Rohde & Schwarz R&S FSP Series, Operating Manual

Page: 351 / 796

Operating Manual 1164.4556.12 - 06 4.279
R&S FSP Instrument Functions
LO/IF ports for external mixers - Option R&S FSP-B21
Tolerance for the comparison of test sweep and reference
Since the LO frequency is displaced downwards in the reference sweep, the conver-
sion loss of the mixer may differ from that of the test sweep. The reasons for this are
the LO output power of the R&S FSP varying with the frequency and the non-ideal
characteristics of the mixer. A certain tolerance should therefore be permitted for the
comparison of the signal levels in the test sweep and reference sweep. The user
can set this tolerance using the function
AUTO ID THRESHOLD .
If the tolerance set is too tight the signal collected with the reference sweep may be
displayed even for the identification of real signals.
Example
A signal with a frequency of 52.5 GHz is applied to the mixer input. Let the signal
level be –30 dBm. The conversion loss of the mixer is 28 dB at this frequency in the
test sweep and 35 dB in the reference sweep. The user has entered a tolerance of 5
dB and a conversion loss of 28 dB. Thus, the signal is recorded with the correct level
of –30 dBm in the test sweep, a signal level of –37 dBm is obtained in the reference
sweep. As the difference (7 dB) between the levels is >5 dB, the signal with the
lower level is displayed, i.e. the signal of the reference sweep. Since the set conver-
sion loss is adapted to the test sweep, the signal displayed on the R&S FSP has a
level of –37 dBm yielding incorrect level display.
The tolerance entered corresponds to the minimum S/N ratio which signals should
have to ensure correct identification. If the S/N ratio of a mixer product is less than
the tolerance, the decision criterion is fulfilled even if only the noise floor is recorded
in the reference sweep at the frequency of this mixer product. Unwanted mixer prod-
ucts are therefore not detected as such by
AUTO ID . They can only be identified by
a visual comparison of the two traces using the function SIGNAL ID .
In order to avoid the visual identification of such unwanted mixer products it is useful
to perform the measurement test in two steps:
In the first step the tolerance is set to the minimum value (0.1 dB). This enables
unwanted mixer products with low S/N ratio to be detected and blanked out.
- [ FREQUENCY : EXTERNAL MIXER : AUTO ID ]
- [ FREQUENCY : EXTERNAL MIXER : AUTO ID THRESHOLD : 0.1 dB ]
To determine the signal levels of real input signals the tolerance is to be set to the
minimum value, e.g. 10 dB, in the second step.
- [ FREQUENCY : EXTERNAL MIXER : AUTO ID THRESHOLD : 10 dB ]
The real input signals are then displayed with the correct level.
Mixer products with low S/N ratio
If the S/N ratio of a mixer product is less than the tolerance set with AUTO ID
THRESHOLD , the level difference between the test sweep and reference sweep at
the frequency of this mixer product is always within limits, even if the signal occurs in
one of the sweeps only. Such mixer products cannot be identified by
AUTO ID . It is
therefore recommended to perform a visual comparison of the test sweep and refer-
ence sweep using the function
SIGNAL ID .
An unwanted signal having a S/N ratio corresponding approximately to the tolerance
set with
AUTO ID THRESHOLD may not be blanked out permanently. Due to the
fact that the noise display varies from one sweep to another, the S/N ratio changes
and thus the level difference between the test sweep and reference sweep mea-