Receiver sensitivity measurement
Figure 1: Oscillogram for sensitivity measurement (MDS)
Receiver sensitivity measurement
The receiver sensitivity measurement determines the lowest possible power (MDS, Minimum Detectable Signal, or Minimum Discernible Signal) at the input of a receiver with which the radar can still detect a target. This measurement provides an important indication of the performance of a radar.
Measurement on analog receivers
The prerequisite for such a measurement is a calibrated RF signal generator, which may also be capable of providing special modulation types. An oscilloscope is required as a further aid.
With the help of a suitable RF signal generator, RF pulses are generated in the rhythm of the pulse repetition frequency, which has a power of 1 mW (= 0 dBm). These pulses are fed into the receive path via a variable attenuator. The receiver video is fed to an oscilloscope. With the help of the variable attenuator, the RF signals are now attenuated until the amplitude of the received signal on the oscilloscope is 3 dB (!) above the noise floor. But be careful! The 3 dB refers to a power level. However, the oscilloscope does not indicate power, but a voltage! One might now think that doubling the power (3 dB) would require a quadrupling of the amplitude (6 dB). In principle: Yes, but here only relative amplitudes are measured in comparison to each other. The level of the noise signal is also only evaluated by its amplitude on the oscilloscope. A doubling of the factor of 0.707 is nevertheless 2 × 0.707. The variable attenuator is therefore adjusted until a signal-to-noise ratio of the amplitudes of about 2 to 1 is obtained. The attenuations of the test setup (cables, connectors, directional couplers) are added to the value readable from the variable attenuator. The result gives the MDS value for the measured receiver.
Figure 2: Measurement setup for the MDS, attenuation in directional coupler, the plug connectors and the cable must be taken into account
Figure 2: Measurement setup for the MDS, attenuation in directional coupler, the plug connectors and the cable must be taken into account.
Figure 2: Measurement setup for the MDS, attenuation in directional coupler, the plug connectors and the cable must be taken into account (interactive picture)
In this test setup example, the attenuation of the cable measured by the calibration laboratory, and the two connectors must be added to the value read (e.g. −100 dB). In this example, only −106 dBm arrives at the receiver input!
Since the noise floor that can be seen on the oscilloscope during the MDS measurement does not have a constant amplitude, an exact measurement will encounter difficulties. To keep the tolerance range of the measurement result as small as possible, the noise signal should be optically averaged, the value determined numerically and the attenuation changed until the amplitude of the test signal increases to twice this value.
Measurement on digital receivers
In digital radars, the measurement of receiver sensitivity can be performed by an internal test routine, which is carried out during the dead time of the radar (i.e. the time in which the phase shifter of a phased array antenna are re-tuned for a new direction of radiation). This internal test routine corresponds to the measuring procedure in an analog radar unit.
An external measurement can usually only be performed under workshop conditions. This requires a large number of measuring instruments which can simulate the signals of the transmitter at any time, but synchronously with the radar. This means that the original waveform must be delayed as much as desired in order to be displayed on the screen at the desired distance (outside the range of an STC).