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Creating and using user defined source files with SPECTRASYS

Question asked by JERRY_MARKETOS on Apr 19, 2005
Spectrasys allows user defined sources in the form of amplitude and phase
at some frequency offset from a given reference.  It's done this way so you can specify
an arbitrary source frequency and have the table defined spectrum profile around that
frequency.  For further information, search the main HELP menu for "sources".

To start, some listing of (usually) amplitude as a function of frequency is needed.
Often, I begin by listing some datapoints or making a simple spreadsheet graph.
See attached bitmap of "charts_combined".jpg  The two charts show essentially
the same amplitude profile, and this is a good occasion to illustrate the need for
sufficient data points in the table file in order to properly outline the desired spectrum.

Custom source files are nothing more than text documents.  They are created from
within Genesys by following a prescribed set of steps (outlined below), and they
are saved as files with extension xxxx.src  in the "my documents\\my models" folder in Windows.
Custom source files can be opened and edited using Windows Notepad.

To create a custom source, do the following:

1) Run Genesys and construct a basic workspace with system analysis.
2) In the System analysis, "General" tab, sources section, click the "ADD" button.
3) From the next window that appears, click the button "New Custom Source with Phase Noise"
4) Supply a name for the file (it creates a file in your directory "My Documents\\My models", so this
file is always available for use in the future.)  If you need access to the file itself for copying to
a different computer, you can access it by right-clicking the windows "START" button and choosing
"explore", then looking under "My Documents", find the folder "My Models" and look in there.
5) After the name is defined, Genesys provides a sample source file from which you can modify
data according to your desired spectral profile.  Be sure to change the frequency units if appropriate !
(most often, the changes made are to frequency units setting and data within the source file.
Each line of data generally consists of frequency (then tab) then power in dBC (then tab) then phase.)
6) Then click the "OK" button as necessary to return to the original source definition.
This creates the xxxxx.src file in the "my documents\\my models" folder.
(the xxxx.src file can be opened with Windows Notepad)
7) Note above steps 1-6 involve creating the sourcefile from within Genesys.  You can also go to the "My Models"
folder and modify an existing sourcefile (using Windows Notepad), then do a "save-as" to new filename.

Now back at the source definition within the Spectrasys analysis, set the source frequency as desired.
The example workspace file I made has source set to 100MHz.

Source files were created as follows:
First source file having relatively sparse data for amplitude as function of frequency.
Second source file with denser data, illustrates the change that can occur simply due to source file density.
(The two sourcefiles above are what's shown in the Genesys bitmap image)

Note the first two source files allow the user to describe just one side of the spectrum, and this
is automatically mirrored to form a symmetrical profile.  It is possible to define an asymmetrical
spectrum, and this is shown in the third source file.  To define an asymmetrical spectrum, the FREQ entry in
the sourcefile is set to some nonzero value, then the data in the sourcefile is entered with both negative and
positive relative frequency values.  For example in the third source file shown here, I entered sourcefile
FREQ of 1MHz, then entered relative frequency values in the data section spanning from -1MHz through
zero (offset), up to +1MHz.
Note also when using a nonzero value of FREQ in the sourcefile to create asymmetrical spectrum, that same
value effectively becomes an offset to the center frequency defined in the System analysis (source tab).