Rohde & Schwarz R&S FSL3 - Page 33

Measuring the Spectra of Complex Signals R&S FSL
1300.2519.12 2.8 E-11
The intermodulation product at f
I2
is generated by mixing the 2nd harmonic of useful signal P
U2
and
s
ignal P
U
1
,
the intermodulation product at f
I
1
b
y mixing the 2nd harmonic of useful signal P
U
1
a
nd signal
P
U
2
.
f
i1
= 2 x f
u1
– f
u2
(6)
f
i2
= 2 x f
u2
– f
u1
(7)
The level of the intermodulation products depends on the level of the useful signals. If the two useful
signals are increased by 1 dB, the level of the intermodulation products increases by 3 dB, which
means that spacing a
D3
between intermodulation signals and useful signals are reduced by 2 dB. This is
illustrated in Fig. 2-5.
Fig. 2-5 Dependence of intermodulation level on useful signal level
The useful signals at the two–port output increase proportionally with the input level as long as the two–
port is in the linear range. A level change of 1 dB at the input causes a level change of 1 dB at the
output. Beyond a certain input level, the two–port goes into compression and the output level stops
increasing. The intermodulation products of the third order increase three times as much as the useful
signals. The intercept point is the fictitious level where the two lines intersect. It cannot be measured
directly since the useful level is previously limited by the maximum two–port output power.
It can be calculated from the known line slopes and the measured spacing a
D3
at a given level
according to the following formula.
IP
a
P
D
N
3
2
3
=+
(8)
The 3
rd
order intercept point (TOI), for example, is calculated for an intermodulation of 60 dB and an
input level P
U
of –20 dBm according to the following formula:
IP dBm dBm3
60
2
20 10= +  =( ) (9)