Rohde & Schwarz EMI Test ReceiverESCI - Measurement of the Third Order Intercept (TOI)

R&S ESCI Analyzer - MEAS
1166.6004.12 4.179 E-1
Measurement of the Third Order Intercept (TOI)
If several signals are applied to a transmission twoport device with nonlinear characteristic,
intermodulation products appear at its output by the sums and differences of the signals. The nonlinear
characteristic produces harmonics of the useful signals which intermodulate at the characteristic. The
intermodulation products of lower order have a special effect since their level is largest and they are
near the useful signals. The intermodulation product of third order causes the highest interference. It is
the intermodulation product generated from one of the useful signals and the 2nd harmonic of the
second useful signal in case of two-tone modulation.
The frequencies of the intermodulation products are above and below the useful signals. Fig. 4-19
shows intermodulation products P
I1
and P
I2
generated by the two useful signals P
U1
and P
U2
.
f
U1
f
U2
f
I2
f
I1
f
∆
f
∆
f
∆
a
D3
P
U1
P
U2
P
I1
P
I2
Frequency
Level
Fig. 4-19 Intermodulation products P
U1
and P
U2
The intermodulation product at f
I2
is generated by mixing the 2nd harmonic of useful signal P
U2
and
signal
P
U1
, the intermodulation product at f
I1
by mixing the 2nd harmonic of useful signal P
U1
and signal
P
U2
.
f
i1
= 2 x f
u1
-
f
u2
(1)
f
i2
= 2 x f
u2
-
f
u1
(2)
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 is reduced by 2 dB. This is
illustrated in Fig. 4-20.