Ametek 7270 Instruction Manual page 54

Chapter 5, FRONT PANEL OPERATION
5-6
For example, if AC Gain = 12 dB and SEN = 2 mV then
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DR 20 log
10
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0.002
DR = 48 dB
TIME CONSTANT
The time constant of the output filters is set using this control. When the Time
Constant Mode on the Output Filters menu (see section 5.3.06) is set to Fast, the
range is 10 µs to 100 ks in a 1-2-5 sequence; when the Time Constant Mode is set to
Normal the minimum time constant is 5 ms.
REF PHASE
This control allows the reference phase to be adjusted over the range -180° to +180°
in 1 m° steps. Adjustment is faster using the Keypad or Active Cursor controls - see
section 4.1.04.
REF PHASE ±90°
This control allows the reference phase to be adjusted in steps of ±90°.
X OFFSET and Y OFFSET
These are the manual X channel and Y channel output offset controls. The offset
levels set by these controls, which can be any value between -300% and +300% in
0.01% steps, are added to the X channel or Y channel outputs when the X channel or
Y channel offsets are switched on using the Output Channels menu. Adjustment is
faster using the Keypad or Active Cursor controls - see section 4.1.04.
The values are set automatically by the Auto-Offset function. Note that the Auto-
Offset function automatically switches on both X and Y channel output offsets.
REF HARMONIC
This control sets the harmonic of the applied reference frequency, either internal or
external, at which the lock-in amplifier's reference channel operates, in the range 1st
(fundamental mode) to 127th. For example, if the control is set to 2nd and a
reference signal of 1.000 kHz is applied, the instrument will measure signals at its
input at a frequency of 2.000 kHz.
RELOCK EXT. REFERENCE
The 7270 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.
AUTO MEASURE
This control allows an auto-measure cycle to be initiated directly from the main
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2
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12
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