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Spectrum Analyzer Tutorial and Basics

Spectrum analyzers are widely used within the electronics industry for analysing the frequency spectrum of radio frequency, RF and audio signals. Looking at the spectrum of a signal they are able to reveal elements of the signal, and the performance of the circuit producing them that would not be possible using other means.

Spectrum analysers are able to make a large variety of measurements and this means that they are an invaluable tool for the RF design development and test laboratories, as well as having many applications for specialist field service.

Why spectrum analysis?

The most natural way to look at waveforms is in the time domain - looking at how a signal varies in amplitude as time progresses, i.e. in the time domain. This is what an oscilloscope is used for, and it is quite natural to look at waveforms on an oscilloscope display. However this is not the only way in which signals can be displayed.

A French mathematician and physicist, named Jean Baptiste Joseph Fourier, who lived from 1768 to 1830 also started to look at how signals are seen in another format, in the frequency domain where signals are viewed as a function of their frequency rather than time. He discovered that any waveform seen in the time domain, there is an equivalent representation in the frequency domain. Expressed differently, any signal is made up from a variety of components of different frequencies. One common example is a square waveform. This is made up from signal comprising the fundamental as well as third, fifth, seventh, ... harmonics in the correct proportions.

In exact terms it is necessary that the signal must be evaluated over an infinite time for the transformation to hold exactly. However in reality it is sufficient to know that the waveform is continuous over a period of at least a few seconds, or understand the effects of changing the signal.

It is also worth noting that the mathematical Fourier transformation also accommodates the phase of the signal. However for many testing applications the phase information is not needed and considerably complicates the measurements and test equipment. Also the information is normally not needed, and only the amplitude is important.

By being able to look at signals in the time domain provides many advantages and in particular for RF applications, although audio spectrum analyzers are also widely used. Looking at signals in the frequency domain with a spectrum analyzer enables aspects such as the harmonic and spurious content of a signal to analyzed. Also the width of signals when modulation has been applied is important. These aspects are of particular importance for developing RF signal sources, and especially any form of transmitter including those in cellular, Wi-Fi, and other radio or wireless applications. The radiation of unwanted signals will cause interference to other users of the radio spectrum, and it is therefore very important to ensure any unwanted signals are kept below an acceptable level, and this can be monitored with a spectrum analyzer.

Spectrum analyzer basics

There are many different types of RF test equipment that can be used for measuring a variety of different aspects of an RF signal. It is therefore essential to choose the right type of RF test equipment to meet the measurement requirements for the particular job in hand.

TEST INSTRUMEN

T TYPE

FREQUENCY MEASUREMEN

T

INTENSITY / AMPLITUDE

MEASUREMENT

APPLICATION

Power meter

N Y Use for accurate total power measurements

Frequency counter

Y N Used to provide very accurate

TEST INSTRUMEN

T TYPE

FREQUENCY MEASUREMEN

T

INTENSITY / AMPLITUDE

MEASUREMENT

APPLICATION

measurements of the dominant frequency within a signal

Spectrum analyser

Y Y Used primarily to display the spectrum of a radio frequency signal. Can also be used to make power and frequency measurements, although not as accurately as dedicated instruments

RF network analyser

Y Y Used to measure the properties of RF devices

Properties of RF measuring instruments in common useThe spectrum analyzer is able to offer a different measurement capability to other instruments. Its key factor is that it is able to look at signals in the frequency domain, i.e. showing the spectrum, it is possible to see many new aspects of the signal.A spectrum analyzer display, like that of an oscilloscope has two axes. For the spectrum analyser the vertical axis displays level or amplitude, whereas the

horizontal axis displays frequency. Therefore as the scan moves along the horizontal axis, the display shows the level of any signals at that particular frequency.This means that the spectrum analyser, as the name indicates analyses the spectrum of a signal. It shows the relative levels of signals on different frequencies within the range of the particular sweep or scan.

General format of the display on a spectrum analyzerIn view of the very large variations in signal level that are experienced, the vertical or amplitude axis is normally on a logarithmic scale and is calibrated in dB in line with many other measurements that are made for signal amplitudes. The horizontal scale conversely is normally linear. This can be adjusted to cover the required range. The term span is used to give the complete calibrated range across the screen. Terms like scan width per division may also be used and refer to the coverage between the two major divisions on the screen.

