The Spectrum Application
R&S
®
FPC
74
User Manual 1178.4130.02 ─ 07
14.2.2
Third Order Intermodulation
Access
: "Meas" > "Measurement Mode" > "Occupied Bandwidth"
Remote commands to control the third order intermodulation measurement:
●
Chapter 19.12.1.3, "Third Order Intermodulation"
Available with the optional "Advanced Measurements" application.
Intermodulation is an effect that occurs when you transmit signals on two or more fre-
quencies close to each other. An ideal DUT would output the same signals that you
apply to its input. In reality, however, DUTs (like an amplifier or mixer) are nonlinear.
This nonlinearity produces additional signals at different frequencies, which can have
negative effects on the system, for example interference in a radio transmission.
●
Harmonics, which are a multiple of the fundamental frequency (f*x).
●
Intermodulation products, which are the sum and difference frequencies of the fun-
damentals (f
x
+ f
y
and f
y
-f
x
).
Frequency
Power
f
1
f
2
2*f
1
2*f
2
f
1
+f
2
3*f
1
3*f
2
2*f
1
+f
2
2*f
2
+f
1
2*f
1
-f
2
2*f
2
-f
1
f
2
-f
1
Fundamentals
2
nd
order products
3
rd
order products
Figure 14-4: Overview of intermodulation products
The third order intermodulation products are of special interest, because:
●
Their signal level is comparatively large.
●
They occur near the carrier frequency and are thus difficult to filter out.
In addition, the level of the intermodulation products increases when you increase the
input level in a ratio of 3:1: Increasing the level of the input signal by 1
dB increases
the level of the third order intermodulation product by 3
dB. Eventually, the level of the
intermodulation product exceeds the level of the input signal. The point where the two
meet is the third order intercept point (IP3).
Input power
Output power
P
f
P
IM3
IP3
Measurements and Result Displays
