Test Procedures R&S SMB
1407.0806.82 1.26 E-5
Test method for low levels
Test principle Low levels can only be measured using a frequency selective
m
easurement instrument. Spectrum analyzers with digital IF are best
suited for this measurement due to their low linearity error. The
absolute accuracy of these analyzers is not sufficient for this
measurement. So a relative measurement referred to the
measurements performed with the power meter is used to increase
the accuracy of the measurement.
Only by switching the input attenuator and preamplifier (when
available) of the analyzer the needed dynamic range of more than 120
dB can be reached. After switching the analyzer attenuator or
preamplifier, a continuity calibration is to be carried out. It is therefore
recommended to switch the attenuator not until reaching 50 dB under
full scale, since the linearity errors are very small in this range.
Test setup Connect the spectrum analyzer to the RF output of the DUT with
hermetically sealed RF measurement cables.
Measurement Settings on DUT
Frequency recommended test frequencies
Level +18 dBm, unmodulated
Setting on the analyzer
Test frequency
SPAN 10 Hz
FILTER TYPE FFT
RES BW 5 Hz
set Marker to test frequency
Reference level P
ref
= +20 dBm
Read the marker level P
Marker
and calculate the correction factor
C = P
absolute
- P
Marker
with P
absolute
from the measurements performed with the power
meter.
Now decrease the DUT level in 5 dB steps and calculate the output
power P by adding the Correction factor C to the marker readout.
As soon as the marker level P
Att1
is lower than P
ref
-45 dB increase
the sensitivity of the analyzer by reducing the input attenuation,
switching on the internal preamplifier if available and reducing the
resolution bandwidth to 1 Hz for levels below -90 dBm. Set the
analyzer reference level to P
Att1
+ 1 dB. After switching the analyzer
sensitivity read out the marker level P
Att2
and recalculate the
Correction factor:
C
new
= C
old
+ P
Att1-
P
Att2
Continue the measurement down to -120 dBm in 5 dB steps.
Recommended test
frequencies.
201 kHz, 512.5 MHz, 1087.5 MHz, 2187.5 MHz, 3187.5 MHz,
4012.5 MHz, 5012.5 MHz, 5987.5 MHz
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