Spectrum Analyzer Types

Just as in the case of other instruments, there are a number of types of spectrum analyzer that can be seen in the manufacturers catalogues.The different types of spectrum analyzer are used in different applications - some are more appropriate for some applications, while others are more appropriate for others.Additionally there are some significant differences in cost between the different types of analyser, making Summary of spectrum analyser types

There are a number of different categories for spectrum analyzers. Some of the main spectrum analyser types are noted below:

Swept or superheterodyne spectrum analysers: The operation of the swept frequency spectrum analyzer is based on the use of the superheterodyne principle, sweeping the frequency that is analysed across the required band to produce a view of the signals with their relative strengths. This may be considered as the more traditional form of spectrum analyser, and it is the type that is most widely used.

Fast Fourier Transform, FFT analysers: These spectrum analyzers use a form of Fourier transform known as a Fast Fourier Transform, FFT, converting the signals into a digital format for analysis digitally. These analysers are obviously more expensive and often more specialised.

Audio spectrum analyzer: Although not using any different basic technology, audio spectrum analyzers are often grouped differently to RF spectrum analyzers. Audio spectrum analyzers are focussed, as the name indicates, on audio frequencies, and this means that low frequency techniques can be adopted. This makes them much cheaper. It is even possible to run them on PCs with a relatively small amount of hardware - sometimes even a sound card may suffice for some less exacting applications.

Spectrum analyser advantages and disadvantages

Both swept / superheterodyne and FFT spectrum analyzer technologies have their own advantages. The more commonly used technology is the swept spectrum analyser as it the type used in a general-purpose analysers enabling these analyzers to operate up to frequencies of many GHz. However a swept frequency analyser is only capable of detecting continuous signals, i.e. CW as time is required to capture a given sweep, and they are not able to capture any phase information.FFT analyzer analyser technology is able to capture a sample very quickly and then analyse it. As a result an FFT analyzer is able to capture short lived, or one-shot phenomena. They are also able to capture phase information. However the disadvantage of the FFT analyzer is that its frequency range is limited by the sampling rate of the analogue to digital converter, ADC. While ADC technology

has improved considerably, this places a major limitation on the bandwidths available using these analyzers.In view of the fact that both FFT and superheterodyne analyzer technologies have their own advantages, many modern analyzers utilise both technologies, the internal software within the unit determining the best combinations for making particular measurements. The superheterodyne circuitry enabling basic measurements and allowing the high frequency capabilities, whereas the FFT capabilities are introduced for narrower band measurements, and those where fast capture is needed. An analyzer will often determine the best method dependent upon factors including the filter settling time and sweep speed. If the spectrum analyser determines it can show the spectrum faster by sampling the required bandwidth, processing the FFT and then displaying the result, it will opt for an FFT approach, otherwise it will use the more traditional fully superheterodyne / sweep approach. The difference between the two measurement techniques as seen by the user is that using a traditional sweep approach, the result will seen as sweep progresses, when an FFT measurement is made, the result cannot be displayed until the FFT processing is complete.These different types of spectrum analyzer technology are described in more detail in further pages of this tutorial.

Superheterodyne, Sweep Spectrum Analyzer

Of the two types of RF spectrum analyzer that are available, namely the swept or superheterodyne spectrum analyzer and the Fast Fourier Transform, FFT spectrum analyzer, it is the swept or sweep spectrum analyzer that is the most widely used.The swept spectrum analyser is the general workhorse RF test equipment of the spectrum analyzer family. It is a widely used item of RF test equipment that is capable of looking at signals in the frequency domain. In this way this form of spectrum analyser is able to reveal signals that are not visible when using other items of test equipment.To enable the most effective use to be made of a sweep spectrum analyzer it is necessary to have a basic understanding of the way in which it works. This will enable many of the pitfalls, including false readings, using an analyzer to be avoided.

Advantages and disadvantages of a swept or sweep spectrum analyzer

The sweep or swept spectrum analyzer has a number of advantages and disadvantages when compared to the main other type of analyzer known as the FFT spectrum analyzer. When choosing which type will be suitable, it is necessary to understand the differences between them and their relative merits. Advantages of the superheterodyne spectrum analyser technology

Able to operate over wide frequency range: Using the superheterodyne principle, this type of spectrum analyzer is able to operate up to very high frequencies - many extend their coverage to many GHz.

Wide bandwidth: Again as a result of the superheterodyne principle this type of spectrum analyzer is able to have very wide scan spans. These may extend to several GHz in one scan.

Not as expensive as other spectrum analyzer technologies: Although spectrum analyzers of all types are expensive, the FFT style ones are more expensive for a similar level of performance as a result of the high performance ADCs in the front end. This means that for the same level of base performance, the superheterodyne or sweep spectrum analyzer is less expensive.

