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
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.
1 Test set up
signal generator in this set was a Siglent and has been set to the
lowest signal output of 1.414Vrms.
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.
receiver under test is that of a TS930S HF transceiver.
AC volt meter is a Fluke 189 multi-meter.
MDS step process
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.
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.
this value of 3mV calculate the value at +3dB.
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
is convenient to convert the signal generator output from mVrms to
dBm as all attenuation values can be simply subtracted from the dBm
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.
Signal Generator output in dBm
dBm=Signal Generator Output dBm-Attenuators dB
Alternately for a µV result;
dBm with Z=50Ω impedance to µV
Convert -125 dBm with Z=50Ω impedance to
between uV and dBm: uV to dBm CONVERSION