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The function
generator -
is one of the most versatile pieces of test &
measurement instrumentation available.
It can generate a variety
of precision wave shapes over a range of frequencies from mHz to MHz.
It
can provide a wide range of controlled amplitudes from a low-impedance
source, and maintain constant amplitude as the frequency is varied.
Voltage control of frequency enables a source of swept frequency to be
generated for frequency response testing. AM and FM modulation
facilities can also be utilised. |
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Analogue or digital - which to choose ?
Function generators fall into two basic categories, analogue and
digital. Analogue generators use a voltage controlled oscillator (VCO)
to generate a triangular waveform of variable frequency. Sinusoidal
waveforms and square waves are generated from this.
Digital generators use a digital to analogue converter (DAC) to generate
a wave shape from values stored in memory. Normally such generators only
offer sine and square waves up to the maximum generator frequency.
Triangle waves and other waveforms are limited to a much lower
frequency. A third type of generator uses digital
techniques to control an analogue VCO. TTi no longer manufactures a
fully digitally controlled analogue generator. However, a simplified
form of digital frequency control, known as Frequency Locking, is
employed in the TG550. |
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Advantages of
analogue generators
Analogue function generators offer several advantages:
1. They provide simple and instantaneous control of frequency and
amplitude.
2. They do not have the high frequency limitations on non-sinusoidal
waveforms such as triangles and ramps that digital generators do.
3. The starting price for an analogue generator is considerably lower
than for a digital generator.
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The TG120, TG210 and TG300 are
pure analogue generators. The TG550 is an
analogue generator with digital frequency locking.
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Advantages of digital
generators Digital generators normally derive the waveform frequency from a crystal
clock using a digital technique. Consequently the frequency accuracy and
stability will usually be higher than can be obtained from an analogue
generator. Digital generators may be able to generate a much greater
number of standard waveforms than analogue generators.
A variety of techniques may be used of which the most versatile is
direct digital synthesis (D.D.S.).
DDS uses a phase accumulator, a look-up table and a DAC. It offers not
only exceptional accuracy and stability but also high spectral purity,
low phase noise and excellent frequency agility.
A DDS generator can be swept over a much wider frequency range than an
analogue generator and can perform phase continuous frequency hopping.
The TG2000, TG1010A and TG4001 are
all DDS based based digital function generators. |
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Disadvantages of
digital generators
The maximum frequency for triangles and other non-sinusoidal waveforms
is limited to a small fraction of the upper frequency for sinewaves.
This is related to the maximum clock rate combined with the filter
characteristics. Rectangular waveforms can be generated from the sinewave using analogue
comparators and can therefore avoid this restriction, but performance
limits will apply to pulse waveforms.
Digital generators are more
complex to use. This can be a drawback in simple or traditional test
environments.
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Arbitrary waveforms
Digital function generators have the potential for generating arbitrary
waveforms.
However, the architecture of a DDS based function generator differs
from that of a true arbitrary generator with consequent limitations to
the arbitrary capabilities.
Nevertheless, DDS function generators with
an arbitrary capability can generate non-standard and custom waveforms
which may be sufficient for many applications.
Universal Waveform Generators
such as the TGA1240 and TGA12100 series combine a DDS function generator
with a true variable clock arbitrary generator.
The TG4001 also uses this architecture, although it
has less sophisticated arbitrary capabilities. |
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The TG1010A is a DDS function
generator with DDS based arbitrary waveform capability.
To learn about the differences
between DDS based an variable-clock based arbitrary generation, use this
link: 
Arbitrary Waveform
Generation Techniques.
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