Disadvantages of the superheterodyne spectrum analyzer technology

Cannot measure phase: The superheterodyne or sweep spectrum analyzer is a scalar instrument and unable to measure phase - it can only measure the amplitude of signals on given frequencies.

Cannot measure transient events: FFT analyzer technology is able to sample over a short time and then process this to give the required display. In this way it is able to capture transient events. As the superheterodyne analyzer sweeps the bandwidth required, this takes longer and as a result it is unable to capture transient events effectively.

Balancing the advantages and disadvantages of the swept or superheterodyne spectrum analyzer, it offers excellent performance for the majority of RF test equipment applications. Combining the two technologies in one item of test equipment can enable the advantages of both technologies to be utilised.

Swept or sweep spectrum analyser basics

The swept spectrum analyser uses the same superheterodyne principle used in many radio receivers as the underlying principle on which its operation depends. The superheterodyne principle uses a mixer and a second locally generated local oscillator signal to translate the frequency.The mixing principle used in the spectrum analyzer operates in exactly the same manner as it does for a superheterodyne radio. The signal entering the front end is translated to another frequency, typically lower in frequency. Using a fixed frequency filter in the intermediate frequency section of the equipment enables high performance filters to be used, and the analyzer or receiver input frequency can be changed by altering the frequency of the local oscillator signal entering the mixer.Although the basic concept of the spectrum analyzer is exactly the same as the superheterodyne radio, the particular implementation differs slightly to enable it to perform is function as a spectrum analyzer.

Superheterodyne or swept frequency spectrum analyzer block diagramThe frequency of the local oscillator governs the frequency of the signal that will pass through the intermediate frequency filter. This is swept in frequency so that it covers the required band. The sweep voltage used to control the frequency of the local oscillator also controls the sweep of the scan on the display. In this way the position of the scanned point on the screen relates to the position or frequency of the local oscillator and hence the frequency of the incoming signal. Also any signals passing through the filter are further amplified, detected and

often compressed to create an output on a logarithmic scale and then passed to the display Y axis.

Elements of a superheterodyne spectrum analyzer

Although the basic concept of the sweep spectrum analyser is fairly straightforward a few of the circuit blocks may need a little further explanation.

1. RF attenuator: The first element a signal reaches on entering the spectrum analyser is an RF attenuator. Its purpose is to adjust the level of the signal entering the mixer to its optimum level. If the signal level is too high, not only may the reading fall outside the display, but also the mixer performance may not be optimum. It is possible that the mixer may run outside is specified operating region and additional mix products may be visible and false signals may be seen on the display.

In fact when false signals are suspected, the input attenuator can be adjusted to give additional attenuation, e.g. +10 dB. If the signal level falls by more than this amount then it is likely to be an unwanted mix product and insufficient RF attenuation was included for the input signal level.

The input RF attenuator also serves to provide some protection to very large signals. It is quite possible for very large signals to damage the mixer. As these mixers are very high performance components, they are not cheap to replace. A further element of protection is added. Often the input RF attenuator includes a capacitor and this protects the mixer from any DC that may be present on the line being measured.

2. Low pass filter and pre-selector: This circuit follows the attenuator and is included to remove out of band signals which it prevents from mixing with the local oscillator and generating unwanted responses at the IF. These would appear as signals on the display and as such must be removed.

Microwave spectrum analyzers often replace the low pass filter with a more comprehensive pre-selector. This allows through a band of frequencies, and its response is obviously tailored to the band of interest

3. Mixer: The mixer is naturally key to the success of the analyser. As such the mixers are high performance items and are generally very expensive. They must be able to operate over a very wide range of signals and offer very low levels of spurious responses. Any spurious signals that are generated may mix with incoming signals and result in spurious signals being seen on the display.

Great care must be taken when using a superheterodyne spectrum analyzer not to feed excessive power directly into the mixer otherwise damage can easily occur. This can happen when testing radio transmitters where power levels can be high and accidentally turning the attenuator to a low value setting so that higher power levels reach the mixer. As a result it is often good practice to use an external fixed attenuator that is capable of handling the power. If damage occurs to the mixer it will disable the spectrum analyzer and repairs can be costly in view of the high performance levels of the mixer.

