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Spectrum analyzer tutorial

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  • 1. 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 of 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 analyser display A key element of using a spectrum analyser is in understand in the display. 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
  • 2. 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. Typical spectrum analyser display Setting the spectrum analyzer frequency When using a spectrum analyser, one of the first settings is that of the frequency. Dependent upon the spectrum analyser in use, there are various ways in which this can be done: 1. Using centre frequency: Using this method, 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 display. 2. Using upper and lower frequencies: Another option that is available on most spectrum analysers 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
  • 3. Adjusting the gain In order to maintain the correct signal levels when using a spectrum analyser, there are two main gain controls available. Their use needs to be balanced to ensure the optimum performance is obtained. RF Attenuator: as the name implies this control provides RF attenuation in the RF section. It is actually placed before the RF mixer and serves to control the signal level entering the mixer. IF Gain: The IF Gain control controls the level of the gain within the IF stages of the spectrum analyser after the mixer. It enables the level of gain to be controlled to allow the signal to be positioned correctly on the vertical scale on the display. The two level controls must be used together. If the signal level at the mixer is too high, then this stage and further stages can become overloaded. However if the attenuation is set too high and additional IF gain is required, then noise at the input is amplified more and noise levels on the display become higher. If these background noise levels are increased too much, they can mask out lower level signals that may need to be seen. 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,
  • 4. 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. Hints and tips There are several hints and tips for using a spectrum analyser to its best effect. Beware input level: IIn order to ensure the optimum performance of the system, the input is normally connected to the primary mixer with only the input attenuator control, often labelled RF level, between them. Accordingly RF can be applied directly to the mixer with no protection. It is therefore very important to ensure that the input is not overloaded and damaged. One major and expensive cause of damage on spectrum analysers is the input mixer being blown when the analyser is measuring high power circuits. Determine if spurs are real: One aspect of using a spectrum analyser that will often be encountered is the spurious signals are often viewed. Sometimes these may be generated by the item under test, but it is also possible that they can be generated by the analyser. To check if they are generated by the item under test, reduce the input sensitivity of the analyser by 10dB for example. If the spurious signals fall by 10dB then they are generated by the unit under test, if they fall by more than 10dB then they are generated by the analyser and possibly as a result of overloading the input. Wait for self alignment: When a spectrum analyser is first switched on, not only does it go through its software boot-up procedure, but most also undertake a number of self-test
  • 5. and calibration routines. In addition to this, elements such as the reference oven controlled crystal oscillator oven need come up to temperature and stabilise. Often manufacturers suggest that fifteen to thirty minutes before it can be used reliably. The crystal oscillator may take a little longer to completely stabilise, but refer to the manufacturers handbook for full details. Power measurement: While the accuracy of a spectrum analyser making power measurements is not as accurate as power meter in terms of absolute accuracy. However it should be remembered that both test instruments make slightly different power measurements. A power meter will make a measurement of the total power within the bandwidth of the sensor head - essentially it will measure the power regardless of the frequency. A spectrum analyser will make a measurement of the power level at a specific frequency. In other words it can make a measurement of the carrier power level, for example, without the addition of any spurious signals, noise, etc. While the absolute accuracy of a spectrum analyser is not quite as good as that of a power meter, they are improving all the time and the difference in accuracy is generally small. Phase noise is a key parameter for many systems, and measuring it accurately is of great importance. One of the easiest methods to measure phase noise is to use a spectrum analyser using what is termed a direct measurement technique. Using a spectrum analyser to measure phase noise can provide excellent results provided that the measurement technique is understood and precautions are adopted to ensure the most accurate results. Phase noise A certain level of phase noise exists on all signals and extends out either side of the wanted signal or carrier. The shape of the phase noise plot will depend upon whether it is a free running oscillator, or locked within a phase locked loop, as this will alter the noise profile.
