Understanding RF signal generator specifications

- a tutorial, information and overview about the basics of RF signal generator specifications of specs - what they mean and how to interpret them for buying a new or used signal generator or obtaining one from a test equipment rental company.

When choosing the correct RF signal generator to purchase, either new or as a used signal generator or possibly to hire from a test equipment rental company is not always easy. A knowledge of RF signal generator specifications is needed to be able to make the right choice for any given application.

RF signal generators are an essential item of test equipment for any RF design or test laboratory. They enable RF signals to be generated that enable signals to be fed into RF circuits so that their operation can be viewed when operating under various signal conditions.

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. When choosing the band required for a signal generator it is necessary to consider all the testing that will be needed. The frequency coverage required for the signal generator may not be just that of the unit under test. For example, when testing radio receivers it is necessary to test their susceptibility to out of band signals at image and other frequencies. These may be will outside the operating frequency range of the unit under test, and the signal generator will need to accommodate these and any other requirements.


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.


Signal generator power output

Another important signal generator specification is its power output. For most RF signal generators, the power output specification is defined in dBm, i.e. dB relative to one milliwatt.


Power level
dBm
Power level
milliwatts
1 1
3 2
10 10
13 20
2 100
23 200

dBm / milliwatts conversion

Although different signal generators have different output levels, the most common maximum output level is +13 dBm, although whatever the exact maximum level is, it is normally in the range 10 to 100 milliwatts, i.e. 10 to 20 dBm.


Power accuracy - relative and absolute:

For many test scenarios it is necessary for the output of the signal generator to be accurately known. This is because the response of the unit under test is most likely to vary according to the signal generator level. In some circumstances it is likely that the response may be very sensitive to the signal generator level. As a result the signal generator level accuracy specification is of great importance.

There are two elements to the output level accuracy of the signal generator. This is as a result of the way that the output level is controlled. The output of a signal generator generally consists of an attenuator and this gives the ability to vary the output level. Prior to the output attenuator in the signal generator, there is an amplifier with a feedback loop which is used to maintain an accurate fixed level. The accuracy of the attenuator then provides the relative accuracy of the individual steps while the maintained level of the amplifier provides the absolute level accuracy.


Phase noise

One items that has to be noted on many signal generators these days is the level of phase noise that is produced. The importance arises because many signal generators are fall into the category of a synthesized signal generator. While a synthesized signal generator offers many advantages from exact frequency selection to stability, and high levels of programmability, the issue of phase noise can be a problem in some generators, and the phase noise spec needs to be carefully considered.

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 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

In order that many tests can be undertaken by the signal generator, it is necessary in many instances that the signal can be modulated. In this way real signals can be more fully simulated and the required tests undertaken. Most signal generators have the ability to modulate signals in a variety of ways, some providing greater levels of flexibility than others. As a result it is necessary to check the signal generator specifications to ensure that it has the required capabilities.

Originally many signal generators had the capability to have amplitude modulation, AM , and frequency modulation, FM applied. However with radio and wireless systems using far more advanced forms of modulation, many signal generators have very comprehensive modulation capabilities. Some of these may be provided by the use of additional options. Today a variety of modulation formats may be available in a signal generator. These may include: 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. Other modulation types including CDMA and OFDM may also be available. It is necessary to ensure that the radio frequency signal generator being considered is able to offer the required modulation formats.


Test equipment calibration interval

The calibration interval for any item of test equipment is important, and this is the case for any signal generator, whether it is new or a used signal generator. Test equipment calibration can add a significant amount to the cost of ownership, so it is necessary to consider the calibration interval. For many signal generators the interval will be a year, but if higher degrees of accuracy of any of the signal generator specifications are needed, then shorter time intervals may be needed.


Summary

Before purchasing an RF signal generator, it is necessary to have a good idea of the signal generator specifications and the actual spec required. A high performance RF signal generators can be very costly (even as a used signal generator) and therefore it is necessary not to set the specification too high otherwise costs will rise. Nevertheless the signal generator specification should not be set too low otherwise it will not be able to fulfil its requirements and again money will be wasted.

By Ian Poole


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