input level at the top of the window.
Because the third-order intermod product rises three times as fast as the wanted signal. As the input increases by one dB, the wanted output rises one dB but the intermod rises three dB, so the intermod closes on the noise floor twice as fast as the signal climbs above it. The spurious-free window therefore grows only two thirds of a dB for every dB of extra intercept. That three-to-one slope is the whole reason the third-order intercept is the headline dynamic-range number for a front end.
In wideband and direct-conversion, or zero-IF, receivers. A channelised receiver with good filtering ahead of the mixer is usually limited by third-order products, but a wideband or zero-IF front end sees the sum and difference of two strong signals fall directly in band, and those second-order products are set by the second-order intercept IP2. The second-order SFDR is one half of the intercept above the noise floor, reflecting the two-to-one slope. The calculator reports both windows so the governing limit for the architecture is clear.
The noise figure and the intercept cascade differently, and they pull in opposite directions. The system noise figure follows the Friis formula, so the first stage dominates and a high-gain low-noise LNA sets a low floor. But the system input intercept follows the intercept cascade, where the gain ahead of a stage refers its intercept back to the input, so that same high-gain LNA pushes the system IIP3 down. Good sensitivity and good dynamic range therefore fight each other, and the SFDR is set by the balance. The Cascade mode computes both and shows the build-up stage by stage.
Turn the site into an intercept requirement. Take the weak wanted signal and the strong two-tone blocker whose third-order product lands on the wanted channel. The blocker intermod must sit a protection ratio below the wanted signal, and the IIP3 that achieves it is the requirement. The Requirements mode computes that required IIP3, then checks a candidate receiver: if its intercept meets or beats the requirement it passes, and the margin tells you how much headroom there is. It is the direct way to match a receiver to a real signal environment.
It is a small-signal two-tone approximation, accurate while the receiver is well below its 1 dB compression point. It assumes a memoryless third-order nonlinearity, that third-order products dominate, and that the datasheet intercepts hold at the operating frequency. Near compression the model breaks down and the real intermod rises faster than the three-to-one law predicts, so a front end being driven hard needs a measured two-tone test. The tool carries these assumptions in a prominent confidence indicator on every screen.
No. The calculator runs entirely in your browser. No intercept points, noise figures, or signal environments are submitted to a server. Useful for commercially confidential work, classified projects, or environments where information security policy prohibits sending engineering data to third party services.
