What is the minimum detectable signal and how do I calculate it for my receiver?
Calculating Minimum Detectable Signal
The minimum detectable signal defines the absolute sensitivity floor of a receiver. It represents the input signal power that produces an output signal-to-noise ratio of exactly 0 dB (signal power equals noise power). Any signal weaker than the MDS is buried in noise and cannot be detected without additional processing gain.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Is MDS the same as sensitivity?
Not exactly. MDS is the signal level at 0 dB SNR. Sensitivity is the minimum signal level for acceptable performance, which requires a specified SNR above the noise floor. Sensitivity is always higher (less negative) than MDS.
Can processing gain improve MDS?
Processing gain (spread spectrum, pulse integration, matched filtering) effectively reduces the noise bandwidth after reception, lowering the noise floor and improving the effective MDS. The physical MDS does not change, but detectable signal levels below MDS become possible.
How does MDS relate to receiver range?
For free-space propagation, received power decreases as 1/R². Improving MDS by 6 dB doubles the detection range. For radar (two-way propagation), received power decreases as 1/R⁴, so 6 dB MDS improvement increases range by a factor of 1.41.