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3 – 10
3.5.2 Auto-gain
3.5.3 Auto-offset
3.5.4 Lock assist
3. From the four-quadrant arctangent of the two preceding mea-
surements, the SR124 determines the nominal phase shift re-
quired for the user input signal, and programs the PHAS setting
accordingly.0
The entire auto-phase cycle requires 16 the output Time Constant
setting to execute. For example, if the Time Constant is set to 3 s, the
auto-phase will require 48 seconds.
The automatic gain cycle adjusts the sensitivity setting of the SR124
to maximize the output signal without causing an overload.
If the instrument is not in an overload state when auto-gain is started,
then the unit will begin decreasing the sensitivity (increasing the
gain), one step at a time, until an overload is detected. At each
setting, the SR124 pauses for 5 output Time Constants, or 500 ms
(whichever is longer), for settling.
If no overload is ever detected, the instrument stops at 100 nV sen-
sitivity. If an overload is detected, the sensitivity is increased one
setting back to the point overload was not firing.
If the SR124 was already in Overload when auto-gain is started, then
the sensitivity is increased (gain decreased), one step at a time, until
the overload is cleared. The same delay of the longer of 5 time
constants or 500 ms is used at each setting.
The automatic o set cycle measures the current output voltage, and
adjusts the output o set setting to attempt to null the current out-
put. Since the measurement and o set driver both occur after the
output filter time constant is applied, this function always executes
promptly. Note that for proper results, the SR124 output should be
stable before starting an auto-o set cycle.
The lock assist function is only used in external mode and can speed
the PLL acquisition of a user's reference signal.
The lock assist first begins a frequency measurement of the Ext. In
signal to determine the user input frequency. This step requres the
greater of 2 s or 2 periods of the input signal.
The SR124 then determines if the frequency is in-range for the current
setting of the reference Range (accounting for 2
Performance Details
f or 3
SR124 Analog Lock-In Amplifier
f operation,
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