Centrad GF 266, Instruction Manual

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4000 4 319 _EN - 02/09
ENGLISH
1-4 TUTORIAL
This unit uses the latest signal synthesis technology.
Combining Direct Digital Synthesis (DDS) with high-quality components, the GF 266 generator is capable of generating
all major waveforms (sine, square, triangle, positive ramp, and negative ramp) over a wide range of frequencies and
with very high accuracy and excellent stability.
This process allows frequency to be changed almost instantaneously, with no loss of phase continuity, which is most
advantageous in some modulation schemes, such as Frequency Shift Keying (FSK) or frequency sweep (linear or
logarithmic sweep).
In addition, this phase continuity offers defect-free Phase Shift Keying (PSK).
Unlike in an analogue generator, frequency sweep is not limited to a particular range and may extend from 0.186 Hz
to the maximum frequency.
Before going into details about the principle of operation, we will introduce beginners in electronics to the basics of
analogue and digital signals.
Specialists can directly go to "Detailed theory of DDS".
CHARACTERISTICS OF AN ANALOGUE SIGNAL:
An analogue signal is continuously variable. In theory, between any two points of its amplitude-time characteristic
curve, it has an infinity of possible values. In practice, the signal is always spoilt by noise, which reduces the number
of usable levels.
The noise level must be as low as possible with reference to the useful signal. This is called the signal-to-noise ratio
( SNR ). A high SNR gives a signal of good quality. A useful signal with an amplitude close to the noise level is not
usable; it is said that the signal is "buried in noise".
Drawbacks of analogue signal synthesis:
To generate a periodic signal, a special circuit – an "oscillator" – is required.
There are multiple forms of analogue oscillators, each with their qualities and shortcomings.
To generate the main waveforms, there is no oscillator possessing all required qualities: low distortion, good frequency
and amplitude stability, and ability to quickly and substantially change frequency (for some modulation schemes).
The implementation of an oscillator is always a trade-off between various parameters.
Digitization principles:
Digitization (also referred to as analogue-to-digital conversion) consists of converting a continuously variable quantity
into a sequence of discrete codes representing the instantaneous value at conversion time.
Mathematically, digitizing a signal is sampling at regular intervals the real number representing its magnitude and
quantizing it to the nearest integer (Fig. 1).
These sequences of numbers are then represented as binary values (0 or 1), which can be manipulated by the computer
and associated tools.
The immediate advantage of this process is that the digital signal is insensitive to noise and distortion, since two
electrical states - 0 or 1 (open or closed) - are sufficient to correctly transmit the information.
The elementary digital information (0 or 1) is called a bit.
The number of bits used for quantizing the signal determines the resolution, i.e. the number of possible amplitude
values:
Coding over 8 bits = 2 to the power of 8 = 256 possibles values
Coding over 10 bits = 2 to the power of 10 = 1 024 possibles values
Coding over 16 bits = 2 to the power of 16 = 65 536 possibles values
Coding over 20 bits = 2 to the power of 20 = 1 048 576 possibles values
Coding over 24 bits = 2 to the power of 24 = 16 777 216 possibles values
fig. 1