- It is not possible to store the displayed spectrum in a file for later computer processing.
- Getting a hard-copy of the displayed spectrum is not an easy job: Dedicated cameras are scarce and expensive and X-Y plotters cumbersome.
In order to solve these two points, I have recently designed and built a simple interface to connect my spectrum analyzer to a PC. This unit, named SAN2PC, digitizes analog spectrum data, then processes and transmits that data to a Personal Computer via the RS232 line.
SAN2PC automatically takes care of the spectrum analyzer scan time.
Any terminal program can be used to receive and store RS232 data in a file. This file can be processed afterward, or simply read by any spreadsheet program: The spectrum can then be displayed on the PC screen and a copy printed if needed.
SAN2PC has been tested on my HP141T analyzer, but I think it could be used with other models and brands, insofar as they have analog spectrum voltage and pen lift signal output connectors. Limited tests have been conducted on a Tektronix 2755P.
I have also written a dedicated software (in the Python language) displaying not only the spectrum, but also the spectrum analyzer's settings (center frequency, scan width, bandwidth, etc...) on the PC screen. This picture can be stored in a file for future use or printed. The program runs under Windows XPTM and has been tested under Linux (the Python software I developed is not limited to the WindowsTM world).
The
heart of the circuit is a
PIC 18F2525 processor associated with a simple analog interface : Click here to get
the schematic
diagram
SAN2PC is connected to the Vertical Output and Pen Lift Output
connectors of the S.A (most analyzers of the kind we are
talking
about have such output lines, usually intended for connection to an X-Y
plotter).
As soon as the Pen Lift Output is active (generally a high to
low
transition), ie, during the S.A sweep time, the Vertical Output signal
is sampled by the PIC internal Analog to Digital Converter
until
the end of the Pen Lift active state. The sampling period is
automatically computed by the PIC from the Pen Lift duration.
About 1000 samples are stored for each S.A sweep time and then
transmitted to the PC via the RS232 line.
-
Remark : This 1000 samples
number leads to an operating limit : when the ratio of the analyzer
settings K
= TotalScanWidth/Bandwidth
is
lower or equal to 1000, the RF level measured by SAN2PC is OK. When K
becomes larger than 1000, the probability of sampling outside the
spectrum amplitude peaks increases, thus leading to amplitude
errors.
March
4th, 2009
:
A new
software, SAN2PC rev 2.00,
with more than 3000
samples/scan, is now
available Click
here for more information
Construction
:
- the circuit
is built on a dual sided PCB, the upper (components side) remaining
unetched while
the lower (solder side) is etched using the artwork mentioned. The
upper side is thus used as the ground plane (see the above inside
picture). As most components pins must be
insulated from ground plane, I use a large drill (6 mm or more) to
remove
copper around holes not connected to ground.
The photo below
is a close-up of two insulated
holes (upper part of the picture) and a grounded hole (lower part).
==> The PCB and pre-programmed PIC are available
from Far
Circuits
- the file san2pc_gnd_holes.png
shows the "grounded holes" and the two wire straps (in red).
- all components are regular ones
except for R7, R8, R13, R14, R16 and C13, C14, C15, C16, C36, C37 which
are SMDs and are soldered on the lower side of the PCB.
- the DC/DC converter (TMA0512D) is a low cost Single In Line Package
made by "TRACO POWER", available from Farnell,
RadioSpares
or other distributors.
You could use an other type, but be sure that the pinout is the same :
on some SIP converters, pin 4 (-Vout) and pin 5(Gnd) are swapped!
==> As mentioned in the QEX
article, if the Vertical Output
voltage
of
your analyzer is unipolar (positive or negative only), you could use a
TS272 op amplifier (or
equivalent), powered by a single supply (+6.6V), instead of
the
TL82 and omit the DC/DC converter U7, the negative regulator U3 and its
related components and the protective diode D7. Don't forget to connect
U4A pin 4 to the ground plane.
Download
the PCB design
(san2pc_w3c.pdf) by clicking here. (This is a top side view)
The
components layout is
shown (san2pc_w3c_lay.pdf)
in this PDF file.
- An
equivalent DC/DC converter available
from Digikey
(part number is 102-1340) can be used instead of the TMA0512D :
it is the V Infinity VASD1 S5 D12
.
Important note :
Buck KC2HIZ tried it but had to
use
1 uF in place of the 10 uF capacitors C1 and C2
connected at the +
and - 6.6V converter outputs.
Thank you for the information Buck.
- Joop, PE1CQP points out that the MAX232 needs 1uF capacitors : he is right. The chip I use in my own SAN2PC is a MAX232A and not the MAX232 shown in the schematic diagram. The MAX232A works well with 0.1uF caps. Thanks Joop.
2.1.1 The PIC
embedded
software.
The
PIC program, named san2pc.c,
is written in C language for
easier development and debugging. I used the compiler proposed by Custom Computer
Services (CCS).
This compiler is easy to use and a lot of useful tools, as
well as
sample programs, are available as part of the package.
Programing the PIC was done with an home made programmer inspired by
the ProPIC2
and using the WinPIC800
software.
==>
The
1000 points software
revision is
san2pc112.hex
: click
here to download it and Click
here to
download the SAN2PC User's
Guide (rev112_2).
2.1.2 A new improved SAN2PC rev 2.00 is now available (March 2009).
As
above-mentioned, the 1000 samples
number leads to an operating limit, when the ratio of the analyzer
settings K
= TotalScanWidth/Bandwidth
is larger than 1000.
