8.2 Aliases and mirror frequencys
Those are typical effects found in spectrum analysers: when you find a rather
strong
signal, additional “phantom” signals appear next to the actual signal, at
fixed
distances. These are not “real” signals! A 400MHz signal also shows up at
800MHz, 1200MHz, 1600MHz, a 1800MHz signal also at 3600MHz and
5400MHz. Those are the so called “mirror frequencys”.
Also “aliases” can show up. The distance between aliases and the main signal
is steps of 20MHz. The level difference between those signals and the main sig-
nal is typically
very large
(20-30dB). When evaluating exposure limits, they
mostly do not even matter at all. Please note when using the 100kHz filter: Two
5MHz Aliases will show up and are almost as strong as the main signal.
HINT:
When using a
sampletime
of
>=1
, aliases (using RBW 300kHz-3MHz)
are
suppressed by almost 100%
. Here, an additional “Anti-alias Interleave
Sampling Algorithm” is hooked up. Obviously, though, this does not work with
activated pulse mode.
8.3 Measuring WLan and cell phones
To measure WLan and cell phones, you need to know their characteristics, as in
practice, both systems are only rarely “really” active to allow measurement:
With
cell phones
, you should actively place a call. Please also note that cell
phones actively decrease transmitting power with good reception conditions or
when you are not talking!
An example for best parameters for a
GSM1800 cell phone
follows:
fLow
=”1710”;
fHigh
=”1781”;
RBW
=”3MHz”;
SpTime
=”0.5”;
Pulse
=”Pulse”
With
Wifi
or
WLan
, you should operate with continuously transfer data because
simple “Pin” mode operation would need a quite long time to detect and measu-
re a signal. An example for best parameters for a
2,4GHz WLan
measurement
follows (or just use “hotkey” 6):
fLow
=”2400”;
fHigh
=”2490”;
RBW
=”3MHz”;
SpTime
=”5”;
Pulse
=”Pulse”
Also note to add about 8dB to the signal strength, because the WLan signal is
20MHz wide, but in this case we measures with a 3MHz RBW.
8.0 Correct measurement
32
Level difference
(typically 20-30dB)
20MHz
Distance between aliases
is around 20MHz
NOTE: All Revision3 SPECTRANs offer a hardware aliases reduction and
have almost no aliases at all if you use RBW 300kHz-3MHz!
This example in GSM900 mode (key 7) shows a rather strong
main signal of -45dBm at 950MHz, and to the left, aliases at
930MHz with -75dBm.
Summary of Contents for SPECTRAN HF-2025
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