4. IF amplifier: Despite the fact that attenuation is provided at the RF stage, there is also a necessity to be able to alter the gain at the intermediate frequency stages. This is often used and ensures that the IF stages provide the required level of gain. It ahs to be used in conjunction with the RF gain control. Too high a level of IF gain will increase the front end noise level which may result in low level signals being masked. Accordingly the RF gain control should generally be kept as high as possible without overloading the mixer. In this way the noise performance of the overall unit is optimised.

5. IF filter: The IF filters restrict the bandwidth that is viewed, effectively increasing the frequency resolution. However this is at the cost of a slower scan rate. Narrowing the IF bandwidth reduces the noise floor and enables lower level spurious signals to be viewed.

6. Local oscillator: The local oscillator within the spectrum analyzer is naturally a key element in the whole operation of the unit. Its performance governs many of the overall performance parameters of the whole analyser. It must be capable of being tuned over a very wide range of frequencies to enable the analyzer to scan over the required range. It must also have a very good phase noise performance. If the oscillator has a poor phase noise performance then it will not only result in the unit not being able to make narrow band measurements as the close in phase noise on the local oscillator will translate onto the measurements of the signal under test, but it will also prevent it making any meaningful measurements of phase noise itself - a measurement being made increasingly these days.

7. Ramp generator: The ramp generator drives the sweep of the local oscillator and also the display. In this way the horizontal axis of the display is directly linked to the frequency.

8. Level detector: The level detector converts the signal from the IF filter into a signal voltage that can be passed to the display. Normally a logarithmic output is required for the display, although occasionally linear displays may be required. Any conditioning and switching for this will be contained within the level detector and associated display circuitry.

9. Display: In many respects the display is the heart of the test instrument as this is where the spectra are viewed. Originally cathode ray tubes were used, but a variety of more modern types of display are used these days. Additionally significant amounts of signal processing are used in spectrum analysers these days, and this enables far higher degrees of functionality to be introduced into these test instruments.

FFT Spectrum Analyzer

There are two main types of spectrum analyser technology. One is the swept frequency, or superheterodyne spectrum analyzer and the other is the FFT spectrum analyzer technology. Of the two, the FFT analyzer technology is the less commonly used on its own, but it is able to offer some distinct advantages over the more common swept frequency analyser. By combining the two technologies the advantages of each can be utilised to offer extremely high performance items of test equipment.In general, spectrum analyzers are used to provide a view of radio frequency, or in some case audio frequency waveforms in the time domain. With other instruments able to provide views of other aspects of signals, the spectrum analyzer is uniquely placed to offer views of the spectrum of a signal, revealing aspects that other instruments are unable to do. With the FFT analyzer able to provide facilities that cannot be provided by swept frequency analyzers, enabling fast capture and forms of analysis that are not possible with sweep / superheterodyne techniques alone.

Advantages and disadvantages of FFT analyzer technology

FFT spectrum analyzer technology has a number of advantages and disadvantages when compared to the more familiar superheterodyne or swept frequency analyzer. When choosing which technology will be suitable, it is necessary to understand the differences between them and their relative merits. Advantages of FFT spectrum analyzer technology

Fast capture of waveform: In view of the fact that the waveform is analysed digitally, the waveform can be captured in a relatively short time, and then the subsequently analysed. This short capture time can have many advantages.

Able to capture non-repetitive events: The short capture time means that the FFT analyzer can capture non-repetitive waveforms, giving them a capability not possible with other spectrum analyzers.

Able to analyse signal phase: As part of the signal capture process, data is gained which can be processed to reveal the phase of signals.

Disadvantages of the FFT spectrum analyzer technology

Frequency limitations: The main limit of the frequency and bandwidth of FFT spectrum analyzers is the analogue to digital converter, ADC that is used to convert the analogue signal into a digital format. While technology is improving this component still places a major limitation on the upper frequency limits or the bandwidth if a down-conversion stage is used.

Cost: The high level of performance required by the ADC means that this item is a very high cost item. In addition to all the other processing and display circuitry required, this results in the costs rising for these items.

