Copyright ©1997, RF Specialties Group.
Last issue, Bill Turney with our Florida office discussed multipath. We received
comments back from many of you offering good advice. John Wilson with Seiko Communications
made a few good points about transmitter tuning. He also discussed SCA injection
levels, and offered clarification: "73.319 allows for a total of 20 percent SCA
injection, but limits injection to 10 percent for frequencies above 75kHz." Thanks
John. Some of you might be interested in knowing that modern SCA technologies
such as his company’s High Speed Data System (HSDS) allow for increased immunity
to multipath-induced crosstalk by "whitening" the SCA data modulation.
This issue, I will expand on one possible cause for what may sound like multipath: AM noise. How do you measure AM noise accurately? You might say to yourself, "That’s easy. I’ll have someone watch the AM noise at my studio monitor while I adjust the transmitter for an AM noise null." Unfortunately, AM noise cannot be read from a studio monitor with any certainty of accuracy and a monitor located at the transmitter probably will not indicate an accurate AM noise null, either. The detectors for these monitors are located at the end of a coaxial cable rather than at the transmitter output and the VSWR on this cable will corrupt the sample.
In order to obtain a clean, accurate reading, the AM noise detector must be located directly at an RF sample port, which must be after the harmonic filter. Radio Design Labs’ ACM-2 AM noise monitor has a detector which is intended to be located external to the monitor, thereby eliminating this problem. Simply add a single-socket line section after the harmonic filter and a Bird sample element. Attach the ACM-2 detector and you are now ready to read, and ultimately reduce, AM noise.