What is the “Coherent-on-receive” operation?
============ In the times of earlier radars, the only available device capable to produce an high-power output at microwave frequencies, and therefore, suitable for use in radar transmitters, was the Magnetron tube. Unfortunately, the magnetron is not an amplifier, but a power oscillator: when a high-voltage pulse is applied to the cathode, it generates at its output a corrensponding pulse of RF energy, with a random initial phase. This is not a problem if coherent operation is not needed: if you don’t have to discriminate the phase of echoes, the only thing you need to make this device working is an Automatic Frequency Control (AFC) circuit to ensure that the transmitter and the receiver are working at the same frequency (usually, tuning the STALO – in this case, used only for the downconversion in the receiver – frequency). Things changes if the radar has to exploit the echo phase information, as in MTI radars. In this case, you may either: – Force the magnetron to start oscillating
