Modularized Spectrum Analyzer Web Site Interested
in owning your own GigaHertz RF Spectrum Analyzer ?
This site is dedicated as a Home Experimenter's Guide to building a
yet, Inexpensive 1000 MHz RF Spectrum Analyzer.
original MSA mated with a laptop computer. Not pretty, but works
Site was Started Jan. 02,
Updated this Main Page, Sept. 1, 2007
your own Home-Brew 1000 MHz Spectrum Analyzer, you already have
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
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
Here are more links supporting the
Main Page for SLIMs.
Explanation and links supporting the SLIM modules Link
to Builder's Group for those interested in
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 Operation Page
Operating the MSA, including instructions for downloading the software.
Brief History of the MSA.
For years, I had wanted a Spectrum Analyzer in my home lab, but the
expense of one, even used, was beyond my means. There were
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. I wanted a graphic
without using an oscilloscope. I wanted the
capability to store and print the spectral display. But most
importantly, I wanted my spectrum analyzer to be frequency and
magnitude stable over time and temperature. Summerized,
these were the Requirements for my home-brew RF Spectrum
Topology To keep
the self generated spurs and IMD to a minimum
Frequency of Operation
455 KHz to 960 MHz
than 10 Hz
-80 dBm, the lowest signal input to be
Range 70 dB
1 dB or better
KHz or 30 KHz
Better than -85 dBc/Hz, @ 10 KHz from
In-Band Image Rejection better than -70 dBc
Two tone, better than -60 dBc
Less than $200 (USA), plus my junk box collection Other Hardware Requirements:
PC or Laptop Computer with LPT 1 standard
port. Windows 95 or later.
Monitor could be any size, but I expected to
use 800 by 600 pixel resolution.
I am using a Toshiba Satellite Laptop, 700 MHz
Celeron. Software Requirements:
Cheap or free. I am not a
software guru. I was
famaliar only with HP Basic and Commodore Basic. Liberty Basic is
very inexpensive, their trial version
free. Go visit their web
site at www.libertybasic.com.
first spectrum analyzer I built was called
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
(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
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
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
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
Olsen, continue with your construction. This new SLIM MSA effort
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,
which was the original design that led me to the creation of the MSA.
Analysis of the Basic Spectrum
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
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
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
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
Logrithmic Detector (Log Det). The output of the Log Detector
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
small increments. A hybrid synthesizer, a combination of a Phase Locked
and a Direct Digital Synthesizer (DDS) is used to obtain these
steps. The DDS, with an
output of approximately 10.7 MHz, is used as the "steering" clock for
PLL1/VCO1 combination. Since the DDS output can step in .015
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 :
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
Here is a very simplified block diagram to look at, as the dynamic
range is analyzed. Notice, there is no amplification added to the
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
-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 =
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
The following will calculate how much noise power is passed through the
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:
log .5 KHz = -120 dBm (minus final filter loss of 3
dB) = -123 dBm
log 2 KHz = -114 dBm (minus final filter loss
of 3 dB) = -117 dBm
log 15 KHz = -105.2 dBm (minus final filter loss of 3 dB) =
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.
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,
I will try to
answer your questions or comments as soon as possible.
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