  • 6. Phase noise of a free running oscillator Note on Phase Noise: Phase noise consists of small random perturbations in the phase of the signal, i.e. phase jitter. An ideal signal source would be able to generate a signal in which the phase advanced at a constant rate. This would produce a single spectral line on a perfect spectrum analyzer. Unfortunately all signal sources produce some phase noise or phase jitter, and these perturbations manifest themselves by broadening the bandwidth of the signal. Click on the link for a Phase Noise tutorial Pre-requisites for measuring phase noise The main requirement for any phase noise measurement using a spectrum analyser is that it must have a low level of drift compared to the sweep rate. If the level of oscillator drift is too high, then it would invalidate the measurement results. This means that this technique is ideal for measuring the phase noise levels of frequency synthesizers as they are locked to a stable reference and drift levels are very low. Free running oscillators are not normally sufficiently stable to use this technique. Often they would need to be locked to a reference in some way, and this would alter the phase noise characteristics of at least part of the spectrum. Phase noise measurement basics
  • 7. The basic concept of using a spectrum analyser to measure the phase noise levels of a signal source involve measuring the carrier level and then the level of the phase noise as it spreads out either side of the main carrier. Typically the measurement is made of the noise spreading out on one side of the carrier, as the noise profile is normally a mirror image of the other and there is no reason to measure both sides. As a result the term single sideband phase noise is often heard. As the level of phase noise is proportional to the bandwidth of the filter used, most phase noise measurements are related to the carrier level and within a bandwidth of 1 Hz. The spectrum analyser uses a suitable filter bandwidth for the measurement and then adjusts the level for the required bandwidth. Typically phase noise measurements are specified as dBc/Hz, i.e. level relative to the carrier expressed in decibels and within a 1 Hz bandwidth. Filter and detector characteristics The filter and detector characteristics have an impact on the spectrum analyser phase noise measurement results. One of the key issues is the bandwidth of the filter used within the analyser. Analysers do not possess 1 Hz filters, and even if they did measurements with a 1 Hz bandwidth filter would take too long to make. Accordingly, wider filters are used and the noise level adjusted to the levels that would be found if a 1 Hz bandwidth filter had been used.
  • 8. Where: L1Hz = level in 1 Hz bandwidth, i.e. normalised to 1 Hz, typically in dBm LFilt = level in the filter bandwidth, typically in dBm BW = bandwidth of the measurement filter in Hz As the filter shape is not a completely rectangular shape and has a finite roll-off, this has an effect on the transformation to give the noise in a 1Hz bandwidth. The type of detector also has an impact. If a sampling detector is used instead of an RMS detector and the trace is averaged over a narrow bandwidth or several measurements, then it is found that the noise will be under-weighted. Adjustments for these and any other factors are normally accommodated within the spectrum analyser, and often a special phase noise measurement set-up is incorporated within the software capabilities. Spectrum analyser requirements When measuring phase noise with a spectrum analyser, there are some minimum requirements for this type of measurement. Spectrum analyser phase noise: In order to be able to measure the phase noise of a signal source using a spectrum analyser, the specification of the analyser should be checked to ensure it is sufficiently better than the expected results for the source. The reason for this is that if a perfectly good signal source was being measured, the phase noise characteristic of the local oscillator in the spectrum analyser would be seen as a result of reciprocal mixing. As a rough guide, the phase noise response of the analyser should be 10dB better than that of the signal source under test. Dynamic range: The dynamic range of the spectrum analyser is also an issue. The analyser must be able to accommodate the carrier level as well as the very low noise levels that exist further out from the carrier. It is easy to check whether the thermal noise is an issue. The trace of the phase noise of the signal source can be taken and stored. Using exactly the same settings, but with no signal, the measurement can be repeated. If at the offset of interest there is a clear difference between the two, then the measurement will not be unduly affected by the analyser thermal noise. Phase noise test precautions
  • 9. When measuring phase noise with a spectrum analyser, there are a few precautions that can be taken to ensure that the test results are as accurate as possible. Minimised extraneous received noise: During a spectrum analyser phase noise measurement, some of the levels that are measured are very low. It is therefore necessary to ensure that levels of extraneous received noise are minimised. The unit under test should be enclosed to ensure that no noise is picked up within the circuit. This is particularly true of any oscillator circuit itself such as a synthesizer voltage controlled oscillator. The use of double screened coax cable between the test item and the analyser may be considered. A screened room could also be used. In this way Use representative power supply: The power supply used to supply the item under test can have a major impact on its performance. Issues such as switching spikes on switch mode regulators can have a major impact on performance. Accordingly a power supply that is representative should be chosen to power the signal source being measured. Analyser set-up: Care should be taken to ensure that the spectrum analyser is correctly set up to measure phase noise. Often in-built phase noise measurement settings will be available for these measurements and they can be used as a starting point. Measuring phase noise with a spectrum analyser is one of the easiest and accurate methods that can be employed. High end analysers are designed with this as a regular measurement that will need to be made, and issues with extraneous noise and many other problems are minimised. Although other methods can be adopted to measure phase noise, special systems often need to be developed, and in view of the very low levels of phase noise involved, these systems may not always be as accurate. A unit that has been developed and optimised with these measurements in mind is bound to have solved the majority of problems and provide a far more time and cost effective method of making these measurements. For any radio frequency, RF amplifier or system, the noise figure is a key parameter. Measuring noise figure may not always be easy and while the easiest way is to use a specialised noise figure meter, one of these may not always be available. Therefore using a spectrum analyser to measure the noise figure can be a very useful option, as these test instruments are often available within RF laboratories. Noise figure measurements with a spectrum analyser Using a spectrum analyser to measure noise figure has a number of advantages:
  • 10. Uses available equipment: Using a spectrum analyser to measure noise figure is often convenient because it utilises test equipment that will be found in many RF development or test laboratories. A dedicated noise figure meter may not be available. Wide frequency range: Spectrum analyser noise figure measurements can be made at any frequency within the range of the spectrum analyser. A variety of frequencies can be used for different devices without changes to the test configuration. Frequency selective noise figure measurements: Measurements can be made to be frequency selective, independent of device bandwidth and spurious responses. Using a spectrum analyser may not as accurate as those obtained when using a noise figure meter, but this is very dependent upon the spectrum analyser and the measurement method used. The "Y" factor method is often accepted as being equally as accurate as that of a specialised noise figure meter, but requires the use of a noise source. Some spectrum analyzers have software built in to run these tests. Noise figure measurement basics Noise figure measurements are important because the limit of sensitivity of a radio or wireless receiver is limited by noise. In an ideal system this would be limited by noise picked up at the antenna, but in reality all systems generate some noise themselves. Note on Noise Figure: The noise figure for an RF system or an element within an RF system is a figure of merit expressed in decibels indicating the level of noise introduced. The ideal noise figure is 1, and anything above this indicates that noise is introduced. Click on the link for a Noise Figure In order to measure noise figure using spectrum analyser it is necessary to manipulate the equations a little.
