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2 – 14
2.3.1.5 10%-ENBW
2.3.1.6 Reserve
same amount of noise as the real filter in question. For the SR124
input filter operating in band pass mode, with the filter frequency
set to f
and Q-factor set to Q, the ENBW is given by
0
Note that when operating in AC Volt mode, the output is an absolute-
value average, and not a true RMS amplitude. The SR124 gain cal-
ibration is set to correct for this when measuring sinusoidal inputs
(see section 3.1.2 for more detail). When measuring noise with the
SR124, this calibration leads to the output being
actual RMS Gaussian noise value.
To correct for this, the 10%-ENBW setting actually programs the Q-
factor to 12.34. When this value is substituted into the ENBW formula
above, and the
(
2) correction is applied, the resulting output in
AC Volt mode with band pass filtering provides an equivalent noise
bandwidth of 10% the filter center frequency. Note again that in
AC Volt mode, the SR124 should be operated in Low Noise reserve
mode.
The SR124 has an additional setting available for the Input Filter Q-
factor to set the Equivalent Noise Bandwidth to 10% (10% ENBW).
In this configuration, the equivalent noise bandwidth for Bandpass
mode is 10% of the filter center frequency: for example, if the input
filter is set to 5.00 kHz, then 10%-ENBW mode provides 500 Hz
equivalent noise bandwidth through the input filter.
To set the SR124 Q-factor to 10%-ENBW, turn the Q-factor knob
clockwise, one "click" past the Q 100 setting.
firmware revision 3.0 and later, the 10% ENBW indicator will il-
luminate. On older units with firmware revision 2.40 through 2.90,
the numeric displays will briefly show the pattern "Enb 10pct" to
indicate 10%-ENBW. On these older units, when configured for 10%-
ENBW, none of the Q-factor indicators will be illuminated.
The remote command QFCT (4 – 12 ) sets the 10%-ENBW mode.
The dynamic reserve setting of the SR124 can be cycled between High
Res., Normal, and Low Noise by successive presses of [Reserve].
(RMOD, 4 – 13 )
f
0
ENBW
2
Q
SR124 Analog Lock-In Amplifier
Introduction
(
2) times the
For units with
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