GUI Reference
R&S
®
ZNB/ZNBT
374
User Manual 1173.9163.02 ─ 38
Image Suppr.
The "Image Suppr." settings define whether the analyzer measures with a local oscilla-
tor frequency LO below or above the RF input frequency. This feature can be used to
eliminate known spurious components in the input signal that can distort the measure-
ment, especially in the low frequency range.
●
In "Auto" mode, the analyzer auto-selects the local oscillator frequency, depending
on the receiver (RF) frequency and the test port. This mode systematically avoids
known spurious signals provided that no frequency conversion occurs in the test
setup.
●
"LO < RF" means that the LO frequency is always below the measured RF fre-
quency. This mode is appropriate for avoiding single, known spurious signals.
●
"LO > RF" means that the LO frequency is always above the measured RF fre-
quency. This mode is appropriate for avoiding single, known spurious signals.
Tip:
In the presence of several spurious signals, setting the "Image Suppr." parameter
globally may not be sufficient. To improve the result, perform a segmented frequency
sweep and assign independent LO frequencies to the individual sweep segments.
Application example
Consider the following test setup with strongly reflecting DUT (e.g. a bandpass in its
stop band) that is measured in transmission. a
1
is generated at a frequency RF. The
reflected wave b
1
falls into the receiver mixer of the analyzer port 1, where a small frac-
tion of the mixer product RF + 2*IF can be reflected back towards the DUT. If this spu-
rious wave a'
1
passes the DUT, then it is received as b'
2
at port 2, together with the
wanted signal b
2
.
LO > RF implies that LO = RF + IF. The mixer at port 2 converts both the wanted signal
b
2
and the spurious signal b'
2
which is at the frequency RF' = IF + LO, to the same IF
frequency. The response of an ideal, infinitely steep bandpass filter with a pass band
between B
-
and B
+
looks as follows:
For a wide bandpass, the spurious response flattens the filter edges.
Channel Config Softtool
