Minimum Detectable Signal is the strength of the weakest signal that a radio receiver can detect and demodulate. It is specified in dBm, Watt or uV.
It is also referred to as Receiver Sensitivity
How to Calculate MDS
Use this tool is used to calculate the sensitivity. Enter the following:
- Temperature
- Bandwidth
- Receiver Noise Figure
- Signal to Noise ratio required to achieve the desired performance
Formula
MDS = 10 * log10 (kTB/(1 mW)) + NF + SNR
Where
- T – Temperature
- B – Bandwidth
- k – Boltzmann’s Constant 1.380649 × 10-23 m2 kg s-2 K-1
- NF – Noise Figure
- SNR – Signal-to-Noise ratio (sometimes referred to as Carrier-to-Noise Ratio)
Noise Figure
Noise figure degrades the receiver sensitivity. Typically a low noise amplifier can be used to improve lower the minimum detectable signal level.
Signal-to-Noise Ratio
As the complexity of the modulation increases the required SNR increases as well. FM for instance requires lower SNR than Bluetooth.
Temperature
As the operating temperature increases, the noise power increases. This degrades the receiver sensitivity
Bandwidth
The larger the bandwidth, the higher the noise power in the band of interest. This in turn degrades the receiver sensitivity.
Example Receiver Sensitivity Calculation
At:
- Room temperature 27oC or 300 K,
- Signal bandwidth of 25 kHz,
- Receiver noise figure of 5 dB,
- Required SNR of 4 dB,
results in a receiver sensitivity or minimum detectable signal level of -121 dBm.
In other words, a signal has to be at least -121 dBm to be demodulated by the receiver.
Bluetooth Receiver Sensitivity
Bluetooth signal has a bandwidth of 1 MHz and requires SNR of 15 dB for a Bit error rate of 0.1%.
Bluetooth requires that compliant devices must be able to achieve a minimum Receiver Sensitivity of -70 dBm. Using the calculator above, the noise figure can be as high as 29 dB.
In practice however the noise figure can be designed to be 8 dB or less. In this case the receiver can detect a signal as low as -90 dBm.