Common Measurement Settings
R&S
®
FSW
358User Manual 1173.9411.02 ─ 19
6.2.5
External Mixer (Option R&S FSW-B21)
If the R&S FSW External Mixer option (R&S FSW-B21) is installed, an external mixer
can be connected to the R&S FSW to increase the available frequency range. In this
case, the input to measure is not taken from the RF input connector, but from the EXT
MIXER connector(s).
● Basics on External Mixers.....................................................................................358
● External Mixer Settings......................................................................................... 366
● How to Work with External Mixers........................................................................ 376
● Measurement Example: Using an External Mixer.................................................379
6.2.5.1 Basics on External Mixers
Some background knowledge on basic terms and principles used with external mixers
is provided here for a better understanding of the required configuration settings.
● Frequency Ranges................................................................................................358
● Two-port and Three-port Mixers............................................................................359
● Bias Current.......................................................................................................... 360
● Conversion Loss Tables........................................................................................361
● Automatic Signal Identification..............................................................................362
Frequency Ranges
In a common spectrum analyzer, rather than providing one large (and thus inaccurate)
filter, or providing several filters to cover the required frequency range of the input sig-
nal (at a high cost), a single, very accurate filter is used. Therefore, the input signal
must be converted to the frequencies covered by the single accurate filter. This is done
by a mixer, which converts and multiplies the frequency of the input signal with the help
of the local oscillator (LO). The result is a higher and lower intermediate frequency (IF).
The local oscillator can be tuned within the supported frequency range of the input sig-
nal.
In order to extend the supported frequency range of the input signal, an external mixer
can be used. In this case, the LO frequency is output to the external mixer, where it is
mixed with the RF input from the original input signal. In addition, the harmonics of the
LO are mixed with the input signal, and converted to new intermediate frequencies.
Thus, a wider range of frequencies can be obtained. The IF from the external mixer is
then returned to the spectrum analyzer.
The frequency of the input signal can be expressed as a function of the LO frequency
and the selected harmonic of the first LO as follows:
f
in
= n * f
LO
+ f
IF
where:
f
in
: frequency of input signal
n: order of harmonic used for conversion
f
LO
: frequency of first LO: 7.65 GHz to 17.45 GHz
Data Input and Output
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