Spectrum Analyzer

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Transcript of Spectrum Analyzer

Spectrum Analyzer

Introduction:Of the two types of RF spectrum analyzer that are available, namely the swept or super heterodyne 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 analyzer 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 analyzer is able to reveal signals that are not visible when using other items of test equipment.

A modern spectrum analyzer display (power vs. frequency plot)

To enable the most effective use to be made of a swept 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.

A spectrum analyzer is a wide band, very sensitive receiver. It works on the principle of "super-heterodyne receiver" (if is is swept spectrum analyzer) to convert higher frequencies (normally ranging up to several 10s of GHz) to measurable quantities. The received frequency spectrum is slowly swept through a range of pre-selected frequencies, converting the selected frequency to a measurable DC level (usually logarithmic scale), and displaying the same on a CRT. The CRT displays received signal strength (y-axis) against frequency (x-axis).

Obviously, signals that are weaker than the background noise could not be measured by a spectrum analyzer. For this reason, the noise floor of a spectrum analyzer in combination with RBW is a vital parameter to be considered when choosing a spectrum analyzer. The received signal strength is normally measured in decibels (dbm). (Note that 0 dBm corresponds to 1 mWatt of power on a logarithmic scale). The primary reasons for measuring the power (in dBm) rather than voltage in Spectrum Analyzers are the low received signal strength, and the frequency range of measurement. Spectrum analyzers are capable of measuring the frequency response of a device at power levels as low as –120dBm. These power levels are encountered frequently in microwave receivers, and spectrum analyzers are capable of measuring the device characteristics at that power levels.

Spectrum Analyzer vs. Oscilloscope:a) A spectrum analyzer displays received signal strength (y-axis) against frequency (x-axis).

An Oscilloscope, displays received signal strength (y-axis) against time (x-axis). b) Spectrum analyzer is useful for analyzing the amplitude response of a device against

frequency. The amplitude is normally measured in dBm in Spectrum Analyzers, whereas the same is measured in volts when using Oscilloscopes.

c) Normally, Oscilloscope cannot measure very low voltage levels (say, -100dBm) and are intended for low frequency, high amplitude measurements. A spectrum analyzer can easily measure very low amplitudes (as low as -120dBm), and high frequencies (as high as 150GHz).

d) The spectrum analyzer measurements are in frequency domain, whereas the oscilloscope measurements are in time domain.

e) Also, a spectrum analyzer uses complex circuitry compared with an Oscilloscope. As a result of this, the cost of a spectrum analyzer is usually quite high.

Working Principle of Swept Spectrum Analyzer:

The swept spectrum analyzer uses the same super heterodyne principle used in many radio receivers as the underlying principle on which its operation depends. The super heterodyne principle uses a mixer and a second locally generated local oscillator signal to translate the frequency.

The mixing principle used in the analyzer operates in exactly the same manner as it does for a super heterodyne 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 super heterodyne radio, the particular implementation differs slightly to enable it to perform its function.

The 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.

Super heterodyne or swept frequency spectrum analyzer block diagram

Elements of a swept spectrum analyzer:

Although the basic concept of the swept spectrum analyzer is fairly straightforward, still a few of the circuit blocks may need a little further explanation.

RF attenuator: The first element a signal reaches on entering the test instrument 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.

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

Mixer: The mixer is naturally a key to the success of the analyzer. 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. Thus the dynamic range performance of the mixer is of crucial importance to the analyzer as a whole.

Great care must be taken when using a swept 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.

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 has 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 test instrument is optimized.

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.

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 analyzer. 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.

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. In other words the ramp generator is controlled by the sweep rate adjustment on the spectrum analyzer.

Envelope or level detector: The envelope detector converts the signal from the IF filter into a signal voltage that can be passed to the display. As the level detector has to accommodate very large signal differences, linearity and wide dynamic range are essential.

The type of detector may also have a bearing on the measurement made. Whether the detector is an average level detector or whether it provides an RMS value.

An RMS detector calculates the power for each pixel of the displayed trace from samples allocated to the pixel, i.e. for the bandwidth that the pixel represents. The voltage for each

sample is squared, summed and the result divided by the number of samples. The square root is then taken to give the RMS value.

For an average value, the samples are summed, and the result is divided by the number of samples.

Display: In many respects the display is the heart of the test instrument as this is where the signal spectra are viewed. The overall display section of the spectrum analyzer contains a significant amount of processing to enable the signals to be viewed in a fashion that is easy comprehended. Items such as markers for minimum signal, maximum peak, auto peak, highlighting and many more elements are controlled by the signal processing in this area. These features and many more come as the result of significant increases in the amount of processing provided.

As for the display screens themselves, cathode ray tubes were originally used, but the most common form of display nowadays are forms of liquid crystal displays. The use of liquid crystal displays does have some limitations, but overall with the level of development in this technology they enable the required flexibility to be provided.

