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Sunday, 23-Jun-2013 12:55:38 PDT
Authored by Ian C. Purdie VK2TIP
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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.
The first thing I sincerely recommend you do is buy this excellent reference book through my amazon affliate program. Introduction to Radio Frequency Design - Wes Hayward W7ZOI - highly recommended. Paperback and published by ARRL - delivery 3 to 5 weeks. Wes Hayward probably taught me 30% of what I know in electronics - an even better reason.
Why do I personally highly recommend this book? Wes Hayward is the sharpest designer I know and is highly respected as the "guru" and I don't make strong recommendations lightly. At the low price it is literally among the cheapest investments you will ever make in your electronics education - believe me, even if you are not especially interested in radio frequency design you will learn a lot of basic principles which are universal in electronics. And you will write and thank me for doing us both a favour.
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.
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 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. You can purchase your copy here.
Wes also wrote "Experimental Methods in Rf Design" which you can also purchase your copy here.
UPDATE - 26/1/09 Bill Kelsey N8ET has advised me "First - As you may know - I was flooded a bit over a year ago and lost all my Kanga US inventory. I am slowly recovering, and part of the process is re-building my web page. I don't have the SA stuff back on-line yet (but it will be). The best solution right now is to give you my main URL, and it will be obvious how to get to the SA stuff when I get it back on-line". Bill's new site is Kanga US.
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. Wes tell's me: "The basic file that you referenced is there, but has been converted to a PDF. It has some recent updates in it that should be useful to the builders. There are other things in my site that should be of interest to the SA folks, especially a new 110 MHz bandpass filter design. That is found in http://w7zoi.net/mixed-bag/mixed_bag.html"
4. An example of the Spectrum Analyzer project constructed by N8QOH, including photographs.
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Updated 29th December, 2000