What is a frequency range?
Imagine a giant motorway, several kilometers wide, with thousands of lanes. On
this motorway, every imaginable kind of vehicle can be found: motorcycles, cars,
trucks, etc. To not let them get into each other’s way, every lane is reserved for
only a single group of road users: e.g., lane 1 ONLY for cyclists, lane 3 ONLY for
pedestrians, lane 40 ONLY for trucks etc. Depending on the traffic caused by the
individual groups, these lanes also have different widths: For example, the lane
reserved for cyclists is far narrower than that reserved for trucks, etc. High fre-
quency works exactly like this, just that here, the lanes are the so-called
fre-
quency ranges
, and the road users are
applications
(for example, a cell
phone, a microwave oven, a radio-controlled car lock, in effect
all
appliances
that somehow work with radio waves).
So, every
application
has its own
frequency range
for EXCLUSIVE use. By
assigning a separate
frequency range
for each
application
, conflicts between
different ‘road users’ can be avoided, so that e.g. a cell phone cannot be dis-
rupted by a microwave oven.
Big differences between exposure limits
Back to our motorway: Of course, all road users also have their own specific
speed limits. For our example, a pedestrian may only walk at up to 5 km/h. Cars,
in change, may speed at up to 300 km/h. Exposure limits for
radio applications
work similarly: Here, however, the word “speed” is replaced by the
transmitting
power
: E.g., a broadcast station may have an enormous
transmitting power
of
1.000.000W or more, in contrast, a radio-controlled car lock only a few mW
(1mW = 0,001W) etc. 3 examples of exposure limits in practice:
Frequency range [MHz]
Application
Power limit [W EIRP]
1.880-1.900
DECT phones
0,25
2.320-2450
Amateur radio (11cm)
750
5.725-5.825
WLan 802.11a
0,025
It is easily visible that each
radio application
may only use one EXACTLY
DEFINED
frequency range
. Also, the high differences in admissible transmit-
ting power are noticeable.
Why spectrum analysis?
There are 2 main reasons:
1.) You would like to know WHICH radio applications are active
14.0 Spectrum analysis basics
46
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