GUI reference
R&S
®
ZNA
633
User Manual 1178.6462.02 ─ 20
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 LO frequency, depending on the
receiver (RF) frequency and the test port. This mode systematically avoids known
spurious signals if no frequency conversion occurs in the test setup.
For
, the selected LO frequency is trace-specific.
Depending on the measured quantity, either "LO < RF" or "LO > RF" is selected.
●
"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 can be insufficient. 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 a strongly reflecting DUT (e.g. a bandpass in its
stop band) that is measured in transmission. The incident wave
a
1
is generated at a
frequency RF. The reflected wave
b
1
falls into the receiver mixer of the analyzer port 1;
a small fraction of the mixer product RF + 2*IF can be reflected back towards the DUT.
If this spurious 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.
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