The Spectrum Analyzer:
An Explanation of Functions and Practical Applications
 
 Definitions
 Audio / P.A. System
 Wireless Microphones/Communications

Recently, a customer requested a "Spectrum Analyzer" to be rented along with a large show package including video monitors, wireless microphones, VTRs, and wireless communications.  The Spectrum Analyzer that was delivered was not the type that the customer needed.   The customer should have been more specific, however, once you read this, you as the Account Executive, will be in a position to ask the customer for more information.  The additional information would have eliminated the confusion.

Definitions:top

Spectrum:
n.A range with defined limits containing particular items with characteristics in common.
adj.Limiting or defining the extremes

Analyzer: n. A device used to seperate into constituent elements so as to elucidate the interrealtion of the parts and the nature or significance of the whole known item or group of items. 

Examples of the Two Types of Spectrum Analyzers
   
Ivie Industries 
Hewlett Packard
Audio / P.A. System: top
The Ivie IE-30 is the industry standard, hand held Audio Spectrum Analyzer. Bexel has these for rent. Years ago, before technology was small enough to build something like the IE-30, acoustic analysis came in the form of a single VU-meter device that could measure only one frequency at a time. To analyze an entire spectrum (low to high), single frequencies were generated by he sound system and the analyzer was set to measure the single frequency. To generate a plot of the room, as many as thirty frequencies were individually generated and measured. This was very slow, and prone to error. The greatest short coming of this method was, however, that the relationship between frequencies could not be identified because only one frequency was ever measured at a time. When true Spectrum Analyzers were developed, the ability to analyze all frequencies at one time was referred to as "Real-Time-Analysis", and hence the audio industry nomenclature of "RTA" to describe the device. RTA's use a test signal called Pink Noise. This broad-band signal contains a collection of many frequencies as does speech or music program material. The combination of frequencies allows the engineer to determine how the acoustics of the venue would effect the program material. The IE-30 kit comes with an external noise generator. Most larger PA audio consoles have a built in noise generator.

RTA's are used to analyze "Acoustic" energy. Acoustic energy generally comes from two sources:
(1) Ambient noise such as air-conditioning rumble, or machinery
(2) Sound system generated energy either as music or a calibrated test signal

In addition to analyzing the volume of individual frequencies, the Real-Time-Analyzer can also indicate Sound-Preasure-Level or SPL, which is used by OSHA and other monitoring organizations for industrial and entertainment health purposes.

Wireless Microphones/Communications:top
The Hewlet Packard Spectrum Analyzer shown above is the type that the customer required. Many of the wireless microphones that Bexel rents are frequency agile. This means that the transmitter can be switched to more than one carrier frequency. The details of wireless and Radio-Frequency operation is outside the scope of this article, however, some discussion is required to explain the use of the device.
Unlike acoustic energy, Radio Frequency energy is NOT dependent on air for the transfer, or propagation of the energy over distances. Likewise, an audio spectrum analyzer uses a microphone to "sense" the measured signal, while the RF-Spectrum Analyzer uses an antenna that is appropriate to the "range" of frequencies to be measured. At an event or show that has a lot of frequencies in both the VHF and UHF ranges, the analyzer gives a "picture" or display of all the RF carriers that are active. More important, however, as the analyzer is presenting "Actual Utilization of Available Spectrum", it indicates which frequencies are NOT in use. These available areas are where the customer can set the switches on the agile transmitters. Without the analyzer, it is anyone's guess which frequencies are available. More detail

In addition, it is important to understand that a single frequency has artifacts called Harmonics. These harmonics manifest themselves as additional RF energy at unique mathematically determined frequencies. These harmonics can be on another receiver's frequency, causing interference.
To conclude, when a customer asks for a "spectrum analyzer", ask if it is for audio or RF purposes.  


Some synonyms for the audio analyzer are:
(1) RTA (what it does)
(2) IVIE (brand name)
(3) SPL meter (alternate use)
 

Some synonyms for the RF Spectrum analyzer are:
(1) Service Monitor (a device that includes a spectrum analyzer in addition to other tools)
(2) IFR (brand name)
 

 
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An Introduction to Radio Frequency Dynamics
 Re-printed with permission by Brad Adams, Communications Service Engineer, The Nashville Network.

Intermodulation, or the "mixing" of two or more RF signals can be a tremendous source of problems for the wireless engineer. When two or more signals "mix" the by-product of this mix WILL BE the sum (f1+f2) and the difference (f1-f2). So, for example let's say that we have two frequencies in operation. Frequency #1 = 513.300 mHz ( a popular wireless mic frequency) and Frequency #2 = 684.500 mHz (a common wireless intercom or wireless mic frequency). So when these two frequencies are operating we will see both of the fundamental frequencies (F1 & F2) and also we will see the sum, 1,197.800 mHz and the difference, 171.200 mHz. Now let's say that we have a wireless IFB operating on 171.200 mHz.........we have a problem! Mixes can occur in receiver front ends, transmitter output stages and even on a corroded screw or bolt in the metal framework of a sound stage. Our example uses only two fundamental frequencies. In a given television production such as an awards show, there may be as many as 50 to 60 wireless frequencies in use! Take some time and do the math on that one! The number of mixes that can occur increases dramatically as more frequencies are placed in sercvice. For this reason, care must be taken to provide adequate physical separation of wireless microphone transmitters and intercom systems and to properly coordinate all RF frequencies in use at a given venue.