2-13 Occupied Bandwidth Measurement Spectrum Analyzer
2-16 PN: 10580-00349 Rev. H Spectrum Analyzer MG
Power density to field strength:
An alternate measure of field strength is power density:
P
d
= E
2
/ 120π
where:
E = field strength in V/m
P
d
= Power density in W/m
2
Power density at a point:
P
d
= P
t
G
t
/ (4πr
2
)
where:
P
d
= power density in W/m
P
t
= power transmitted in Watts
G
t
= gain of transmitting antenna
r = distance from the antenna in meters
This equation is only valid in the far field, where electric and magnetic fields are related by
the characteristic impedance of free space.
2-13 Occupied Bandwidth Measurement
Occupied Bandwidth (OBW) is a common measurement performed on radio transmitters.
This measurement calculates the bandwidth containing the total integrated power occupied
in a given signal bandwidth. There are two different methods of calculation depending upon
the technique used to modulate the carrier.
• % Integrated Power Method: The occupied frequency bandwidth is calculated as the
bandwidth containing the specified percentage of the transmitted power.
•>dBc Method: The occupied frequency bandwidth is defined as the bandwidth
between the upper and lower frequency points at which the signal level is a desired
number of dB below the peak carrier level.
Required Equipment
• Test Port Extension Cable, Anritsu part number 15NNF50-1.5C
• 30 dB, 50 Watt, bi-directional, DC –18 GHz, N(m) – N(f), Attenuator, Anritsu
42N50A-30 (required if the power level being measured is > +30 dBm)
Procedure
1. Using the test port extension cable and the 30 dB, 50 watt, bi-directional attenuator
(if needed) connect the RF In port to the appropriate transmitter test port or signal
source.
2. Press the Freq main menu key followed by the Center Freq submenu key and enter the
center frequency using the keypad, the arrow keys, or the rotary knob. If entering a
frequency using the keypad, the submenu key labels change to GHz, MHz, kHz,
and Hz. Press the appropriate units key. Pressing the Enter key has the same effect as
the MHz submenu key.
3. If an attenuator was connected in Step 1, then press the Amplitude main menu key then
press the RL Offset submenu key, enter 30 then select dB External Loss to compensate
for the loss in the attenuator.
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