Appendix
R&S
®
Spectrum Rider FPH
465
User Manual 1321.1011.02 ─ 06.00
Applying the Fourier transform to the periodic square wave transforms it into the fre-
quency domain. The spectrum analyzer would show the fundamental (or frequency of
the square wave) and its harmonics.
The spectrum analyzer uses a narrow bandpass filter for measurements in the fre-
quency domain. Only at frequencies containing a signal there is a reading that gives
the amplitude of the frequency component.
shows the basic principle of how a spectrum analyzer works.
Figure 15-2: Block diagram showing the basic functionality of a spectrum analyzer
The precision attenuator at the R&S Spectrum Rider input attenuates the signal to a
level that the mixer can handle without overdriving the mixer. The attenuator is directly
coupled to the reference level. You can attenuate the signal in the range from 0 dB to
40 dB in steps of 5 dB.
The mixer converts the RF signal to a fixed intermediate frequency (IF). This process
usually involves several stages. It lasts until you get an IF for which good narrowband
filters are available. The R&S Spectrum Rider needs three mixing stages to get an IF
that the filter can handle. Figure 11-3 graphically shows the mixing process.
For models with a frequency limit of 3.6 GHz, the IFs are 4892.8 MHz, 860.8 MHz and
54.4 MHz. The conversion from a specific input frequency to the first IF is done by a
local oscillator (LO). This LO can be tuned from 4.8 GHz to 8.4 GHz. All other conver-
sions are handled by single-frequency oscillators.
In case of models with a frequency limit of 8 GHz, the IFs are 8924.8 MHz, 860.8 MHz
and 54.4 MHz. The conversion from the first to the second IF for these models is done
by a second local oscillator.
The frequency of the local oscillator determines the input frequency at which the spec-
trum analyzer performs measurements: fin = f
LO
– f
IF
.
The first mixer produces the sum frequency f
LO
+ f
in
(= image frequency f
image
) as well
as the difference frequency f
LO
– f
in
.
The image frequency is rejected by the bandpass at the IF so that it does not interfere
with the subsequent frequency conversions.
How a Spectrum Analyzer Works