Fast Fourier Transform

At the very heart of the concept of the FFT analyzer is the fast Fourier Transform itself. The fast Fourier Transform, FFT uses the same basic principles as the Fourier transform, developed by Joseph Fourier (1768 - 1830) in which one value in, say, the continuous time domain is converted into the continuous frequency domain, including both magnitude and phase information.However to capture a waveform digitally, this must be achieved using discrete values, both in terms of the values of samples taken, and the time intervals at which they are taken. As the time domain waveform is taken at time intervals, it is not possible for the data to be converted into the frequency domain using the standard Fourier transform. Instead a variant of the Fourier transform known as the Discrete Fourier Transform, DFT must be used.As the DFT uses discrete samples for the time domain waveform, this reflects into the frequency domain and results in the frequency domain being split into discrete frequency components of "bins." The number of frequency bins over a frequency band is the frequency resolution. To achieve greater resolution, a greater number of bins is needed, and hence in the time domain a large number of samples is required. As can be imagined, this results in a much greater level of computation, and therefore methods of reducing the amount of computation required is needed to ensure that the results are displayed in a timely fashion, although with today's vastly increased level of processing power, this is less of a problem. To ease the processing required, a Fast Fourier Transform, FFT is used. This requires that the time domain waveform has a the number of samples equal to a number which is an integral power of two.

FFT spectrum analyzer

The block diagram and topology of an FFT spectrum analyzer are different to that of the more usual superheterodyne or swept spectrum analyzer. In particular circuitry is required to enable the digital to analogue conversion to be made, and then for processing the signal as a Fast Fourier Transform. This means that the block diagram for the FFT spectrum analyzer is very different to that of the more familiar superheterodyne spectrum analyzer.The FFT spectrum analyzer can be considered to comprise of a number of different blocks:

FFT Spectrum Analyser Block Diagram

Analogue front end attenuators / gain: The FFT analyzer requires attenuators of gain stages to ensure that the signal is at the right level for the analogue to digital conversion. If the signal level is too high, then clipping and distortion will occur, too low and the resolution of the ADC and noise become a problems. Matching the signal level to the ADC range ensures the optimum performance and maximises the resolution of the ADC.

Analogue low pass anti-aliasing filter: The signal is passed through an anti-aliasing filter. This is required because the rate at which points are taken by the sampling system within the FFT spectrum analyzer is particularly important. The waveform must be sampled at a sufficiently high rate. According to the Nyquist theorem a signal must be sampled at a rate equal to twice that of the highest frequency, and also any component whose frequency is higher than the Nyquist rate will appear in the measurement as a lower frequency component - a factor known as "aliasing". This results from the where the actual values of the higher rate fall when the samples are taken. To avoid aliasing a low pass filter is placed ahead of the sampler to remove any unwanted high frequency elements. This filter must have a cut-off frequency which is less than half the sampling rate, although typically to provide some margin, the low pass filter cut-off

frequency is at highest 2.5 times less than the sampling rate of the FFT spectrum analyzer. In turn this determines the maximum frequency of operation of the FFT spectrum analyzer.

Sampling and analogue to digital conversion: In order to perform the analogue to digital conversion, two elements are required. The first is a sampler which takes samples at discrete time intervals - the sampling rate. The importance of this rate has been discussed above. The samples are then passed to an analogue to digital converter which produces the digital format for the samples that is required for the FFT analysis.

FFT analyzer: With the data from the sampler, which is in the time domain, this is then converted into the frequency domain by the FFT analyzer. This is then able to further process the data using digital signal processing techniques to analyze the data in the format required.

Display: With the power of processing it is possible to present the information for display in a variety of ways. Today's displays are very flexible and enable the information to be presented in formats that are easy to comprehend and reveal a variety of facets of the signal. The display elements of the FFT spectrum analyzer are therefore very important so that the information captured and processed can be suitably presented for the user.

Real Time Spectrum Analyzer

In recent years a form of spectrum analyser, termed a real-time spectrum analyser, RSA has grown in popularity.These real-time spectrum analyzers are particularly useful in looking at waveforms where changes may be seen, and need to be captured. Often spectrum analysers that take time to process the waveforms may miss spurious signals and these can be particularly important when testing for compliance and out-of-band signals.As the name implies, real-time spectrum analysers operate in real time.

Realtime spectrum analyser basics

A realtime spectrum analyser operates in a different way to that of a normal swept or superheterodyne spectrum analyser.The realtime spectrum analyser is set to the required centre frequency. They then acquire a particular bandwidth or span either side of this. The realtime spectrum

analyser captures all the signals within the bandwidth and analyses them in real time.To achieve their performance a real time spectrum analyser captures the waveform in memory and then uses a fast Fourier transform technology to analyse the waveform very quickly, i.e. in real-time.By analysing the waveform in this way, transient effects that may not be visible on other forms of spectrum analyser can be captured and highlighted.There are a number of characteristics of realtime spectrum analyzers:

They are based around an FFT - Fast Fourier Transform spectrum analyser. This will have a real-time - very fast - digital signal processing engine capable to processing the entire bandwidth with no gaps.

