www.radartutorial.eu Radar Basics

Exciter

A radar exciter provide coherent frequency and timing relationships performed by (an older) analog synthesizer or (a newer) direct digital synthesis (DDS), capable of creating high-resolution wideband waveforms for radar systems. The exciter provides fully coherent receiver local oscillator signals at radar frequency band as well as requisite, auxiliary high frequency clock signals. The exciter function is divided into an internal frequency synthesizer and an upconverter.

Master
Oszillator
Waveform
Generator
Timing and
control
Exciter
to receiver (CW)
to transmitter
(pulsed)
TP 1
TP 2
TP 3
TP 4
Masterclock
Start Waveform
Pulse Width Gate
Waveform (IF)
60 MHz
180 MHz
540 MHz
1260 MHz
2700 MHz
120 MHz
360 MHz
720 MHz
1440 MHz
75 MHz
2700 MHz
2775 MHz

Figure 1: Diagram of an analogue radar exciter and the up-converting of frequencies for use in transmitter and receiver.

TP1:

TP2:

TP3:

TP4:

Master
Oscillator
Waveform
Generator
Timing and
control
Exciter
to receiver (CW)
to transmitter
(pulsed)
TP 1
TP 2
TP 3
TP 4
Masterclock
Start Waveform
Pulse Width Gate
Waveform (IF)
60 MHz
180 MHz
540 MHz
1260 MHz
2700 MHz
120 MHz
360 MHz
720 MHz
1440 MHz
75 MHz
2700 MHz
2775 MHz

Figure 1: Diagram of an analogue radar exciter and the up-converting of frequencies for use in transmitter and receiver.

M1 F1 M2 F2 M3 F3 M4 F4 Synth FD 1 FD 2 FD 3 FD 4 M5 LD 1 LD 1 TP4 TP5 TP6 TP7
Exiter
to receiver (CW)
to transmitter
(pulsed)
TP 1
TP 2
TP 3
TP 4
Masterclock
Start Waveform
Pulse Width Gate
Waveform (IF)
60 MHz
180 MHz
540 MHz
1260 MHz
2700 MHz
120 MHz
360 MHz
720 MHz
1440 MHz
75 MHz
2700 MHz
2775 MHz

Figure 1: Diagram of an analogue radar exciter and the up-converting of frequencies for use in transmitter and receiver.
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The low phase noise frequency synthesizer provides the different basic frequencies to achieve frequency agile capabilities. In the Figure the frequency synthesizer is adumbrated as a simple switch between 60 and 120 MHz. In reality this is a very complex subblock. Recent radar systems use even phase lock loop (PLL) architecture.

Exciter and receiver of ATC-radar ASR-E
(click to enlarge: 600·530px = 75 kByte)
Figure 2: Exciter and receiver of ATC-radar ASR-E (Manufacturer: EADS)

The upconverter produces the system’s transmit carriers and the local oscillator frequencies for receivers downconverter and detectors, to convert the received RF-backscatter into wideband first IF and narrowband second IF. System functions included in both synthesizer and upconverter include; switches, limiters, variable attenuators and filtering to reduce spurious signals.

Output signal spectral performance is achieved by use of quartz crystals in the master oscillator and frequency synthesis techniques as cascaded stages of narrowband resonator filters, regenerative frequency division, and low noise, diode doublers and step recovery diode frequency multiplication.

The sub-assemblies frequency synthesizer, upward mixer, waveform generator and master oscillator are integrated under the denomination Exciter or Signal Generator in some radar sets. This radar assembly is combined with the receiver in a single rack often, as shown in figure 2.

Description of the modules in the block diagram