All electronic circuits and equipment receive input signals
and process them into new and different output signals.
When you’re designing and testing circuits and equipment,
where do you get those input signals? You could build your own
signal source for a specific application, but that isn’t necessary.
That’s because there’s a signal generator available for any
type of signal, no matter what type of equipment is being
designed or under test. The signal generator is like the scope,
multimeter, and power supplies on your bench. With analog
and digital models alike, it’s an essential instrument that saves
time and ensures that your product works properly (Fig. 1).
FUNCTION GENERATORS
A basic function generator produces sine, square, and triangular
waves from about 0.2 Hz up to 20 MHz or so. Some units
offer linear ramps and positive and negative pulses. They’re
used for basic audio, ultrasonic frequencies, and low RF testing.
Pulse outputs are TTL/CMOS levels, while linear outputs are
variable up to about ±20 V p-p.
Low-cost generators are implemented with analog circuits
that feature continuously variable frequency and output
voltage. While some low-cost analog function generators are
still available, most modern function
generators today use digital signalgeneration
methods and frequencysynthesis
techniques.
In fact, a majority of engineers prefer
one of the digital models. These
are more commonly known as arbitrary
function generators (AFGs)
or arbitrary waveform generators
(AWGs), both generically referred to
as ARBs (Fig. 2).
The AFG, the simpler of the two,
is set up to produce only the most
commonly used signals, such as sine,
triangle, sawtooth, or square waves.
Meanwhile, the AWG can be set up
to produce virtually any type of signal.
Most AFGs employ direct digital
synthesis (DDS) along with a waveform
storage memory containing
standard waveforms and DAC output
(see “DDS Basics” at www.electronicdesign.com, Drill Deeper 19147).
The output signal is stored in a RAM or ROM as a sequence
of binary samples of the desired waveform. This data is output
to the digital-to-analog converter (DAC) that generates a
stepped approximation of the desired output signal. Some
AFGs can produce sine and other waves up to 300 MHz.
An AFG has all of its standard waveforms, which are selected
via the front-panel control, pre-stored in the memory. Standard
waveforms are also available with an AWG, but users can enter
any desired waveform into RAM. External software is used to
create the binary file that defines the desired waveform.
A frequency synthesizer provides an incremental address to
the RAM, which delivers the waveform samples to the DAC.
Also, an analog low-pass filter eliminates residual digital artifacts.
An output level control sets the desired amplitude.
Some function generators can also supply basic modulation.
These include amplitude modulation (AM), amplitude shift
keying (ASK), on-off keying (OOK), frequency modulation
(FM), frequency shift keying (FSK), phase modulation (PM),
phase-shift keying (PSK), and some digital modulation types.
Examples include Tektronix’s AWG 5000, which uses a
standard fractional-N phase-locked-loop (PLL) synthesizer
(Fig. 3). It also features two channels of output that can be single-ended or differential.
With a DAC sampling rate to
1.2 Gsamples/s, it can produce
output waveforms with a maximum
frequency of 600 MHz.
Because of its high frequency
capability, it can be used for RF
testing in some applications.
Its key specification is
dynamic range, which is determined
by the 14-bit DAC resolution.
Maximum waveform
storage capability is 32 Msamples.
The two outputs are set
up so that the I and Q signals
are available simultaneously for
digital modulation tests.
The AWG 5000 brings flexibility due
to its wide frequency range and waveform
programmability. It can perform virtually
any form of digital modulation, and it’s
well-suited for testing DACs and analogto-
digital converters (ADCs) thanks to
excellent bit resolution. For DAC testing,
the 14-bit parallel digital words that are
output from the waveform memory to the
internal DACs are available as outputs.
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