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SPECTRUM ANALYZER

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What is a Spectrum Analyzer?

A RF Spectrum Analyzer is a very special kind of superhetrodyne receiver which receives a chosen range of signals and displays the relative signal strength on a logarithmic display, usually a cathode ray oscilloscope - CRO. In a dedicated Spectrum Analyzer the CRO display is incorporated into the instrument itself.

Presented here is a review of a project presented by the masters themselves, Wes Hayward, W7ZOI, and Terry White, K7TAU. This Spectrum Analyzer project was featured in the magazine QST during August and September, 1998.

This Spectrum Analyzer project is available as a relatively inexpensive kit and is highly recommended as probably the best piece of test equipment you will ever own. Professional Spectrum Analyzers usually cost tens of thousand of dollars and for good reason. Refurbished second hand units frequently cost $10,000+.

Thanks to the skills of Wes Hayward and Terry White, this Spectrum Analyzer project costs a mere fraction of those prices and sacrifices very little in performance.

Basics of a Spectrum Analyzer

As I said earlier a Spectrum Analyzer is simply a specialised superhetrodyne receiver which usually employs a multiple mixing scheme to achieve reception of the higher frequency signals. This Spectrum Analyzer project has an upper frequency limit of 70 Mhz although this can be extended by usng converters. It is a double conversion scheme.

The general idea is to slowly sweep through a range of desired signals, all the while converting them to a much lower frequency where they are individually (within practical limits) converted to a DC level representing a logarithmic or decibel level for display on the cathode ray oscilloscope - CRO. Here meaningful comparisons and information about the signals can be discerned. Indeed a Spectrum Analyzer is a very poweful piece of test equipment.

The Wes Hayward and Terry White Spectrum Analyzer project

The first IF is set at 110 Mhz, the swept first oscillator, 110 - 180 Mhz, utilises a commercial voltage controlled oscillator. The second IF is fixed at 10 Mhz. Quoting in part from Wes Hayward's and Terry White's article:

"The RF spectrum analyzer is essentially a swept receiver with a visual display. The display shows the strength of all signals within a user-defined frequency span. Each signal is represented by a line or blip that rises out of a background noise, much like the action of an S meter. Commercial analyzers are calibrated for signal power, with all signals referred to a reference level at the top of the screen. Our analyzer is designed for a basic reference level of –30 dBm, a common value in commercial analyzers."

A very important feature, in my humble opinion, about this article is the general discussion on aspects of receiver design which apply in all instances and, some very sage advice is offered throughout the article, in particular:

"We strongly discourage building the entire analyzer before testing specific sections. Such an approach may work for casual kits, but is not suitable when careful control of signal levels is required. That approach also robs you (the builder) of the excitement of the process: the learning that comes from detailed examination."

"Before jumping into the circuit details, we reemphasize that this analyzer although simple is intended for serious measurements."

After double conversion, along with gain equalisation to compensate for losses in the filters and double balanced mixers, this Spectrum Analyzer project has the main gain (as it should be) at the lower IF of 10 Mhz as well as after the logarithmic detection.

Extensive filtering is provided throughout the whole Spectrum Analyzer project with both multi-pole resonator filters and a crystal filter. Broad banding techniques are employed in the critical stages of amplifier design.

Wes Hayward W7ZOI, is the very highly respected author of a number of technical publications including "Introduction to Radio Frequency Design". This is a book I thoroughly recommend with absolute confidence and I consider it relatively inexpensive through my Amazon affiliate program at $US30.00. You can purchase your copy here.

The Spectrum Analyzer project kit

A complete kit of parts for the Spectrum Analyzer project, costing $US 192.00 + S/H is available from Kanga.

Alternatively, if you prefer you can just buy the PC boards for $US 42.00 + S/H. Also available is the later (QST - November, 1999) companion tracking generator kit.

The Spectrum Analyzer Project

1.  A complete PDF reprint of the QST - August, 1998 article of the Spectrum Analyzer project: PDF - 386 kB and;

2.  A complete PDF reprint of the QST - September, 1998 article (Part 2) of the Spectrum Analyzer project: PDF - 192 kB

3.  Updates, improvements and additions to the Spectrum Analyzer project may be had at Wes Hayward's site.

4.  An example of the Spectrum Analyzer project constructed by N8QOH, including photographs.

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Related topics on Spectrum Analyzers

band pass filters

low pass filters

narrow band filters

decibels

superhetrodyne receivers

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the author Ian C. Purdie, VK2TIP of www.electronics-tutorials.com asserts the moral right to be identified as the author of this web site and all contents herein. Copyright © 2000, all rights reserved. See copying and links. These electronic tutorials are provided for individual private use and the author assumes no liability whatsoever for the application, use, misuse, of any of these projects or electronics tutorials that may result in the direct or indirect damage or loss that comes from these projects or tutorials. All materials are provided for free private and public use.
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Updated 29th December, 2000

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