Spectrum-analyzer Measurements

An Introduction to Spectrum Analyzers

1. What is a Spectrum Analyzer?

A spectrum analyzer is a wide band, very sensitive receiver. It works on the principle of "super-heterodyne receiver" 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.

2. Spectrum Analyzer Vs. Oscilloscope

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).
Spectrum analyzer is useful for analyzing the amplitude response of a device against frequency. The amplitude is normally measured in dBm in Spectrum Analyzers, where as the same is measured in volts when using Oscilloscopes.
Normally, Oscilloscope can not 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).
The spectrum analyzer measurements are in frequency domain, whereas the oscilloscope measurements are in time domain.
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.

3. Simplified Block Schematic of Heterodyne Receiver:

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
Voltage Controlled Oscillator
Sawtooth Generator
IF Amplifier
Detector
Video Amplifier
Cathode Ray Tube (CRT)

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.

4. Key Features to Consider When Buying a Spectrum Analyzer:

Resolution bandwidth
Frequency range
Frequency stability
AC/DC Operation
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.

5. Spectrum Analyzer Applications:

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:

Return-loss measurement
Satellite antenna alignment
Spurious signals measurement
Harmonic measurements
Inter-modulation measurements

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

Color display
Continuous 30 Hz to 26.5 GHz sweep
Fast digital resolution bandwidths of 1, 3, 10, 30 and 100 Hz
Adjacent channel power, channel power, carrier power, occupied bandwidth percentage and time-gated measurements standard
Precision timebase and 1 Hz counter resolution
MIL-PRF-28800, Class 3 rugged
Measurement personalities for digital radio and phase noise measurements
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.

6. Spectrum Analyzers that you may consider buying:

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.

Beginner-range:
1. Avcom PSA Series
2. BK Precision 3.3 GHz Spectrum Analyzer
3. Instek 2.7 GHz Spectrum Analyzer

Mid-range:
1. Anritsu Handheld 7 GHz Spectrum Analyzer
2. Anritsu MS2681A Spectrum Analyzer (9 kHz to 3 GHz), High Performance
3. R&S 6 GHz Spectrum Analyzer

High-end:
1. Agilent ESA-L Spectrum Analyzers
2. Agilent ESA-E Spectrum Analyzers
3. Anritsu MS2668C Spectrum Analyzer (9kHz to 40GHz)

7. More Information and places to buy spectrum analyzers:

Given below are some of the sources to buy spectrum analyzers:

www.tessco.com
www.testmart.com
www.sigview.com