  • 11. Where: N = noise power output k = Boltmann's constant - 1.374 x 10^-23 joule/°C T = temperature in ° Kelvin, i.e. 290° is room temperature B = bandwidth in Hz G = device gain F = Noise factor (Noise figure is the noise factor expressed in decibels) It is possible to set the equation based on the Noise Figure in dB and split it into the different elements of the equation. This noise figure equation can be further reorganised. In this equation the noise power bandwidth is set by the resolution bandwidth of the spectrum analyser. Therefore the equation be reorganised as follows:
  • 12. In some instances it may be necessary to add some factors to accommodate for the real responses, etc of the analyser against the theoretical requirements for making noise figure measurements with a spectrum analyzer: Filter: As the filter response cannot be a complete rectangular shape and the noise power bandwidth and the resolution bandwidth are not the same. A typical figure quoted for some analyzers was that the filter bandwidth was around 1.2 times the resolution bandwidth which equates to an adjustment of 0.8 dB. Noise level: The averaging effect of the video filters, etc on analogue spectrum analyzers tended to give a reading that was around 2.5 dB below the RMS noise level. The overall adjustment is around +1.7 dB (i.e. 2.5 - 0.8). However most modern spectrum analyzers will have corrections for these discrepancies and the makers literature or help material should be consulted regarding them. To make the noise figure measurements required, the spectrum analyzer should have a noise floor that is 6dB lower than the noise emanating from the device under test. As this will typically be an amplifier, its noise level is likely to be greater. If not a further low noise amplifier can be added after the device under test to bring the noise level up - note that the noise will tend to be at the input to the device under test if it is an amplifier. The noise figure can then be calculated as: Where: N = noise power output Gd = Gain of device under test in dB Gamp = Gain of additional amplifier in dB B = Bandwidth in Hz In this way the noise figure can be measured with a knowledge of the device gain, measurement bandwidth and noise power output.
  • 13. Noise figure measurement procedure The test for measuring noise figure using this method is quite straightforward. It consists of simple stages: 1. Measure gain of system: One of the key elements in the noise figure equation is the system gain. This needs to be measured by the system. Typically this is achieved by using a signal generator fed into the device under test. The gain can be measured simply by measuring the signal level directly from the output of the signal generator and then with the amplifier in circuit. 2. Measure noise power: The next step is to disconnect the signal generator and terminate the input to the device under test with a resistance equal to its characteristic impedance. With the lower signal level, i.e. just the noise power from the device, the input attenuator level of the spectrum analyser may need to be adjusted, e.g. to 0dB, and sufficient video averaging applied to obtain a good reading for the noise level. 3. Calculate noise figure: Using readings for the average noise power and the bandwidth for the analyzer, these can be substituted in the equation above to give the noise figure for the device under test. Pulse signals are being used increasingly for a variety of radio and other RF applications. As a result, spectrum analysis of these pulse signals is required. Traditionally pulse spectrum analysis techniques and approaches are normally aimed at steady analogue RF signals. However pulse spectrum analysis demands a little understanding of the signals being analysed, and this can enable additional information to be gained. Pulse signals Pulse signals take a variety of forms, but have a number of common traits. As a result it is possible to apply these to pulse spectrum analysis techniques. Accordingly the basics of pulse signals will be analysed. The basic waveform of a pulse is modulated onto the RF carrier. A look at the nature and makeup of the pulse waveform will provide an understanding the spectrum of the modulated pulsed waveform.
