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[N329]Noise Measurement Spectrum Analyzer

by Kevin Mcmenamy, Kev

The name spectrum analyzer is used to refer to the spectral analyzer instrument which is employed to measure a waveform spectrum – that applies to the waveforms of the optical waveform, the acoustic waveform, and the electrical waveform. At times, the spectrum analyzer might also be used to gauge the power spectrum.

Basically, a spectrum analyzer can either be of the analog variety, or of the digital variety. It is an extremely sensitive measuring equipment that functions like a car radio since it can detect waveform frequencies (though your radio can only identify radio waves) and then uses a display to show these incoming frequencies to you.

When you have an analog spectrum analyzer on the job, it measures spectrum frequencies using either the variable band-pass filter, or the superheterodyne receiver instrument.

The digital spectrum analyzer relies on a mathematical process dubbed the discrete Fourier transform (or DFT) to interpret a waveform into its respective frequency spectrum parts.

A new variation on these two basic spectrum analyzer categories is the hybrid method for analyzing spectrum frequencies. A hybrid spectrum analyzer system can rely on syperheterodyne methods to down-convert the signal input so that it becomes transformed into a lower frequency, which then is studied using the FFT (fast fourier transformation) methods.

When an incoming signal is perceived and is to be measured, the spectrum analyzer will show what the frequency of the signal is through the display. The display should be able to indicate the fluctuation of the signal input over a certain period of time. The display tells us the degree of strength of the incoming signal – when we get past the incoming signal, our spectrum analyzer will reflect low-level noise and not a signal.

A digital spectrum analyzer is believed to be superior to an analog spectrum analyzer, because a digital spectrum analyzer can produce better frequency resolution over the prescribed acquisition time frame.)

A spectrum analyzer is used to check how strongly and frequently your transmitter can send out signals and how well these signals can be perceived. A spectrum analyzer can also check for the presence of interference (which can be a powerful signal in the area that blocks the signals you are transmitting) or if the frequency bandwidth you chose to transmit in is congested already.

Your spectrum analyzer might also be useful for a host of other test applications such as component characterization tests, test alignment of microwave and satellite antenna frequencies, intermodulation, how much bandwidth is occupied, checking power of adjacent channels, co-channel interference, and antenna isolation.

If you purchase the less expensive spectrum analyzers, you may get an instrument geared for limited frequencies or use only specific bands. There are also portable (or hand-held spectrum analyzers) which are powered by batteries.

One important concept in use of spectral analyzers is the signal-to-noise ratio concept. Signal- to-noise ratio will measure how strong your signal is being transmitted and received compared to the level of noise present in the environment. If your signal is strong enough, the background noise can be drowned out and be negligible.

It also displays the received signal and compares the bandwidth to the frequency. A comparison is often done with an Oscilloscope, which compares the strength of the signal against the time.

Spectrum analyzers are also useful in analyzing amplitude against the frequency. Amplitude is normally measured in power or in dBm instead of volts, which is what is normally used in most spectrum analyzer.

The reason behind this is the fact that there are low signal strengths and frequency of movements that may not be measured. Spectrum Analyzers can only measure the frequency of the response at powers as low as 100 dBm. These are the levels that are frequently seen in microwave receivers. Oscilloscopes, on the other hand, cannot measure such very low voltage. The device can only deal with very low frequency levels and high amplitude.

The analog analyzer uses a filter with a mid-frequency that can be automatically shifted through a series of frequencies where the spectrum will be measured. The digital spectrum, on the other hand, utilizes a mathematical process called the Fast Fourier Transform (FFT), which is used to transform a wavform into the different components of its frequency spectrum. This way, computer programs who do the transformations will make the audio processing much easier. FFTs, however, are not only used for this purpose. They also have applications in other fields.

There are also spectrum analyzers that makes use of a technique wherein the incoming signal is converted into a lower frequency. This hybrid technique uses first the superheterodyne and then the FFT techniques. Examples of spectrum analyzers with this technique are those made by the Tektronix from the real-time spectrum analyzer series.

Spectrum Analyzers have so many applications. One application is the device frequency response measurement, which refers to the amplitude response of a machine against frequency of device.

Another application is Microwave Tower Monitoring, which measures the transmitted power of the machine as well as the power that it receives. This is utilized for the verification if the signal strengths and frequency of the transmitter. A directional coupler is used to tap the power. This is done so as not to disturb or interrupt communications.

A spectrum analyzer is one instrument that is often used in the conversion of higher frequencies, often those that range up to 10s gigahertz. It is a sensitive receiver that works based on the super-heterodyne receiver principles.

Once received by the spectrum analyzer, the frequency signal is swept through a pre-selected set of frequencies. The selected frequency is then converted into a DC level, a logarithmic scale, that can be measured. It is also displayed on the CRT, where the y-axis contains the signal strength while the frequency is seen on the x-axis.

Spectrum analyzers, however, cannot detect signals that are too weak or weaker than the noise in the background. This is the reason why the spectrum analyzer is often used in tandem with an RBW. In fact, RBW is one of the vital considerations in choosing or buying a spectrum analyzer.

Here, the received signal strength is measured in dBm or what is called decibels, the zero of which corresponds to 1mWatt on the logarithm scale. The reason for the use of power or decibels instead of the usual voltage is the fact that what is being measured are the low signal strengths and the frequency range of measurement.

Spectrum analyzers can only measure the response of a device powered at 120 dBm. These are the power levels that are normally seen in microwave receivers.In addition to the RBW, there are other key features of spectrum analyzers that people need to consider before buying one. One of these vital components is the resolution width, which affects the sensitivity of the spectrum analyzer. In fact, the sensitivity is directly dependent on this feature. For instance, if the measurements are the over a wide band, a 3 KHz RBW will normally be effective.

However, if you need to analyze a much narrower spectrum, such as with filters, then you may need a bandwidth resolution of 300Hz or a 10 Hz RBW. All depends on how the spectrum analyzer will be used. Another feature is the frequency range, which refers to the frequencies that you will be needing in order to take measurements. Spectrum analyzers have ranges from 100 Hz to 50.

Another is the frequency stability, which allows the spectrum to maintain its frequency within a specific levels that is precise and accurate. Often, the frequency stability is entirely dependent on the stability of the oscillator. A provision for Narrow band measurements for instance is an important parameter because spectrum analyzers do not usually have very high stability clocks.

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