R&S FSQ Signal Analyzer - Rohde & Schwarz

R&S FSQ Instrument Functions
FFT FILTER
MODE
VBW MODE
(LIN LOG)
Analyzer Mode
The softkey FFT FILTER MODE allows to specify between the following three
options for FFT filters:
WIDE: The FFT filters with the greater partial span (this is the span which is
covered with one FFT analysis) are used always.
AUTO: The firmware decides whether to use wide or narrow filters to achieve
the best performance of the measurement.
NARROW: The FFT filters with the smaller partial span are used, this allows measurements
near a carrier with reduced reference level, because of a
more narrow analog prefilter.
Remote command: SENS:BWID:FFT WIDE | AUTO | NARR
The VBW MODE (LIN LOG) softkey determines the position of the video filter in the
signal path for resolution bandwidths ≤ 100 kHz:
• If LINear is selected, the video filter will be in front of the logarithmic amplifier
(default).
• If LOGarithmic is selected, the video filter will be behind the logarithmic amplifier.
The essential difference between the two operating modes relates to the settling in
case of falling signal edges:
With LINear, the falling signal edge will be "flatter" than with LOGarithmic.
This is due to the conversion from linear power to logarithmic level units: a reduction
of the linear power by 50% reduces the logarithmic signal level by only 3 dB.
This is important for EMI measurements above 1 GHz. Below 1 GHz, CISPR 16-1-1
defines the linear average detector only. Above 1 GHz both, the linear and the logarithmic
average detector may be specified in product standards. Some standards
(e.g. ANSI C63.4:2000) require the linear average detector, whereas microwave
oven measurements may be made with the log average detector. CISPR 11 specifies
weighted measurements with a VBW of 10 Hz.
Fig. 4.2 shows the response of the linear and the logarithmic average detector for
pulse-modulated signals.
Fig. 4.2 Reponses of the linear and the logarithmic average detectors as a function of pulse
repetition frequency (PRF) for pulse-modulated signals with pulse durations of 400 ns
and 1 ms. Resolution bandwidth is 1 MHz.
Remote command: BAND:VID:TYPE LIN
4.30 Operating Manual 1313.9681.12 - 01