Tektronix 2715, Руководство пользователя

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Spectrum Analyzer Menus
6–10
2715 User Manual
The decibel relative to a microvolt per meter (dB
m
V/m) is an electric field
strength unit that characterizes the intensity of radiated RF energy. Typically, the
radiated signal amplitude is measured at the terminals of a calibrated antenna to
determine the field strength.
With most spectrum analyzers, you then correct the measured signal amplitude
for any external gain or attenuation, convert signal amplitude to radiated
intensity using the antenna factor (often referred to as the K-factor) for your
antenna, and scale the field strength for the difference between the measurement
distance and the required reference distance.
However, the 2715 performs the correction, conversion and scaling for you.
Input the external gain or attenuation, antenna factor, and measurement distance
using the INPUT menu. Then, select the dB
m
V/m reference unit and the 2715
does the rest. The signal intensity is read out on screen using the marker and
corrected for distance, in either dB
m
V/m or V/m. Using the DISPLAY menu, you
can have the 2715 sound an alert if the measured signal exceeds a threshold that
you set. This feature facilitates go/no-go or present/absent tests.
Figure 6–4 shows how to set up your equipment. A balun can be included as part
of your antenna. The filter and external matching network are optional. The filter
is intended primarily to prevent off-the-air signals, such as radio and television,
from swamping the 2715. The matching network may be necessary for maxi-
mum accuracy. If in doubt, try the measurement with and without the network. If
there is no difference, omit the network for maximum sensitivity.
The following formula relates the radiated field strength in dB relative to a
microvolt per meter (dB
m
V/m) to the measured signal amplitude in dB relative to
a millivolt (dBmV), and scales the result measured at a distance d
meas
to a
reference distance, d
ref
. (The reference distance is often specified by the
regulatory agencies.)
P
dB
m
V
ń
m
+
P
dBmV
)
13
)
20 log
ƪ
d
meas
d
ref
ƫ
*
A
)
K
d
meas
= distance from radiation source at which the measurement is carried out
d
ref
= reference distance at which the intensity is desired
A = attenuation or gain between antenna and 2715. If the filter is used, its
gain or attenuation should be included in this number. If balun losses
are not included in the antenna factor, they should be included here.
Cable loss, if significant, can also be included here.
K = antenna factor; supplied by manufacturer or calculated as follows:
K = 20 log f – G – 10 log (19 R
ant
)
G = antenna gain as a function of frequency
f = frequency of signal in MHz
R
ant
= output resistance of the antenna or the balun, if the balun is
treated as part of the antenna
Using the DBUV/M