We can
increase the samples number to mitigate this issue : however we are
limited by the size of the PIC internal SRAM (#3900 bytes) and by the
data transmission time from
SAN2PC
to the associated computer.
Thus I modified the SAN2PC file format as
follows :
- The samples number is now around 3000 points, compatible with the
available PIC internal memory.
- There are no more any frequency information associated with each
sample : each frequency is computed by
the Drawsan program from the start and stop frequencies found in the
header data : we thus save a lot of transmission time.
- The amplitude data for each frequency is now the A/D generated byte
and not the RF power
in dBm, thus saving transmission time too.
- The file format being different of the one generated by the first
revision SAN2PC (e.g 1.14 and earlier) , Drawsan must have a way to
know if
the file processed is a new or an old type. The header of the
new
data files is preceded by the figure '2' to point out that it
has
been
generated by a rev. 2.0x SAN2PC software.
- Because of the larger samples number, it is necessary to select a
Spectrum Analyzer ScanTime/div
equal or greater than 10 ms
before running SAN2PC.
Click here to download the new 3000 points san2pc200.zip file and Click here to download the new SAN2PC User's Guide rev 2.00_2
2.2.1 The PC plotting software.The
plotting program named Drawsan
relies on the header
of the file sent
by san2pc
via the RS232 line
and stored on the PC hard disk : this header includes main analyzer
settings (Reference Level, Center Frequency, Scan Width,...). I chose
to write it in the Python language
mainly because of the availability of a first class graphics library,
Matplotlib available
from the Sourceforge site.
This library emulates MatlabTM
commands (but does not
require MatlabTM)
and creates very nice plots. The drawsan
user has the opportunity to
save the picture as a png, pdf or other formats file.
==> The
1000 points revision is drawsan114.zip (click here to
download it).
- I would like to acknowledge Joe HOPSTER WB8DNO who wrote
the interpolated frequency computation .
Important
note : Before using
drawsan.py, you must install Python of
course, but also the Matplotlib and
Numpy libraries.
==> A
Python
Installation and Drawsan user's Guide
(rev 2.2) is available (Click
here to download)
2.2.2 The new Drawsan rev. 2.00.1 (April 2009).
This
new version works only with
3000 points files created by the new SAN2PC
rev.
2.00 and the user is warned
if
he tries to process an old file type with this new Drawsan.
Click here to download the drawsan200_1.zip file
April 2010 : Contribution of Buck KC2HIZ : He wanted a way to perform several captures on the HP141T and then process them in a batch, without having to specify file names and 'save as.'Thus he modified the Python source code and gave me the authorization to propose his work (Drawsan 2.01B) to other interested people. His batch file is included in the zip file. Thank you very much Buck !!! Click here to download his Drawsan201B-Batch.zip
==> Your comments about the new SAN2PC and Drawsan programs are welcome !
Comparison
: The
pictures below show the same signal (Analyzer settings : Total
Scan Width = 50MHz and Bandwidth = 30kHz , thus K # 1666)
processed by the new SAN2PC 2.00/Drawsan 2.00 programs (on
the left) and by the old ones (on the right). With the new method, there is no amplitude
underestimation compared to that observed on the
right (the first and second peaks on the right picture are 2 dB lower,
while the
third is 5dB lower, etc...).
February
2009 : Important
note about Python
2.6 : a
new version of the Python language (V.2.6.x) is
available since the beginning of 2009.
Unfortunately, the old version of Numpy (V.1.2.x) used with
Matplotlib doesn't work with Python 2.6.x
Thus please, don't use Python
2.6.x until
the new
Numpy is available !
A Matplotlib tutorial is available at : http://matplotlib.sourceforge.net/users/pyplot_tutorial.html
In order to check that your Python, Matplotlib and Numpy are ready to go, I strongly suggest you download (click here) the "A simple program" file and use it before running drawsan.py.When this little program works, you can start drawsan : if you don't have any spectrum data file, you can download the test1.zip file.
Sample pictures of spectra plotted with drawsan.py are shown below (click on the pictures to enlarge). The analyzer settings are automatically displayed in the upper part of the picture.
-
QEX
article (2.3
Mbytes PDF).
-
Schematic diagram.
-
PCB artwork.
(san2pc_w3c.pdf)
-
Components
layout.
(san2pc_w3c_lay.pdf)
-
Grounded
holes and wire
straps. (san2pc_gnd_holes.png)
- PIC
Hex file.
(san2pc112.hex) (Nov. 26,
2007)
- SAN2PC
User's Guide
rev 1.12_2 (==> updated : Jan. 29,
2010)
- New SAN2PC200.hex PIC zip file. (March 2009)
- New SAN2PC User's Guide rev
2.00_2 (==> updated : Jan. 29, 2010)
- How to
install Python
and use Drawsan Guide (rev. 2.2) (450 kbytes PDF)
-
Drawsan114.zip
Python program.
(April
19, 2008)
- Python program Drawsan200.1 zip file (April 27, 2009)
- New : Drawsan 201B_Batch.zip
courtesy of Buck KC2HIZ (April
30, 2010)
-
Simple program Python test file
-
Test1.zip file
4. FAQ
Not available now.
This simple interface may help to solve the hard-copy problem associated with many secondhand spectrum analyzers. It also provides the user with a file for subsequent computer processing.
It could be probably used with others equipments, like network analyzers, that don’t have any printing function but only analog output connectors.