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A few comments from a retired
engineer.
Discussion on Modulation Process: I would offer a
simple explanation with out getting into the mathematics for the non-engineering
professionals in the group. FM and PM refers only to the modulation process in
the transmitter. FM is where the frequency deviation is directly proportional to
the amplitude of the modulating wave form. PM or Slope modulation is where
the frequency deviation is directly proportional to the phase change or
slope of the modulating wave form. Both FM and PM are detected in the
receiver by one of many different processes, Frequency Discriminator, Ratio
Detector, Quadrature Detector, and other digital processes of more recent
design.
FM was forced upon the land mobile mfg as users
wanted to modulate digital signals directly onto the RF carrier and not go
through the modem/demod process. FM and PM are equally linear, its the
additional low pass & high pass filters, audio limiters, audio amplifiers
etc that introduce the distortion products into the system. Land Mobile was
designed to sent understandable information between two points, not Hi-Fi audio.
Look at what we are now accepting in cell phones.
RF Links: The Land Mobile radio equipment was never
designed to be used for RF point to point links, but as there are people that
want to get by with as little expense as possible most anything will be put to
use. If you are fortunate enough, you should use 900 mhz and higher microwave
equipment with baseband multiplexing. Then you won't have to worry about the
links audio quality.
For those that still want to use mobile radio
equipment for linking I would offer the following practical suggestion from my
experience. One should look at the manufactures specifications of their
pre/de-emphasis, you should find something like +3/-8 db of a standard 6 db per
octave curve. This presents you with a considerable problem. The audio levels
and sounds can vary considerably over the radios audio pass band, even from
radios of the same manufacture. Also, different radios become non-linear at
different levels of deviation. High quality radios will normally have good
linearity up to +/- 4 KHz deviation; this is one reason why system distortion is
measured at 2/3 system deviation.
1. Link RF
levels should have a fade margin greater than 15 db between Tx and Rx. NO RF
levels at "YEA it's OK, SET IT THERE!".
2. Match the
Tx/Rx pre/de-emphasis as closely as practical. Component variation between the
same mfg. sets can cause wide variations.
3. Do not let
the RF links modulation exceed +/- 3.3 KHz deviation (Set the links Tx max
Deviation for +/- 5 KHz per the mfg instructions. Set the audio coupler between
the repeaters Rx and the links Tx such that +/- 5 KHz deviation on the received
signal into the repeater Rx produces only +/- 3.3 KHz deviation on the links
Tx.)
Yahoo! Groups Links
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- Re: [Repeater-Builder] FM/PM & RF Links Fred Seamans
