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Modularized Spectrum Analyzer Wiki

Interested in owning your own GigaHertz RF Spectrum Analyzer ?
This site is dedicated as a Home Experimenter's Guide to building a Quality,
yet, Inexpensive 1000 MHz RF Spectrum Analyzer.

The original MSA mated with a laptop computer. Not pretty, but works quite well.

If you want to build your own Home-Brew 1000 MHz Spectrum Analyzer, you already have half of it built. That half is the computer you are using to read this. The processors and displays are very expensive portions of modern spectrum analyzers, and those functions can be replaced by your home computer. Your computer's monitor is the Spectrum Analyzer's display. Therefore, you can save and print spectrum plots, even in color if you desire. The only hardware to be constructed for the spectrum analyzer is the RF portion. The computer software is free to download and is written in Basic. It will operate on all Microsoft Windows platforms.

For previous visitors to this Site, you may notice that this Main Page has been significantly changed. This Main Page is called "Scotty's MSA Web Site". It has been modified from previous versions to simplify the navigation of my other web pages. This page will present some history of the Modularized Spectrum Analyzer and, more specifically, analysis of the Basic Spectrum Analyzer using Modular Construction. On this page, I will include links to two other pages that support the construction for the two versions of the Modularized Spectrum Analyzer.

One page is for the Original MSA version. It is a construction guide, to support the Original MSA. It includes more links to pages for the addition of a Tracking Generator and expansion of the Original MSA/Tracking Generator to a Vector Network Analyzer (MSA/VNA).

The second page is for the new SLIM MSA version. It is a construction guide, to support the SLIM MSA. It includes more links to pages for the addition of a Tracking Generator and expansion of the SLIM MSA/Tracking Generator to a Vector Network Analyzer (MSA/VNA).

Links supporting the MSA

Main Page for SLIMs. Explanation and links supporting the SLIM modules Link to Builder's Group for those interested in sharing ideas on the MSA. There are several people in the process of building the MSA and can offer suggestions and comments. This is a Yahoo Group page and you are welcome to join and contribute. Coaxial Cavity Bandpass Filter. A page for construction of the MSA bandpass filter. Step Recovery Diode frequency multiplier scheme that can replace LO 2 in the MSA spectrumanalyzer.bas Software for MSA. This is version 110 and will be updated for the new SLIM system as soon as possible. It will download as a text file. Change the name from spectrumanalyzer.txt to spectrumanalyzer.bas so that Liberty Basic will recognize it.

The History of the MSA

For years, I have wanted a Spectrum Analyzer in my home lab, but the expense of one, even used, was beyond my means. There are several good designs of home brew spectrum analyzers on the internet, but no single one would satisfy all of my requirements. I wanted an SA that would cover from 455 KHz to 1000 MHz. It needed a graphical display without using a separate oscilloscope. I certainly wanted the capability to store and print the spectral display. But most importantly, I wanted a frequency stable spectrum analyzer that would not drift over time and temperature.

Summed up, these were the Desired Requirements for a home-brew RF Spectrum Analyzer

Other Hardware Requirements

PC or Laptop Computer with LPT 1 standard parallel port. Windows 95 or later.

Monitor can be any size, but must be set for a minimum of 800 by 600 pixel resolution. More is fine.

I am using a Toshiba Satellite Laptop, 700 MHz Celeron.

Software Requirements

Liberty Basic 3.01 or more recent. I am not a software guru. I was famaliar only with HP Basic and Commodore Basic and this is very similar. Liberty is also very inexpensive, their trial version 4.0 is free. Go visit their web site at www.libertybasic.com.

The first spectrum analyzer

I built was called the SSA Prototype (SSAP). It used junk box parts that were connectorized. It worked fine and I decided to publish it on this web site. However, the components I used were expensive and hard, if not impossible, to find. I redesigned the Prototype and layed it out a single circuit board using common and inexpensive components.

I called this second spectrum analyzer, the SSA Board (SSAB). It also worked quite well, but restricted the builder to specific components. I thought a good compromise between the Prototype and Board Spectrum Analyzer would be suggested components on several seperate circuit boards. Each board could be connected together to create a Spectrum Analyzer. This way, the individul boards could be customized at the descretion of the builder, as long as the basic operational concept is maintained.

