Ametek 7280 Instruction Manual page 124

Chapter 5, FRONT PANEL OPERATION
5-78
Offset function automatically switches on all output offsets
HARMONIC 1
HARMONIC 2
These controls set the harmonic of the applied reference frequency, either internal or
external, at which the lock-in amplifier's two reference channels operate, in the range
1 st (fundamental mode) to 32 nd. For example, if a reference frequency of 1 kHz is
applied, Harmonic 1 is set to 3 and Harmonic 2 to 5, the instrument will measure
signals at both 3 kHz and 5 kHz.
RELOCK EXT. REFERENCE
The 7280 includes frequency-dependent calibration parameters. When operating in
Internal reference mode the correct parameters can be chosen because the reference
frequency is equal to the specified oscillator frequency. In External reference mode
the applied frequency is measured, and the measured value is used to select the
correct parameters. If , however, the external reference frequency drifts or changes
with time then the lock-in amplifier may need to use different calibration parameters.
It therefore includes an automatic algorithm that detects significant changes in
reference frequency, and if these occur, updates all the frequency-dependent
calibration values.
Pressing this key has the effect of manually updating these calibration parameters.
This should be done when operating in External reference mode after each intentional
change to the applied reference frequency.
AUX DAC 1 and AUX DAC 2
These two controls set the voltage appearing at the DAC1 and DAC2 output
connectors on the rear panel to any value between +10 V and -10 V with a resolution
of 1 mV. Adjustment is faster using the Keypad or Active Cursor controls - see
section 4.1.04.
OSC FREQUENCY
The frequency of the instrument's internal oscillator may be set, using this control, to
any value between 0.5 Hz and 2.000 MHz with a 1 mHz resolution. Adjustment is
faster using the Keypad or Active Cursor controls - see section 4.1.04.
OSC AMPLITUDE
This control may be set to any value between 1 mV and 1 V rms. Adjustment is faster
using the Keypad or Active Cursor controls - see section 4.1.04.
Since the instrument is now generating two sets of outputs (one for each harmonic of
the reference) there are more output display choices in this mode than in single
reference mode, and these are listed in table 5-4. The outputs corresponding to the
two harmonics are identified by the suffices "1" and "2" respectively.