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 analog radar exciter and the up-converting of frequencies for use in transmitter and receiver.

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 analog radar exciter and the up-converting of frequencies for use in transmitter and receiver.

Master
Oscillator

Waveform
Generator

Timing and
control

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 analog 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 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.

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.