Therefore, the third spectrum analyzer, the Original MSA, uses simplified, modular building blocks. It is functionally equivalent to the SSA Prototype and the SSA Board. The modules can be built independently and mixed and matched, according to the builder's preference. Click here to go to the Original MSA Page. I show simplified schematics and suggested board layouts, which give the builder choices as to which modules he wishes to use. On that page, there are links to add a Tracking Generator and to expand the original MSA into a VNA. To support the original MSA, Cash Olsen created some PC Boards and some kits, available at www.zianet.com/erg.

The Modular System Evolves

After the original MSA had been published, I began receiving questions from potential builders as to what modules should be used. It seems that many builders do not like too many choices in modules. Therefore, I created an MSA topology with specific modules. I redesigned the modules for simplicity and lower cost integration. These modules became SLIMs, Standardized Laboratory Integration Modules. Go to this page to see the SLIMs.

So now, there is a forth MSA, the SLIM MSA. It is functionally identical to the original MSA, in that, it has the same Requirement Specifications, such as input frequencies, dynamic range, etc. However, there are some differences. The topology is different, in that, multiple original modules have been combined into a single modules, the SLIMs. It is operationally different, in that, the software can command the modules concurrently, allowing faster sweep times. It is electrically different, two ways. The power input to the SLIM MSA is +12 volts, allowing mobile or battery operation. The individual SLIM modules have lower input voltage requirements, +10 volts. Click here to go to the SLIM MSA Page. On that page, there are links to add a Tracking Generator and to expand the SLIM MSA into a VNA. The SLIM MSA is a blind design, that is, the design is complete, but I have not ordered the parts and pwbs to construct it yet. I have no reason to believe that any failure mechanism exists in this design, but I will report any errors as I construct it.

If you have already ordered original MSA parts and kits from Cash Olsen, continue with your construction. This new SLIM MSA effort does not obsolete my original MSA design. It is just a new way to do things, but, I recommend it for new builders starting from scratch.

I would like to thank Wes Hayward for his support in my early progress of creating this web site. Wes, W7ZOI, and Terry White, K7TAU are the creators of the very fine, Wesalyzer, which was the original design that led me to the creation of the MSA.

Analysis of the Basic Spectrum Analyzer

The following will describe the Signal Flow and and how Dynamic Range is determined in a basic spectrum analyzer. I will use the block diagrams associated with the Original MSA.

Block Diagram for 0 to 1 GHz, Basic MSA

Signal Flow in a basic Spectrum Analyzer

The signal (to be measured) is input to the I port of the first mixer (Mxr 1). The I port is used instead of the R port because it is much more responsive at very low frequency inputs. It is high side mixed with the first Local Oscillator (LO 1) to create the first intermediate frequency of 1013.3 MHz (1st IF=LO-RFin). This I.F. output, from the first mixer R port, is passed through the cavity filter to the R port of the second mixer (Mxr 2). The main purpose of the cavity filter is to attenuate the I.F. Image Frequency that will occur at 1034.7 MHz. The second mixer, mixes the 1st IF with the second Local Oscillator (LO 2) to obtain the final I.F. frequency of 10.7 MHz. This final I.F. is amplified, then filtered by the Final Xtal Filter. The Final Xtal Filter determines the Resolution Bandwidth of the MSA. Only one Final Xtal Filter is shown in the Block Diagram. However, several different bandwidth filters can be switched in and out of circuit to have a multiple resolution bandwidth MSA.

The final IF is sent to the amplitude measuring device, the Logrithmic Detector (Log Det). The output of the Log Detector (video) is a DC voltage corresponding to the log power of the incoming signal, and has a dynamic range of about 100 dB. This video voltage is converted to a digital data word by the action of the Control Board's Analog to Digital Converter (A/D) and the computer. The computer program converts the digital data word to a power level and is graphed on the computer's monitor in a window called Graph Window.

For the Spectrum Analyzer to be frequency agile, the 1st LO is changed by computer control and is able to change (step) in very small increments. A hybrid synthesizer, a combination of a Phase Locked Loop and a Direct Digital Synthesizer (DDS) is used to obtain these small steps. The DDS, with an output of approximately 10.7 MHz, is used as the "steering" clock for the PLL1/VCO1 combination. Since the DDS output can step in .015 Hz increments, PLL1 / VCO 1 will step approximately 1.5 Hz at the lower end of its' range (1000 MHz) and 3 Hz at its' upper end (2000 MHz) for every step of the DDS output.