  • 14. The basic pulse waveform is shown in the diagram below. It has a repetition time of T and the pulse duration of t. Basic pulse waveform Using Fourier analysis it can be seen that this waveform is made up from a fundamental and harmonics. The basic waveform of a square wave can be made up from the fundamental sine wave with the same repetition rate as the square wave and then odd harmonics with the amplitudes of the harmonics inversely to their number. A rectangular pulse is just an extension of this basic principle. The different waveform shape is obtained by changing the relative amplitudes and phases of harmonics, both odd and even. Harmonics making up a pulse waveform These base-band signals can then be plotted and the amplitudes and phases of the infinite number of harmonics, both odd and even result in the smooth envelope shown below.
  • 15. Spectrum of a perfectly rectangular pulse The envelope of this plot follows a function of the basic form: γ = sin (x) / x This single can then be modulated onto an RF waveform to give a spectrum. As the harmonics of the baseband signal, extend out to infinity, so too do the sidebands of the modulated signal. In reality, however, the bandwidth will never be infinite and the harmonics, especially higher order ones are attenuated. Although this results in some distortion of the signal, the levels are generally acceptable. Spectrum of a pulse waveform modulated onto an RF carrier with phase inversions Pulse spectrum analysis It has been possible to see how pulse signals are generated and the resulting spectra. While the phase of the sidebands is accommodated on the plots above, spectrum analysers are scalar test instruments and do not normally give an indication of the phase of a signal. Accordingly the plots from spectrum analysers are only shown "above the line.".
  • 16. Spectrum of a pulse waveform modulated onto an RF carrier There are a number of points can be noted for this: Spectra lines: The individual spectra lines shown on the graph of the modulated waveform are separated by a frequency equal to 1/T. Nulls in envelope: The nulls in the envelope or overall shape of the spectra occur at intervals of 1/t. Further nulls occur at n / t Envelope null distinctness: The nulls in the pulse spectrum shape are not always particularly distinct because of the finite rise and fall times in the modulating signals and the resulting asymmetries that exist. Pulse desensitisation Sometimes the issue of pulse desensitisation is referred to in terms of pulse spectrum analysis. The issue is that when the modulation is applied to the carrier, the peak level of the envelope is reduced, appearing that the signal has been reduced in overall power. The apparent reduction in peak amplitude occurs because adding the pulse to the signal and modulating it with a square wave results in the power being distributed between the carrier and the sidebands. As the level of the modulation increases, so does the level of the sidebands. As there is only limited power available and each of the spectral components, i.e. carrier and sidebands, then contains only a fraction of the total power. The overall effect as seen on a spectrum analyser is that the peak power reduces, but it is spread over a wider bandwidth. It is possible to define a pulse desensitisation factor α. This can be described in the equation: It should be noted that this relationship is only really valid for a true Fourier line spectrum. For this to be applicable the resolution bandwidth of the analyser should be < 0.3 PRF.
  • 17. The average power of the signal is also dependent on the duty cycle as the power can only be radiated when the signal is in what may be loosely termed the "ON" condition. This can be defined by the equation below: Where: α = Pulse "desenitisation factor T = pulse repetition rate PRF = Pulse Repetition Frequency (1 / T) t = pulse length teff = effective pulse length taking account of rise and fall times Pavg = Average power over a pulse cycle Ppeak = Peak power Triangular and trapezoidal waveforms While pulse spectrum analysis is normally applied to square or rectangular waveforms, similar principles also apply to triangular and trapezoidal waveforms. The format of the waveform has many similar characteristics to those of a pulse waveform but with different levels of the different constituent signals and hence the sidebands. It is therefore possible to analyse these waveforms in a similar way. Pulse spectrum analysis measurement tips When looking at a pulsed signal using a spectrum analyser it is necessary to employ techniques to ensure that the signal is displayed to reveal the aspects that are required. Some of the chief aspects are: Measurement bandwidth less than line spacing: To resolve the individual spectral lines, the measurement bandwidth must be small relative to the offset of the lines, i.e. Bandwidth < 1 / T. If the measurement bandwidth is reduced further, them the spectral lines will retain their value (as expected) but the noise level will be reduced, although measurement time will be longer. Measurement bandwidth between line spacing and null spacing : The next stage occurs when the measurement bandwidth is greater than the spectral line spacing, but less
  • 18. than the null spacing. For this condition the spectral lines are not resolved and the amplitude height of the envelope depends upon the bandwidth. This is because a greater number of spectral lines, each with their own power contribution re contained within the measurement bandwidth. For this case 1 / t > B > 1 / T. Measurement bandwidth greater than null spacing: For this case where the measurement bandwidth is greater than the null spacings on the signal spectrum envelope, i.e. B > 1 / T, the amplitude distribution of the signal cannot be recognised. With pulse transmission being widely used, pulse spectrum analysis is an important element of characterising and testing any equipment that is developed and the signals they produce.