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It is often necessary to understand the specifications for a spectrum analyzer.
Not only is this useful when buying an analyzer, but it also helps to understand its limitations when using an analyzer as well.
There are several key spectrum analyzer specifications that are useful to understand, each one having its own bearing on performance.
Spectrum analyzer specifications summary
There are very many different spectrum analyzer specifications. For any given application, some will be more important than others.
When selecting or using a given piece of test equipment it is very useful to have a good understanding of which one to choose, but also to understand its limitations and thereby operate it more effectively.
Spectrum analyzer frequency coverage
Possibly one of the most important headline specifications for a spectrum analyzer is its frequency coverage.
Typically a spectrum analyzer will be able to measure from very close to zero Hertz right up to its top frequency.
Normally the bottom frequency limit is not an issue for most applications as RF spectrum analyzers are normally used for frequencies well into the RF spectrum and seldom for much below 100kHz. But it is always worth checking.
The main parameter required for the frequency coverage specification is the top limit. This should obviously include at least the fundamental of the signals of interest, but remember that spectrum analyzers are often required to measure spurious signals like intermodulation distortion and harmonics. Often it is advisable for the spectrum analyzer specification for frequency to reach at least the third harmonic.
This may increase the costs and therefore a balance between the specification / performance and the cost may need to be made.
Frequency accuracy specification
The frequency accuracy is an important specification for any analyzer.
Often it is called the Frequency Readout Accuracy, and it is made up from errors from a number of sources:
- Frequency reference inaccuracy: This error is determined primarily by the internal timebase oscillator within the analyzer. Today virtually all spectrum analyzers use a high performance crystal oven oscillator so this term is normally quite small. Also the internal architecture of the analyzer will also have a bearing on this term. However, when using a spectrum analyzer for any frequency measurements, it is worth remembering that the oven does take time to warm up and settle, so any measurements should only be taken once the analyse has settled. Full details for this will be given in the spectrum analyzer specification sheet.
- Span error: On older analyzers that may not have used digital techniques, a span error was also a key issue. This error was often split into two specs, based on the fact that many spectrum analyzers were fully synthesized for small spans, but are open-loop tuned for larger spans. Check out the operation of the analyzer, but for most modern ones this is not applicable
- Centre frequency error : Again, this form of error specification was applicable to older analyzers. In most cases it was much smaller than the span error.
Amplitude accuracy specification
The spectrum analyzer specification for amplitude accuracy is of great importance for any measurements made by the test instrument.
There are two analyzer specifications associated with amplitude accuracy:
- Absolute accuracy specification: This spectrum analyzer specification refers to measurements where the absolute level is required. It may be a measurement of the power level of a signal expressed in terms of dBm, etc.
- Relative accuracy specification: The relative accuracy specification is slightly different. This specification is used when signals are expressed in terms of decibels when compared to another signal. For example a harmonic may be expressed in terms of decibels down on the carrier. These measurements are generally more accurate than the absolute measurements because the accuracy of the whole signal chain is
Resolution bandwidth specification
The resolution bandwidth specification for a spectrum analyzer is important when it is necessary to measure signals that are close together.
The resolution bandwidth is chiefly determined by the bandwidth of the filter used within the analyzer, but other factors like filter type, residual FM, and noise sidebands are factors to take into consideration when determining he useful resolution available.
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