The master clock is a 64 MHz, CMOS oscillator. It is used to clock the DDS, to provide a reference for PLL 2, and to provide a reference for the optional tracking generator. Since only one clock is used, precise frequency measurements can be maintained with software control. PLL 1, PLL 2, and the DDS are all controlled by the software routine, running on the home computer. The Control Board is the interface between the home computer and the RF hardware.

The frequency conversions outlined are a suggestion. Others frequency schemes can be used. The Final Xtal Filter can be any frequency between 9 MHz and 15 MHz. LO 2 can be any frequency between 1010 MHz and 1100 MHz.

Dynamic Range of a Basic Spectrum Analyzer

By definition, the dynamic range is the range of minimum detectable input signal to maximum input signal. I have specified the dynamic range of this MSA to be from -110 dBm to -20 dBm. However, several factors will determine the actual dynamic range. These factors are, mixer and filter losses, amplifier noise figure and gain, the Final Xtal Filter's bandwidth, and choice of Log Detector.

Here is a very simplified block diagram to look at, as the dynamic range is analyzed. Notice, there is no amplification added to the system.

Assume the Log Detector has a dynamic range from -90 dBm to 0 dBm. -90 dBm is the noise floor of the Log. Detector. 0 dBm is the input level to the Log. Detector at saturation.

Also assume that each mixer has -7 dB of loss. The first filter has -7 dB of loss and the final filter has 3 dB of loss. Since the total loss preceeding the Log Det is -24 dB, then the input dynamic range, at the input of the MSA, would be from -66 dBm to +24 dBm.

Since the maximum input to the first mixer can be no more than 0 dBm, a gain stage is added, to decrease the maximum input to the system and so that the full range of the Log Det. can be utilized.

The following simplified block diagram adds amplification.

Adding the amplifier, with a gain of 24 dB, will allow the input to have a dynamic range from -90 dBm to 0 dBm. However, the amplifier will also, add broad band noise to the system. The following will calculate how much noise power is generated by the amplifier:

Using the noise formula : -174 dBm/sqrt Hz +3 +24 = -147 dBm/sqrt Hz. This is the noise power at the output of the amplifier, measured in a 1 Hz bandwidth. The +3 in the formula is the noise figure of the amplifier. The final filter will limit the amount of noise power entering the Log Det. The following will calculate how much noise power is passed through the filter:

Total noise power (dB) = noise + 10logBW (Hz), where BW is the bandwidth of the filter.

The following will show how different filter bandwidths change the total noise power:

-147dBm + 10 log .5 KHz = -120 dBm (minus final filter loss of 3 dB) = -123 dBm

-147dBm + 10 log 2 KHz = -114 dBm (minus final filter loss of 3 dB) = -117 dBm

-147dBm + 10 log 15 KHz = -105.2 dBm (minus final filter loss of 3 dB) = -108.5 dBm

For any of the above 3 filters, the total noise power is below the -90 dBm noise floor of the Log Det and will not interfere with signal measurement. The spectrum analyzer will have a dynamic range of -90 dBm to 0 dBm, free of added broad band noise. However, even though the first mixer will tolerate a 0 dBm input signal, it is subject to high intermodulation distortion (IMD) at this power level. It would be better to have a maximum signal of -20 dBm (to decrease IMD). Therefore, increase the gain of the amplifier from +24 dB to +44 dB.

The input dynamic range now becomes -110 dBm to -20 dBm. The 20 dB gain increase will also increase the total noise power by 20 dB.

• 5 KHz filter noise power increases from -123 dBm to -103 dBm. 2 KHz filter noise power increases from -117 dBm to -97 dBm. 15 KHz filter noise power increases from -108.5 dBm to -88.5 dBm. This filter bandwidth will bring the noise power into the dynamic range of the Log. Det., but only 1.5 dB above the noise floor of the Log. Det. This would be considered minor interference.

When using narrow band filters, the spectrum analyzer will have a dynamic range of -110 dBm to -20 dBm, free of added broad band noise. When using a wide band filter (15 KHz filter), the spectrum analyzer will have a dynamic range of -108.5 dBm to -20 dBm, due to the minor amount of broad band noise contribution of the final I.F. amplifier.

A little about me: My name is Scotty Sprowls. I am a retired RF Design Engineer from E-Systems / Raytheon. Although I am not an Amateur Radio Operator, I do repair radios as a hobby. My frustration in tuning cavity filters in diplexers for a couple of Hams caused me create this Spectrum Analyzer to aid me. You can get in touch with me, via email, at wsprowls(at)yahoo.com

I will try to answer your questions or comments as soon as possible.