An ADC - analogue to digital converter capable of digitising he entire bandwidth of the passband.

Sufficient capture memory to enable continuous acquisition over the desired measurement period.

How to use a Spectrum Analyzer

Spectrum analyzers are an invaluable item of electronic test equipment used in the design, test and maintenance of radio frequency circuitry and equipment. Spectrum analysers, like oscilloscopes are a basic tool used for observing signals. However, where oscilloscopes look at signals in the time domain, spectrum analyzers look at signals in the frequency domain. Thus a spectrum analyser will display the amplitude of signals on the vertical scale, and the frequency of the signals on the horizontal scale.In view of the way in which a spectrum analyzer displays its output, it is widely used for looking at the spectrum being generated by a source. In this way the levels of spurious signals including harmonics, intermodulation products, noise and other signals can be monitored to discover whether they conform to their required levels. Additionally using spectrum analysers it is possible to make measurements of the bandwidth of modulated signals can be checked to discover whether they fall within the required mask. Another way is using a spectrum analyzer is in checking and testing the response of filters and networks. By using a tracking generator - a signal generator that tracks the instantaneous frequency being monitored by the spectrum analyser, it is possible to see the loss at any given frequency. In this way the spectrum analyser makes a plot of the frequency response of the network.

Spectrum analyzer

The purpose of a spectrum analyzer is to provide a plot or trace of signal amplitude against frequency. The display has a graticule which typically has ten major horizontal and ten major vertical divisions.The horizontal axis of the analyzer is linearly calibrated in frequency with the higher frequency being at the right hand side of the display. The vertical axis is calibrated in amplitude. Although there is normally the possibility of selecting a linear or logarithmic scale, for most applications a logarithmic scale is chosen. This is because it enables signals over a much wider range to be seen on the spectrum analyser. Typically a value of 10 dB per division is used. This scale is normally calibrated in dBm (decibels relative 1 milliwatt) and therefore it is possible to see absolute power levels as well as comparing the difference in level between two signals. Similarly when using a linear scale is used, this is often calibrated in volts to enable absolute measurements to be made using the spectrum analyzer.

Setting the spectrum analyzer frequency

To set the frequency of a spectrum analyser, there are two selections that can be made. These are independent of each other. The first selection is the centre frequency. As the name suggests, this sets the frequency of the centre of the scale to the chosen value. It is normally where the signal to be monitored would be located. In this way the main signal and the regions either side can be monitored. The second selection that can be made on the analyzer is the span, or the extent of the region either side of the centre frequency that is to be viewed or monitored. The span may be give as a given frequency per division, or the total span that is seen on the calibrated part of the screen, i.e. within the maximum extents of the calibrations on the graticule. Another option that is often available is to set the start and stop frequencies of the scan. This is another way of expressing the span as the difference between the start and stop frequencies is equal to the span.

Adjusting the gain

There are many other controls on a spectrum analyser. Most of these fall into one of two categories. The first is associated with the gain or attenuation of sections

within the spectrum analyzer. If sections are overloaded, then spurious signals may be generated within the instrument. If this occurs then false readings will be given. To prevent this happening it is necessary to ensure that the input stages in particular are not overloaded and an RF attenuator is used. However if too much attenuation is inserted, additional gain is required in the later stages (IF gain) and the background noise level is increased and this can sometimes mask lower level signals. Thus a careful choice of the relevant gain levels within the spectrum analyzer is needed to obtain the optimum performance..

Filter bandwidths

Other controls on the spectrum analyzer determine the bandwidth of the unit. There are two main controls that are used:

IF bandwidth: The IF filter, sometimes labelled as the resolution bandwidth adjusts the resolution of the spectrum analyzer in terms of the frequency. Using a narrow resolution bandwidth is the same as using a narrow filter on a radio receiver. Choosing a narrow filter bandwidth or resolution on the spectrum analyzer will enable signals to be seen that are close together. It will also reduce the noise level and enable smaller signals to be seen.

Video bandwidth: The video filters enable a form of averaging to be applied to the signal. This has the effect of reducing the variations caused by noise and this can help average the signal and thereby reveal signals that may not otherwise be seen.

Adjustment of the IF or resolution bandwidth and the video filter bandwidths on the spectrum analyzer has an effect on the rate at which the analyzer is able to scan. The controls should be adjusted together to provide a scan that is as accurate as possible as detailed below.

