Rohde & Schwarz R&S FSP Series, Operating Manual

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Operating Manual 1164.4556.12 - 06 2.3
R&S FSP Getting Started
Measuring the Spectra of Complex Signals
Example:
f
u1
= 100 MHz, f
u2
= 100.03 MHz
f
s1
= 2 × f
u1
– f
u2
= 2 × 100 MHz – 100.03 MHz = 99.97 MHz
f
s2
= 2 × f
u2
– f
u1
= 2 × 100.03 MHz – 100 MHz = 100.06 MHz
The level of the intermodulation products depends on the level of the useful signals.
If the level of the two useful signals is increased by 1 dB, the level of the intermodu-
lation products is increased by 3 dB. The intermodulation distance d
3
is, therefore,
reduced by 2 dB. Fig. 2.9 shows how the levels of the useful signals and the 3 rd
order intermodulation products are related.
Fig. 2.9 Level of the 3
rd
order intermodulation products as a function of the level of the useful
signals
The behavior of the signals can be explained using an amplifier as an example. The
change in the level of the useful signals at the output of the amplifier is proportional
to the level change at the input of the amplifier as long as the amplifier is operating
in linear range. If the level at the amplifier input is changed by 1 dB, there is a 1 dB
level change at the amplifier output. At a certain input level, the amplifier enters sat-
uration. The level at the amplifier output does not increase with increasing input
level.
The level of the 3 rd order intermodulation products increases 3 times faster than the
level of the useful signals. The 3 rd order intercept is the virtual level at which the
level of the useful signals and the level of the spurious products are identical, i.e. the
intersection of the two straight lines. This level cannot be measured directly as the
amplifier goes into saturation or is damaged before this level is reached.
The 3 rd order intercept can be calculated from the known slopes of the lines, the
intermodulation distance d
2
and the level of the useful signals.
TOI = a
D3
/ 2 + P
n
(10)
with TOI ( T hird O rder I ntercept) being the 3rd order intercept in dBm and P
n
the
level of a carrier in dBm.
Input level
Output
level
Intercept
point
Intermodulation
products
1
a
D3
1
3
Compression
Carrier
level
1