R&S FSQ Signal Analyzer - Rohde & Schwarz

R&S FSQ Getting Started
Output
level
1
Carrier
level
1
Intercept
point
Compression
aD3
Measuring the Spectrums of Complex Signals
Intermodulation
products
Input level
Fig. 2.2 Level of the 3 rd order intermodulation products as a function of the level of the useful
signals
The behavior of the signals can 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 saturation.
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 d2 and the level of the useful signals.
TOI = aD3 / 2 + Pn
(8)
with TOI (Third Order Intercept) being the 3rd order intercept in dBm and Pn the
level of a carrier in dBm.
With an intermodulation distance of 60 dB and an input level, Pw, of –20 dBm, the
following 3rd order intercept is obtained:
TOI = 60 dBm / 2 + (-20 dBm) = 10 dBm.
2.4 Operating Manual 1313.9681.12 - 01
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