The MDS (Minimum Discernible Signal) receiver sensitivity measurement technique.

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The MDS (Minimum Discernible Signal) is defined as a signal that achieves a 3dB audio voltage gain over the internally generated noise from within the receiver alone. 

The below set up shows a signal generator connected to the receiver’s antenna input via a step attenuator. A voltage meter is attached to the audio output of the transceiver. With the signal generator switch off the no signal voltage from the audio output of the receiver is recorded. With the signal generator set to the same frequency as the receiver, switched on and set to a level that produces a 3dB rise in the audio output voltage. Determine the signal voltage applied to the receiver’s antenna connection. The voltage at the antenna connection is the MDS (Minimum Detectable Signal) rating of the receiver for the frequency tested.

Fig 1 Test set up   

Equipment:

The signal generator in this set was a Siglent and has been set to the lowest signal output of 1.414Vrms.

The step attenuator is a home brew unit. The step attenuator has a maximum attenuation of 41dB which is nowhere near enough and there for a fixed 70dB attenuator was connected in series with the step attenuator achieving an attenuation range of -70db to -111dB.

The receiver under test is that of a TS930S HF transceiver.

The AC volt meter is a Fluke 189 multi-meter.

MDS step process

Step 1 

Test set up for receiver and test equipment as shown in Fig#1. The Signal generator connected to the step attenuator plus fixed attenuator, the attenuator connected to the antenna socket of the receiver and the AC volt meter connected to the audio output of the receiver. The volt meter will be set to the mV AC range.  

Step 2

With the signal generator turned off and the receiver on, measure the audio voltage with the AC volt meter. Adjust the audio gain until there is a voltage measured. The voltage is not critical; however a reasonably rounded value is preferable. Assume a value of 3mV is recorded. This voltage represents a bench mark for the white noise generated from within the receiver’s electronic circuitry.

With this value of 3mV calculate the value at +3dB.

Vout=Vin×10(dB/20)

Vout=3mV×10(3dB/20)=4.24mV

Step 3

The signal generator is set to the same output frequency as the receiver and the output level is set to the lowest signal output of -50dBm in this case.

It is convenient to convert the signal generator output from mVrms to dBm as all attenuation values can be simply subtracted from the dBm value.  

Step 4

With the signal generator turned on and tuned to the same frequency as the receiver and with the receiver on and set to either the CW or SSB mode, select the attenuation settings that produce a 4.24mV reading on the AC volt meter. Add all the attenuation settings including the value from the signal generator output of -50dBm and that is the receiver MDS (Minimum Discernible Signal) capability. A slightly better MDS result will be achieved in the CW mode with CW bandpass filtering than for SSB.

Reciver dBm=Signal Generator Output dBm-Attenuators dB

                           -50.0dBm-40dBm -35dBm=-125dBm

Alternately for a µV result;

Convert dBm with Z=50Ω impedance to µV

Convert -125 dBm with Z=50Ω impedance to µV  

Reference

Conversion between uV and dBm: uV to dBm CONVERSION

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Page last revised 05 May, 2025