R&S SM300
Manual Test Procedure
Version 04
81
Service Manual SM300: 1147.1652.00
Instrument Part No SM300: 1147.1498.03
Measurement
procedure
Since the SWR of a source is to be measured, a purely passive
measurement using the SWR bridge is only possible at levels for which the
SWR is determined by the output impedance of the electronic attenuator.
For higher levels, the effect of level control has to be considered. This is done
by means of an auxiliary generator which sends a wave with a slightly offset
carrier frequency (difference frequency within the level bandwidth of level
control) to the EUT. The carrier frequency is superimposed by the outgoing
wave. Given an ideal internal impedance, the outgoing wave of the EUT alone
flows back to the SWR bridge. At any other internal impedance, there is a
superposition of the two components which, due to the frequency offset, results
in a beat. The SWR can be concluded from the amplitude ratio of this beat.
Settings on SM300
LEVEL 0 dBm, -26 dBm
FREQ test frequency SWR
Test frequency: 100 MHz, 500 MHz, 1 GHz, 1.3 GHz, 1.7 GHz, 2.0 GHz,
2.2 GHz, 2.7 GHZ, 3.0 GHz
Settings on
spectrum analyzer
Center frequency = test frequency
Span = 0 Hz
Reference level= test level
Resolution and video bandwidth = 10 kHz
Linear level scale
Sweep time = 30 ms
Settings on 2nd
signal generator
Frequency = test frequency – 100 Hz
First RF OFF
Measurement
1.
Now bring the displayed line to the center of the screen by changing the
reference level and subsequently note it down.
2.
Unscrew SWR bridge from SM300and increase level on second signal
generator so that the reference level is again measured on the analyzer.
3.
Screw SWR bridge or directional coupler again to SM300.
A more or less undulating line can now be seen on the spectrum
analyzer. This line represents the SWR of SM300.
Calculate SWR from the maximum and minimum voltage according to
the following equation: SWR = u
max
/u
min
.
Passive measurement of SWR at output levels of SM300 below -25 dBm
Settings on SM300
LEVEL –26 dBm
FREQ far from test frequency (>10 MHz)
Settings on 2nd
signal generator
Frequency = test frequency
Level = 10 dBm
Measurement
1.
Unscrew SWR bridge from EUT and note down level measured on
analyzer as reference value.
2.
Screw on SWR bridge or directional coupler again and determine new
level on analyzer.
The test level/reference level voltage ratio is the output reflection
coefficient r of the EUT.
3.
Determine the standing wave ratio (SWR) according to the following
formula SWR = (1+r)/(1-r).
