To distinguish a moving target of a fixed
object with help of the Doppler frequency, at least two periods of the
deflection must be compared with each other.
Since the Doppler- frequency (few Hertz) is small relatively to the
transmitted frequency (much Mega-Hertz), therefore a phase comparison is
more easily to carry out than a direct frequency comparison technically.
The storage of a deflection is carried out in suitable memory media, in the
past in special analogous vacuum memory tubes, later also with a chain of
condensers (distance: digital, signal: analogous) and today only in digital
memory cells.
Figure 1: functional
block circuit diagram of a coherent receiver
Well, a fixed target suppression happens by
the phase comparison of the echoes received by several pulse periods (pulse-
pair processing). If the phase relationship is always equal, then there
isn't any phase difference and the target will be suppressed. If the target
has moved, the phase difference is unequally zero and the target will be
shown on the screen.
To get the necessary frequency-reference for the phase-detector, a high
correct coherent oscillator (called: �Coho�) is synchronized with the down
converted on the IF- frequency transmitting pulse.
| The echo signal of a moving target at the output of the
phase-detector changes it's value and also the polarity in every
pulse period. A fixed cluttersignal will keep it's value and
polarity in every pulse period.
A pulse period is stored in a
memory and than it is subtracted from the following period. On this
way the moving target produce an output signal and the fixed clutter
don't do this. |
|
Figure 2:Oscillogram of an output signal of a phase-detector |