Scan rate

The spectrum analyser operates by scanning the required frequency span from the low to the high end of the required range. The speed at which it does this is important. The slower the scan, obviously the longer it takes for the

measurements to be made. As a result, there is always the need to ensure that the scans are made as fast as reasonably possible.However the rate of scan of the spectrum analyzer is limited by a number of factors:

IF filter bandwidth: The IF bandwidth or resolution bandwidth has an effect on the rate at which the analyzer can scan. The narrower the bandwidth, the slower the filter will respond to any changes, and accordingly the slower the spectrum analyzer must scan to ensure all the required signals are seen.

Video filter bandwidth: Similarly the video filter which is used for averaging the signal as explained above. Again the narrower the filter, the slower it will respond and the slower the scan must be.

Scan bandwidth: The bandwidth to be scanned has a directly proportional effect on the scan time. If the filters within the spectrum analyzer determine the maximum scan rate in terms of Hertz per second, it follows that the wider the bandwidth to be scanned, the longer the actual scan will take.

Normally the processor in the spectrum analyzer will warn if the scan rate is too high for the filter settings. This is particularly useful as it enables the scan rate to be checked without undertaking any calculations.Also if the scan appears to be particularly long, an initial wide scan can be undertaken, and this can be followed by narrower scans on identified problem areas.

Understanding Spectrum Analyzer Specifications

Spectrum analyzers are an essential item of test equipment for any RF design or test laboratory. They enable a view of the spectrum of an RF signal to be seen and in this way they provide a unique insight into the way an RF circuit or piece of equipment is working. However choosing the correct spectrum analyzer to purchase, either as new equipment or as used test equipment or possibly to hire from a test equipment rental company is not always easy. A knowledge of spectrum analyzer specifications is needed to be able to make the right choice for any given application.There is a wide variety of different types of spectrum analyzer ranging from an audio spectrum analyzer for low audio frequencies right through to RF spectrum

analyzers capable of measuring frequencies up to many GHz. For each category there are many different types for a variety of different manufacturers. In addition to the manufacturers offering a wide variety of instruments, used test equipment suppliers as well as test equipment rental companies will hold stocks of a wide variety of instruments. Accordingly it is necessary to be able to judge which might be most suitable by consulting the spectrum analyzer specifications.

Frequency coverage

The frequency coverage of the spectrum analyser is one of the basic specifications or parameters. When determining whether a particular spectrum analyzer is suitable for the application, it is necessary to consider the maximum frequencies that will need to be viewed. It is worth remembering that the maximum frequency to be viewed should include the harmonics and intermodulation products of the wanted signals.

Amplitude accuracy

The amplitude accuracy is a major spectrum analyzer specification. While the accuracy of a spectrum analyzer itself will not match that of a dedicated power meter for example, the accuracy of the individual level measurements need to be accurate to enable useful measurements to be made.The amplitude accuracy specification of a spectrum analyzer is determined by a number of factors, including the basic accuracy of the instrument as well as its frequency response. This means that the frequency elements should also be taken into consideration. Often accuracy levels of the order of ± 0.4 dB are achievable.For microwave spectrum analyzers a YIG oscillator is normally used. As YIGs are highly non-linear devices, and as a result the amplitude accuracy specification figures will be less (typically ± 1 dB) when the YIG oscillator is used.Some spectrum analyzers incorporate a power meter which operates with the analyzer to provide a very accurate measurement specification. For this, the spectrum analyser has a special power sensor that calibrates the input level at a number of absolute level points, then uses the very good linearity of the analyser to very accurately measure levels over the full range which may be in excess of 100dB.

Frequency accuracy specification

Most spectrum analyzers today employ frequency synthesized sources. This means that the accuracy of the frequency measurement is governed by that of the peak detection circuitry, detecting where the centre of a signal is, and also the accuracy of the reference source within the frequency synthesizer.Spectrum analyzers can be used as extremely accurate frequency counters with relatively high specifications. They locate a signal and track it, simultaneously with measuring it's absolute frequency. This can be particularly advantageous in many applications.

Spectrum analyzer sensitivity specification

In order to determine the low signal performance of spectrum analyzer a sensitivity specification is normally given. This is normally specified in terms of dBm / Hz at a given frequency.If a noise figure specification is required, then this can be calculated:

Noise Figure = Sensitivity (dBm/Hz) - Noise floor at room temp (-174 dBm/Hz)

If a further improvement in the sensitivity or noise figure specification is required, then it is possible to add a low noise pre-amplifier.

