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The figure above shows a simplified block diagram of a super-heterodyne receiver. As seen from the above figure, it consists of the following parts:
Front-end mixer is where the rf input is combined with the local oscillator (VCO) frequency to give IF (Intermediate Frequency) output. The IF frequencies are then fed to the an IF amplifier, then to a detector. The output of the detector is fed to the video amplifier. The output from the video amplifier is given to CRT (vertical axis), and the output of the sawtooth generator is given to the horizontal axis of the CRT. Thus we see the signal amplitude against the time sweep (which in turn represents the frequency).
Normally, the frequency conversion takes place in multiple stages, and band-pass filters are used to shape the signals. Also, precision amplifiers, and detectors are used to amplify and detect the signals.
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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.
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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:
Given below are some important features available with a 8563EC Portable Spectrum Analyzer, 9 kHz to 26.5 GHz:
Note: The above specifications are given as an example only, and may not accurately represent the actual equipment specifications.
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The price is primarily determined by the frequency range (i.e. the range of operation of the instrument), the resolution bandwidth, and the frequency stability. Some instruments have additional options such as in-build tracking generator, frequency counter, or power meter that may also add to the overall cost.
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Given below are some of the sources to buy spectrum analyzers: