www.radartutorial.eu Radar Basics

Waveform-Generator

A waveform generator generates the transmitting signal on an IF- frequency. It permits generating predefined waveforms by driving the amplitudes and phase shifts of carried microwave signals. These signals may have a complex structure for a pulse compression. Since these signals are used as a reference for the receiver channels too, there are high requirements for the frequency stability.

Addresscounter
Memory
D/A Converter
Mixer
set
reset
carry
FF
11-bit
Counter
sine
PROM
cosine
PROM
D
A
D
A
HY
IF-Waveform

Figure 1: an e.g. Block diagram of a waveform generator for a non-linear compressed pulse

Addresscounter
Memory
D/A Converter
Mixer
Clock 25 MHz
Waveform Start
set
reset
carry
FF
11-bit
Counter
sine
PROM
cosine
PROM
IF
IF-Waveform
D
A
D
A
HY

Figure 1: an e.g. Block diagram of a waveform generator for a non-linear compressed pulse

Addresscounter
Memory
D/A Converter
Mixer
set
reset
carry
FF
11-bit
Counter
sine
PROM
cosine
PROM
IF-Waveform
D
A
D
A
HY

Figure 1: an e.g. Block diagram of a waveform generator for a non-linear compressed pulse 1) (interactive picture)

Memory-based Arbitrary Waveform Generator (AWG)

The desired waveform may be described by a mathematical function, and each discrete value of the function is stored as a digital word in a memory. The memory will be counted using the system clock and provides the values continuously at the output. There, they are converted to an analogue voltage. The individual values of time in quick succession give the synthesized waveform.

The finally waveform is constructed of e.g. 2048 discrete voltage steps here. Its values of amplitude and phase are stored in programmable memories (PROMs). A change of the waveform is only possible by replacing the PROMs by the manufacturer.

In the by the Radartutorial self-developed Didactical Multifunction Radar are used instead of two PROM fast static RAM with a capacity of 64 Kbytes each with 16 bits. Provided by a laptop via USB, an ATMEL processor can load these memories with the needed values. Thus, it is possible to generate any waveform, ranging from ultra-short pulses for classical pulse radar, via intrapulse modulation (with subsequent pulse compression) to all forms of modulation of frequency modulated continuous wave radar (FMCW radar).

The processing of an I & Q- phase- detector is arranged reverse virtually. This method of design the transmitting pulses gives the advantage, that the waveform is digitally described for a computer-controlled signal processing. A digital processor can execute the pulse compression now.

1) A very (unusual) low intermediate frequency of 470 kilohertzes was choosen here to visualize the high-frequencies in the schreen-shoots of the oscilloscope.

Description of the modules in the block diagram