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1986-11-21
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INTERFER.BAS DOCUMENTATION
ANALYZING RECEIVER INTERFERENCE
Receiver interference caused by cross-modulation products can be
extremly difficult, if not impossible, to analyze without the help of a
computer. This type of interference is often generated by non-linearity,
which causes the mixing of sum, difference and/or harmonics of various
signals.
In an environment with a large number of communication and broadcast
transmitters, the potential for interference to an STL receiver is likely.
The cause may be difficult to trace, particularly if the offending
transmitters are only on the air intermittently. Using a calculator in a
complex multi-user site could take days, or even weeks.
A better approach relies on the use of a computer and a program that
will analyze the various combinations of frequencies. These tools will
enable the engineer to determine the most likely cause for current or
potential interference.
Audience complaints about receiver interference can be a frequent
source of irritation to the engineer. These complaints are often passed
off as receiver inadequacy when in reality they may be because of spurious
emission. Running an interference analysis on these complaints will
certainly remove any doubt.
THE PROGRAM
The analysis program is written to run on Microsoft Basic and is IBM-
PC compatible. If you manually key in the program, be careful to follow
the listing. Any errors in entering the program will, of course, affect
the results. There is no quarantee that the program will work with other
versions of Basic. It is likely, however, that it can be modified if you
are familiar with your particular version of Basic.
The program begins by gathering transmitter frequency and bandwidth
data until terminated by the operator by entering a zero frequency.
Receiver information is then requested. The receiver's IF frequencies and
bandpass must be known. The program requires that data be entered in
megahertz. The line printer then prints all the entered test data
followed by the interference analysis report.
The frequencies are paired in such a way that each signal is tested
with each other frequency in the database. The frequencies are calculated
to their extremes based on bandwidth, then are added, subtracted and
multiplied by the harmonic number.
The result is 27 test frequences to be compared with 14 receiver
parameters. The receiver parameters are frequency extremes based on
bandwidth. The receiver frequency, first and second IF, first and second
IF image, and both local oscillators are used as receiver test parameters.
The computer must make a total of 400 checks on each pair of frequencies.
The test is made within the five levels of nested loops. Line 222 is th
heart of the program and tests for an overlap of the low and high receiver
parameter with the high and low frequency parameter.
Don't expect an instant solution from even the fastest computer.
Because of the large quantity of calulations performed, a 4Mhz
microprocessor will require ((n/2)*(n/2))/650 hours to complete the tests.
A test of a site with 60 transmitters requires approximately two hours. A
test of 10 requires only about two minutes.
USING THE PROGRAM
Enter all the transmitter frequencies that are in close proximity to
the problem receiver along with their associated bandwidths in megahertz.
Enter a zero frequency when finished and the screen will request receiver
information. Enter the receive frequency, both IF frequencies and the
bandpass of the receiver, all in megahertz. If a single conversion
receiver is involved, use a zero in the second IF prompt.
The printer will then begin printing an analysis. If you have
entered a large number of transmitters for analysis, plan on having the
system tied up for a few hours. Be sure to enter data for all
transmitters at the site; their relationships must be checked at the same
time.
RESULTS
Study the printout carefully, considering the power levels, possible
strength of the harmonics and proximity to the receiver antenna. The
suspected offending frequencies should then be tested by keying the two
frequencies simultaneously and carefully monitoring the receiver output.
A spectrum analyzer would be helpful at this point, but not
mandatory. In high-level RF environments, however, a low-level signal may
often be masked by the noise level. This is especially true if the exact
frequency is not known. If a pair of frequencies are proved to cause
interference, a trap may be required. Frequently, an ac power-line filter
will prevent interference without the use of expensive trps. Some STL
receivers have ineffective or non-existent EMI filters. Ground loops in
the receiver coaxial cables are also frequent sources of problems.
Relocating the receive antenna can solve some problems, but this approach
is often impractical.
As broadcasting becomes more competitive, the need for technical
excellence is of the utmost importance. The pressures on the station
engineer to provide that extra measure of excellence may require the use
of new tools, such as the computer. In this particular example, you can
see that the application of a computer to a relatively complex problem was
actually quite simple. The key was in letting the computer perform the
large number of calulations for you. There may be other tasks where the
computer can help. Look around your station and see what the computer can
do for you in your job.
Re-printed by permission of Intertek Publishing Corporation.he most likely cause for current or
potential interference.