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.

This RF signal generator tutorial is split into several pages each of which addresses different aspects of RF signal generator technology and operation:

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

There are 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.

These specifications are detailed more fully in our page on RF signal generator specifications.

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.

Further pages from this tutorial
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