2 – 14
Introduction
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
0
and Q-factor set to
Q
, the ENBW is given by
ENBW
=
π
2
×
f
0
Q
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
p
(
π/
2) times the
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
p
(
π/
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.
2.3.1.5
10%-ENBW
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.
For units with
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.
2.3.1.6
Reserve
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 )
SR124
Analog Lock-In Amplifier
