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Chapter 5, FRONT PANEL OPERATION
5-62
its frequency is equal to the specified oscillator frequency. The parameters for the
external reference channel are selected by measuring the applied frequency. 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 Dual reference mode after each intentional
change to the external reference frequency.
AUTO MEASURE 1 and AUTO MEASURE 2
These controls allows an auto-measure cycle for reference channel 1 or 2 to be
initiated directly from the main display.
REF2 HARMONIC
This control sets the harmonic of the reference frequency at which the lock-in
amplifier's reference 2 channel operates, in the range 1 st (fundamental mode) to
127 th.
For example, if Ref1 Harmonic is set to 3rd, Ref2 Harmonic is set to 1st, Reference 1
is set to External Front Panel, an external reference signal of 12 kHz is applied to the
front panel connector and the internal oscillator set to 1 kHz, the instrument will
measure signals at its input at frequencies of both 3 kHz and 12 kHz.
AUX DAC 1, AUX DAC 2, AUX DAC 3, and AUX DAC 4
These four controls set the voltage appearing at the DAC1, DAC2, DAC3, and
DAC4 output connectors on the rear panel to any value between +10 V and -10 V
with a resolution of 1 mV, assuming that the corresponding output is set to the
USER DAC mode on the DAC Menu - see section 5.3.31. Adjustment is faster using
the Keypad or Active Cursor controls - see section 4.1.04.
OSC FREQUENCY 1
The frequency of the instrument's internal oscillator may be set using this control to
any value between 0.001 Hz and 250.0 kHz with a 1 mHz resolution. Adjustment is
faster using the Keypad or Active Cursor controls - see section 4.1.04. The number
indicates which channel is configured to use the internal oscillator as its source of
reference.
OSC AMPLITUDE
This control sets the amplitude of the signal at the 7270's instrument OSC OUT
connector to any value between 1 µV and 5 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 the external and
one for the internal reference signals) there are more output display choices in this
mode than in single reference mode, and these are listed in table 5-2. The outputs
corresponding to the two references are identified by the suffices "1" and "2"
respectively.
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