Atec Rohde-Schwarz-EB500 - Manual (Page 16)

Block diagram of digital signal processing
Display and LAN
IF spectrum
Digital audio
via LAN
I/Q data
via LAN
Clear/write
Analog audio
3 IF
Demodulation
bandwidths
100 Hz to 5 MHz
DDC
IF spectrum
1 kHz to 20 MHz
DDC
Display and LAN
4096 points
16 bit
HF direct
IF panorama
path
Demodulation
path
Video spectrum
Video/IF
analog output
16 bit
AGC
MGC
ABS
value
Demod
FFT
16 bit
A
D
Digital up-
converter
A
D
ITU
measurement
Average
Min.
hold
Max.
hold
Fast
Peak
RMS
Average
Level
measurement
A
D
Lowpass filter
16
IF spectrum
FFT calculation of the IF spectrum is performed in a num­
ber of steps. These are described below in simplified form
for an IF bandwidth of 20 MHz ( BW
IF spectrum
= 20 MHz),
which yields high spectral display.
Due to the finite edge steepness of the IF filter, the sam­
pling rate f
S
must be larger than the selected IF bandwidth
BW
IF spectrum
. The quotient of the sampling rate and the
IF bandwidth is thus a value > 1 and is a measure of the
edge steepness of the IF filter. This relationship is ex­
pressed by the following two formulas (for the AUTO
setting):
or
f
S
= BW
IF spectrum
× const
The value of the constant is dependent on the selected
IF bandwidth, i.e. it may vary as a function of the IF
bandwidth.
Analog signals are reconverted from the I/Q data by a
16 bit D/A converter; they are then available as analog
IF signals or analog video data.
High receiver sensitivity, high signal resolution
The R&S®EB500 features an IF bandwidth of up to
20 MHz. This allows even very short signal pulses to be
captured since the receiver displays the wide bandwidth
of 20 MHz in a single spectrum around the set center fre­
quency without any scanning being required.
Using the AUTO setting, the widest IF bandwidth of
20 MHz yields the widest spectral display; the narrow­
est IF bandwidth of 1 kHz yields maximum sensitivity and
resolution.
The receiver’s IF spectrum is digitally calculated using Fast
Fourier Transform (FFT). The use of FFT computation at the
IF offers a major advantage: The receiver sensitivity and
signal resolution are clearly superior to those of a conven­
tional analog receiver at the same spectral display width.
const
BW
IF spectrum
f
s =