Key Features of a Spectrum Analyzer:i. Resolution bandwidth

ii. Frequency rangeiii. Frequency stabilityiv. AC/DC Operationv. Service warranty

Resolution bandwidth: This is an important parameter to consider when buying a Spectrum Analyzer. The sensitivity of the spectrum analyzer is directly dependent on the resolution bandwidth of the analyzer. If your measurements are over a wide band, a 3 KHz RBW is normally sufficient. If you need to make very narrow band measurements (such as filters), then consider a 300Hz or even a 10Hz RBW spectrum analyzer. Obviously, a spectrum analyzer with lower RBW costs more than a spectrum analyzer with 3 KHz RBW.

Frequency range: This is the range of frequencies that you need to make measurements. Spectrum analyzers are available from 100 Hz to 50 GHz range. If you require measurements up to, say IF to 2.4 GHz, a spectrum analyzer from 10MHz-2.4 GHz would be suitable.

Frequency Stability: Frequency stability is the ability of the spectrum analyzer to maintain the frequencies within a specified accuracy. The frequency stability is dependent on the Local

Oscillator stability of the spectrum analyzer. For narrow band measurements, this is a very important parameter. Spectrum analyzers do not normally have very high stability clock. If high accuracy of measurement is required, consider buying a spectrum analyzer with provision for external frequency reference. In such an event, the accuracy of the spectrum analyzer is as good as the external reference.

Input Power Range: This is the range of input power that could be fed to the spectrum analyzer input connector. Normally, this ranges from -100 dBm to +10 dBm. Beyond the lower limits, the spectrum analyzer may not be able to identify the signal from back ground noise. If you feed signals beyond the maximum specified range, it is possible that the input mixer is saturated and the reading shown on the spectrum analyzer may not represent the actual power levels accurately. There is also a likelihood of damaging the front-end component of the spectrum analyzer. Use an external attenuator if it is required to measure power levels beyond the specified limits. Please note that spectrum analyzers are available for various input signal power levels.

Harmonics: The frequency harmonics is a measure of accuracy of the spectrum analyzer. Normally, the harmonics are greater than 30 dB below the desired signal. The harmonics add to the measurement uncertainty, and should be kept to the minimum.

AC/DC operation: If you need to make measurements out-doors, you may require DC operation. Check if it is available.

Service warranty: Normally, spectrum analyzers are very expensive. A comprehensive warranty is recommended when buying a spectrum analyzer. Also ensure that the rf input connection has dc protection.

Applications of Spectrum Analyzer:

Device Frequency Response Measurements: You can use spectrum analyzers for measuring the amplitude response (typically measured in dbm) against frequency of device. The unit of frequency is Hertz. 1000Hz=1KHz, 1000Kz=1MHz, 1000MHz=1GHz. The device may be anything from a broadband amplifier to a narrow band filter.

Microware Tower Monitoring: You can measure the transmitted power and received power of a Microware tower. Typically, you use a directional coupler to tap the power without interrupting the communications. In this way, you can verify that the frequency and signal strength of your transmitter are according to the specified values.

Interference Measurements: Any large RF installations normally require site survey. A spectrum analyzer can be used to verify identify and interferences. Any such interfering signals need to be minimized before going ahead with the site work. Interference can be created by a number of different sources, such as telecom microwave towers, TV stations, or airport guidance systems etc.

Other measurements that could be made using spectrum analyzer include the following:

1. Return-loss measurement2. Satellite antenna alignment3. Spurious signals measurement4. Harmonic measurements5. Inter-modulation measurements

Given below are some important features available with a 8563EC Portable Spectrum Analyzer, 9 kHz to 26.5 GHz:

Keysight (Agilent/HP) 8563EC 9 kHz – 26.5 GHz Spectrum Analyzer

1. Color display2. Continuous 30 Hz to 26.5 GHz sweep3. Fast digital resolution bandwidths of 1, 3, 10, 30 and 100 Hz4. Adjacent channel power, channel power, carrier power, occupied bandwidth percentage

and time-gated measurements standard5. Precision timebase and 1 Hz counter resolution6. MIL-PRF-28800, Class 3 rugged

7. Measurement personalities for digital radio and phase noise measurements8. Easily transfer screen image or trace data to PC with E4444A BenchLink software

Note: The above specifications are given as an example only, and may not accurately represent the actual equipment specifications.

Experimental Results:

Following is the experimental work done shown which was performed in person in EMC lab.

For 40 KHz, Square wave:

On Signal Generator

On Spectrum Analyzer

For 1 MHz, Sinusoidal wave:

On Signal Generator

On Spectrum Analyzer

For 1 MHz, Triangular wave:

On Signal Generator

On Spectrum Analyser

References: http://www.radio-electronics.com/info/t_and_m/spectrum_analyser/superheterodyne-

sweep-swept-analyzer.php http://www.tutorialsweb.com/rf-measurements/spectrum-analyzer.htm