| RF Radio Frequency Signal Generator |
RF Radio Frequency Signal Generator
- an overview or tutorial of the basics of radio frequency signal
generator or RF signal generator, the way radio frequency signal generators
can be used, and the important parameters in RF signal generators and their
specifications.
Radio frequency signal generators (RF signal generators)
are a particularly useful item of test equipment widely used in RF design
and test applications. These RF signal generators come in a variety of forms
and with a host of specifications. Accordingly it is necessary to decide
whether the specifications of the RF signal generator meet the requirements
for the required test equipment or not. If the RF signal generator is
over-specified, then this can result in a considerable increase in cost.
Types of RF signal generator
It is possible to design radio frequency signal
generators in a variety of ways. Also with developments that have been made
in electronics circuitry over the years, different techniques have evolved.
It can be said that there are two forms of signal generator that can be
sued:
- Free running RF signal generators: These signal
generators are rarely used these days as their frequency tends to drift.
However they do have the advantage that the signal produced is very
clean and does not have the level of noise (phase noise) either side of
the main signal that is present on some other radio frequency signal
generators.
- Synthesized radio frequency signal generators:
Virtually all radio frequency signal generators used today employ
frequency synthesizers. Using this technique enables frequencies to be
entered directly from a keypad, or via remote control and it also
enables the output signal to be determined very accurately. The accuracy
being dependent upon either an internal reference oscillator that can
have a very high degree of accuracy, or the signal can be locked to an
external frequency reference which can be exceedingly accurate.
Radio frequency signal generator specifications
This is a number of important specifications associated
with any RF signal generator. While some are fairly common to all
applications, it is necessary to ensure that all the requirements for the
signal generator are captured and noted. However a list of some of the more
common RF signal generator parameters are listed below:
- Frequency range: One of the obvious key
specifications for any radio frequency signal generator is the frequency
range that it covers.
- Harmonics and spurious signals: All signal
generators produce some level of spurious signals. Harmonics are
generally much higher as considerable effort is spent in reducing
intermodulation and other non-harmonically related spurious signals.
- Power output: The output from most radio frequency
signal generators is of the order of between 10 and 100 milliwats. This
is generally measured in dBm (i.e. dB relative to 1 mW)
- Power accuracy - relative and absolute: The output
of a radio frequency signal generator generally consists of an
attenuator. Prior to this an amplifier with a feedback loop is used to
maintain an accurate fixed level. The accuracy of the attenuator then
provides the relative accuracy of the individual steps. However it is
also necessary to ensure that the absolute accuracy of output is
maintained.
- Phase noise: One items that has to be noted on many
RF signal generators these days is the level of phase noise that is
produced. When making general noise measurements of a system, the phase
noise of a signal generator used may affect the measurements.
Accordingly it is necessary to know what can be tolerated.
The level of phase noise from a radio frequency signal generator will
generally fall as the offset from the carrier increases. The actual
levels may be given at several points in a specification, and sometimes
a plot of the phase noise may be given.
Phase noise levels are measured in terms of dBc / Hz. This is the level
of noise in a 1 Hertz bandwidth relative to the level of the carrier. As
noise is not on a single frequency but distributed over the frequency
range, the wider the measurement bandwidth, the more noise is seen.
Accordingly it is necessary to specify a bandwidth and 1 Hertz is taken
as the standard.
- Accuracy - short and long term: The accuracy of an
RF signal generator is often important. With most RF signal generators
using frequency synthesizers, this means that the frequency accuracy is
determined by the frequency standard used within the generator.
Frequency standards have their accuracy defined with a number of
different specifications and these have to be combined in the correct
manner to give the overall "accuracy". All accuracy measurements are
specified in terms of parts per million (PPM). However there are
elements including temperature stability, line voltage stability, ageing
(i.e. the steady drift with time over many months of the crystal within
the reference standard, etc. These need to be added statistically to
gain the overall "accuracy" for the radio frequency signal generator.
- Modulation formats supported: Originally many RF
signal generators had the capability to have amplitude modulation, AM ,
and frequency modulation, FM applied. However a large number of
modulation formats including various forms of phase shift keying, PSK
(including BPSK, QPSK, 8PSK, etc) as well as other more complicated
modulation formats including quadrature amplitude modulation, QAM
(including 16 and 64 point QAM) need to be used. It is necessary to
ensure that the radio frequency signal generator being considered is
able to offer the required modulation formats
Before purchasing an RF signal generator, it is necessary
to have a good idea of the specification required. High performance radio
frequency signal generators can be very costly and therefore it is necessary
not to set the specification too high otherwise costs will rise>
Nevertheless the specification should not be set too low otherwise it will
not be able to fulfill its requirements and again money will be wasted.
Summary
Radio frequency signal generators are a form of
electronics test equipment found in virtually every radio frequency design
or test laboratory. These signal generators are used wherever an RF signal
needs to be supplied to a circuit or unit that is being developed or tested.
As such RF signal generators are essential items for RF development and
testing.
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