8.0 Correct measurement
Firmware V 1.0 / © 2005-2013 by Aaronia AG, D-54597 Euscheid, www.aaronia.com
39
8.9 DFT mode (rapid measurements with small RBWs)
All series 50xx SPECTRAN models feature a “superfast“
DFT
mode (
d
igital
f
ou-
rier
t
ransformation).
DFT mode is automatically activated when the follo-
wing settings are used:
a) an RBW of 0,3Hz, 1Hz or 3Hz
AND
b) a SPAN smaller than 200Hz (with 10Hz being the minimum)
The DFT mode allows a significantly faster sweep when using extremely narrow
filters and is especially suitable for measurement of mains power (50Hz) or trac-
tion power (16.7Hz). In contrast to regular FFT,
DFT
does NOT produce the infa-
mous aliases known from FFT.
ATTENTION:
For quick searching of signals, you could even use a
too low set-
ting
for sweep time in DFT mode. This allows very fast sweeps, but inherently
may produce
wrong results
. For preventing users from misinterpreting these,
the
UNCAL display
is automatically activated as soon as a too short sweepti-
me is chosen, indicating that measurement results are not reliable. However, it
is still possible to locate quickly a signal source or verify its presence.
More information on DFT mode can be found in the section“The 0Hz spike and
the noise floor“.
8.10 Choosing the right attenuator
SPECTRAN contains a highly accurate internal attenuator, allowing the
attenuation of input signals to prevent overdrive of the internal amplifier.
Without using the attenuator, serious measurement errors and even destruction
of the internal amplifier could occur. To choose the right attenuator for your mea-
surement, proceed as follows:
Initially, always select the highest attenuator (40dB) and, if there is a usable rea-
dout, remember its value. Afterwards change to the next smaller attenuator
(30dB). If the new readout value is significantly lower than the previous one, the
amplifier is already suffering from overdrive, so you should change back to the
higher attenuator. However, should the readout be almost identical, this attenua-
tor is the correct choice.
Further indications of having chosen the wrong attenuator is an unsteady read-
out and a cluttered graphical display.
When supplying
external signals
via the
SMA connector
, we recommend the
following attenuator settings (100kHz signal):
>=1mV:
10dB
, >=5mV:
20dB
, >=10mV:
30dB
, 50mV and above:
40dB
.
