HAM Radio 1/HamRadio.cdr/freq/interfer/interfer.doc

Text File | 1986-11-21 | 6KB | 98 lines

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.