The “Coho”
The COHO (COHerent Oscillator) is a very stable continuous wave oscillator locked to the nominal IF frequency and constitutes the internal phase reference for a moving target indication in a pseudo-coherent radar.
Figure 1: View of the phase detector output signals
A directional coupler provides a sample of the magnetron output on every pulse. This down-converted signal locks the actual phase of the COHO to the same phase of the magnetron. The phase synchronization of the COHO by means of a sample of the magnetron output is mandatory because there is no phase correlation between two successive RF pulses of the magnetron. The COHO takes over the phase of the magnetron and provides it to the receiver part of the system. This is the reason why the pseudo-coherent radar is also-called “coherent on receive”.
The COHO provides the coherent reference signal to the phase sensitive detector where the intermediate frequency is converted to base band i.e. the intermediate frequency carrier is removed. The figure clearly shows the difference between fixed and moving target echoes following phase-sensitive detection. A fixed target produces the same output on every pulse. The amplitude may be positive or negative depending on the actual phase shift but the pulse is stable in every case. A moving target causes an output that fluctuates at Doppler frequency when viewed on an oscilloscope.