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Chapter 6, COMPUTER OPERATION
6.5.03 Frequency Response Measurement
6.5.04 X and Y Output Curve Storage Measurement
6-38
required output.
In this example, the lock-in amplifier's internal oscillator output signal is fed via the
filter stage under test back to the instrument's signal input. The oscillator frequency
is stepped between a lower and an upper frequency and the signal magnitude and
phase recorded. At sampling rates of up to a few points per second, there is no need
to use the internal curve buffer or oscillator frequency sweep generator. The
commands to achieve this would therefore be similar to the following sequence:
IE 0
Set reference mode to internal
VMODE 1
Single-ended voltage input mode
AUTOMATIC 1
AC Gain control automatic
FLOAT 1
Float input connector shell using 1 k to ground
LF 0
Turn off line frequency rejection filter
OA. 1.0E0
Set oscillator amplitude to 1.0 V rms
OF. 100.0
Set oscillator frequency to 100 Hz (starting frequency)
SEN 27
Set sensitivity to 1 V full-scale
TC 11
Set time constant to 50 ms
AQN
Auto-Phase
The frequency sweep would be performed and the outputs recorded by sending the
following commands from a FOR...NEXT program loop:
OF. XX
Set oscillator frequency to new value XX hertz
Software delay of 200 ms (4 × 50 ms) allowing output to
stabilize
MAG.
Read Magnitude in volts
PHA.
Read Phase in degrees
FRQ.
Read reference frequency in hertz. This would be same as
the oscillator frequency since the unit is operating in
the internal reference mode.
until the stop frequency is reached.
In this example, the lock-in amplifier is measuring a current input signal applied to
the B input connector and the measured X channel output and Y channel output are
recorded for 10 seconds at a 100 Hz sampling rate. The acquired curves as read back
to the computer are required in floating point mode.
The sequence of commands is therefore as follows:
IE 2
Set reference mode to external front panel input
IMODE 1
Normal current input mode
AUTOMATIC 1
AC Gain control automatic
FLOAT 1
Float input connector shell using 1 k to ground
LF 0 0
Turn off line frequency rejection filter
SEN 18
Set sensitivity to 1 nA full-scale
TC 11
Set time constant to 50 ms
AQN
Auto-Phase
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