This tool computes the Spurious (or Spur) Free Dynamic Range of a Radio. It is often abbreviated as SFDR and is a key indicator of the linearity of the radio.
Enter:
- Input Third Order Intercept (dBm)
- Minimum Detectable Signal (dBm)
Formula
SFDR = 2/3 * (IP3 – MDS)
Background
The IP3 is a measure of how strong of an input signal the receiver can tolerate before it generates spurs or non-linear products. Ideally the receiver should be operated well below the IP3.
The MDS or receiver sensitivity, is the weakest signal that can be detected by a radio. It can be calculated using the Noise Floor Calculator and depends on various factors including the Noise Figure of the receiver.
To maximize the SFDR, the IP3 should be as high as possible and the MDS should be as low as possible. In practice it’s very difficult to meet both these constraints at the same time.
Example Application
Using an IIP3 of +10 dBm and a Minimum Detectable Signal of -90 dBm, the SFDR is 66.7 dB. As the IIP3 increases or the MDS decreases, the SFDR will increase.
One way to increase the IIP3 is to add attenuation at the front end of the receiver. However, the impact of this attenuation, specially at the input, is to increase the noise figure. RF designers have to work with amplifiers and attenuators to carefully balance these quantities and maximize the SFDR.
Notes
[1] SFDR is a key specification of Analog-to-digital converters. For software defined radios or any radio that relies on digitization, the SFDR of the ADC can limit the performance of the entire radio receiver chain