Phase noise specification

There are many instances when the phase noise of a signal source, e.g. a transmitter, receiver local oscillator, etc needs to be measured. When this is the case, the phase noise specification of the spectrum analyzer is of particular importance. It should be better than the signal source being measured, typically by at least 10 dB for it not to affect the readings being made. For these applications, the spectrum analyser specification for phase noise needs to be carefully considered.

Techniques apart from a straight measurement can be sued to improve the operation of the spectrum analyzer. These techniques include a noise correction process, where the noise of the spectrum analyzer is subtracted from the measurement. For higher performance it is possible to utilise cross-correlated phase noise measurements where the spectrum analyzer is effectively able to remove the phase noise of its internal local oscillators from the measurement. This process allows phase noise measurements to be made below the physical thermal limit, i.e. better than -174dBm/Hz.

Spectrum analyzer dynamic range

Dynamic range is a particularly important parameter for any spectrum analyzer. This type of test equipment is normally used on a logarithmic scale and is required to look at signals with enormously wide level ranges. Therefore the ability of the spectrum analyzer to accurately look at small signals in the presence of relatively close strong signals is particularly important.

Spectrum analyzer tracking generator

The spectrum analyzer tracking generator is an additional capability beyond the basic spectrum analyzer that provides an additional measurement capability. The spectrum analyzer tracking generator enables some basic network measurements to be made. In view of this a tracking generator considerably extends the applications for which a spectrum analyzer can be used, making them more flexible and versatile.

Spectrum analyzer tracking generator basics

Normally spectrum analyzers are what may be termed passive instruments, making measurements of signals applied to them. Typically they may be used for measuring the spectra of oscillators, transmitters or other signals in RF systems. They measure signals in the frequency domain rather than the time, and this makes them ideal for looking at many RF signals.In their basic form, spectrum analyzers are not able to make response or network measurements. These types of measurements require signals to be applied to a particular device or network under test, and then measuring the response or output.

In order to make a network measurement like this, it is necessary to have a source to stimulate the device under test, and then a receiver is needed to measure the response. In this way it is possible to make a variety of network measurements including frequency response, conversion loss, return loss, and other measurements such as gain versus frequency, etc..There are two items of test equipment that can be made to make these stimulus-response measurements. Possibly the most obvious type of test equipment is an RF network analyzer and the other is a spectrum analyzer with a tracking generator. If phase information is required, then it is necessary to use a vector network analyzer, but it possible to use a spectrum analyzer tracking generator arrangement for many other measurements. As many laboratories will already use a spectrum analyzer, the tracking generator approach is particularly attractive. In addition to this, tracking generators are incorporated into many spectrum analyzers as standard. This means that it is possible to use the spectrum analyzer tracking generator to make many network measurements at no additional cost.

Spectrum analyzer tracking generator

A spectrum analyzer tracking generator operates by providing a sinusoidal output to the input of the spectrum analyzer. The by linking the sweep of the tracking generator to the spectrum analyzer, the output of the tracking generator is on the same frequency as the spectrum analyzer, and the two units track the same frequency.

Spectrum Analyser and Tracking GeneratorIf the output of the tracking generator was connected directly to the input of the spectrum analyzer, a single flat line would be seen with the level reflecting the output level of the tracking generator.If a device under test, such as a filter is placed between the output of the tracking generator and the input of the spectrum analyzer, then the response of the device under test will alter the level of the tracking generator signal seen by the spectrum analyzer, and the level indicated on the analyzer screen. In this way the response of the device under test will be seen on the analyzer screen.

Using a spectrum analyzer tracking generator

Using a spectrum analyzer tracking generator is normally very easy. As a tracking generator is either built into the spectrum analyzer, or is manufactured as an external option for a specific analyzer, then there are few issues with their use. However there are a few standard precautions to remember when using a tracking generator:

Adjust tracking generator to centre of analyse passband: There is often an adjustment for the tracking oscillator to trim its frequency. Before using the tracking generator, it is wise to adjust the frequency trim adjustment to ensure that it is on exactly the same frequency as the spectrum analyzer.

This is achieved by maximising the reading on the spectrum analyzer display.

Calibrate system using direct connection: To ensure that any cable losses are known, it is always wise to replace the device under test with a back-to-back connector, or other short connecting line. In this way, the system will reveal any losses which it may be possible to "calibrate out".

When using a spectrum analyzer tracking generator it is possible to make many measurements very easily. A few precautions, when making the measurements will enable inaccuracies to be counteracted, and reliable measurements made.