7 Non-coherent and Pseudo-coherent Radar Systems
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oscillator is phased again through the phasing mixer by the transmitter, the pulses of
which are incoherent with respect to each other.
The duration of the generation time of the coherent local oscillator exceeds the
delay time t R of reflected pulse arriving from the maximum range. Before arrival of
the next phasing pulse, the coherent local oscillator is locked by a special control
circuit and is unlocked a little later than the moment of phasing pulse arriving. In
this case, the phasing pulse reliably imposes its phase and frequency on the coherent
local oscillator.
By providing a phasing of coherent local oscillator at intermediate frequency,
the stable local oscillator simultaneously serves as a conventional local oscillator
of superheterodyne receiver. However, the continuous oscillations generated by it
are characterized by increased frequency stability in order to ensure a constant phase
shift between the voltage of coherent local oscillator (5) and receiving signal (4). The
receiving signal of intermediate frequency and reference voltage (5) is delivered to
a phase detector that multiplies these voltages. After filtering out high combination
frequencies (by low-pass filter at output of phase detector), an output video signal
represents a sequence of pulses that are modulated in amplitude (6) by Doppler
frequencies, determined by velocities of targets movement, since the delay time in
adjacent probing periods is different (t R1 and t R2 ), and accordingly, the phase shift
between signal of coherent local oscillator and the reflected signal has a different
meaning.
In the overwhelming majority of cases, coherent equipment is used for moving
target discrimination by suppressing (compensating) of reflections from stationary
(e.g., ground) or low-moving (e.g., passive radar chaffs/dipoles) objects, or solving
the problem of fast discrimination, detecting the type and degree of moveability of
reflected objects (“wind shear,” turbulence).
An automatic phase-locked-loop frequency control (PLL) loop can be used as a
device that implements the function of memorizing and storing the value of initial
phase of sounding pulse during operation period of the radar. The block diagram of
a pseudo-coherent transmitter using a PLL is shown in Fig. 7.5.
From synchronizer output, the pulses are delivered to input of pseudo-coherent
transmitting unit providing a formation of frequency repetition via subsequent division into FrD and further into M for ensuring of pulse generation of specified shape
and duration. Further, a signal comes to PA where the main amplification is provided
up to a value ensuring a magnetron operation. Magnetron is a microwave generator
(oscillator) operating in intercrossed by magnet electrical fields, capable to shape
signals of high pulsed power.
Shaped sounding signals through a circulator come to antenna unit and are radiated
into an air space. Since the formed oscillations are non-coherent, it is possible to
provide coherent processing via registration of initial phase value at one repetition
period. This is possible by shifting into intermediate frequency in mixer and signal
delivering to a unit, providing this value storage, representing a phase-locked-loop
frequency control. Circuit includes VCO, which is tuned by a signal in such way that
it corresponds to a signal on loop input. PD allocates a signal of phase difference
of sounding signal and VCO. Integrator smooths random HF oscillations induced
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oscillator is phased again through the phasing mixer by the transmitter, the pulses of
which are incoherent with respect to each other.
The duration of the generation time of the coherent local oscillator exceeds the
delay time t R of reflected pulse arriving from the maximum range. Before arrival of
the next phasing pulse, the coherent local oscillator is locked by a special control
circuit and is unlocked a little later than the moment of phasing pulse arriving. In
this case, the phasing pulse reliably imposes its phase and frequency on the coherent
local oscillator.
By providing a phasing of coherent local oscillator at intermediate frequency,
the stable local oscillator simultaneously serves as a conventional local oscillator
of superheterodyne receiver. However, the continuous oscillations generated by it
are characterized by increased frequency stability in order to ensure a constant phase
shift between the voltage of coherent local oscillator (5) and receiving signal (4). The
receiving signal of intermediate frequency and reference voltage (5) is delivered to
a phase detector that multiplies these voltages. After filtering out high combination
frequencies (by low-pass filter at output of phase detector), an output video signal
represents a sequence of pulses that are modulated in amplitude (6) by Doppler
frequencies, determined by velocities of targets movement, since the delay time in
adjacent probing periods is different (t R1 and t R2 ), and accordingly, the phase shift
between signal of coherent local oscillator and the reflected signal has a different
meaning.
In the overwhelming majority of cases, coherent equipment is used for moving
target discrimination by suppressing (compensating) of reflections from stationary
(e.g., ground) or low-moving (e.g., passive radar chaffs/dipoles) objects, or solving
the problem of fast discrimination, detecting the type and degree of moveability of
reflected objects (“wind shear,” turbulence).
An automatic phase-locked-loop frequency control (PLL) loop can be used as a
device that implements the function of memorizing and storing the value of initial
phase of sounding pulse during operation period of the radar. The block diagram of
a pseudo-coherent transmitter using a PLL is shown in Fig. 7.5.
From synchronizer output, the pulses are delivered to input of pseudo-coherent
transmitting unit providing a formation of frequency repetition via subsequent division into FrD and further into M for ensuring of pulse generation of specified shape
and duration. Further, a signal comes to PA where the main amplification is provided
up to a value ensuring a magnetron operation. Magnetron is a microwave generator
(oscillator) operating in intercrossed by magnet electrical fields, capable to shape
signals of high pulsed power.
Shaped sounding signals through a circulator come to antenna unit and are radiated
into an air space. Since the formed oscillations are non-coherent, it is possible to
provide coherent processing via registration of initial phase value at one repetition
period. This is possible by shifting into intermediate frequency in mixer and signal
delivering to a unit, providing this value storage, representing a phase-locked-loop
frequency control. Circuit includes VCO, which is tuned by a signal in such way that
it corresponds to a signal on loop input. PD allocates a signal of phase difference
of sounding signal and VCO. Integrator smooths random HF oscillations induced