Advanced Measurement Technology 273A, Руководство пользователя

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Chapter 5—Operating Instructions 43
a. Press SET IR. Then key and enter an R, of "0." It is always advisable to start with
zero IR compensation. With a higher starting value, there is a definite risk factor in
that the system may oscillate when the Positive Feedback Mode is activated. HIGH
POWER, HIGH AMPLITUDE OSCILLATIONS CAN BE DESTRUCTIVE AND
DEFINITELY SHOULD BE AVOIDED.
b. Press the POSITIVE FEEDBACK key, establishing Positive Feedback mode
operation.
c. Program the Model 273A to apply a 62.5 Hz, ±50 mV square wave to the cell. The
following parameter values can be used. Note: The ±50 mV figure is not critical.
Smaller or larger amplitudes, if more appropriate to the chemistry, can be used.
Note: If LINE SYNC is ON (OVERRIDE 14), the frequency will be 15 Hz (60 Hz
power) or 12.5 Hz (50 Hz power). The procedure will generally be easier to
perform with the LINE SYNC off (less flicker on the oscilloscope display).
E / I 1 -0.05 V with respect to the potential of interest
DELAY 1: PASS (see Note)
SCAN 1 STEP
E / I 2 +0.05 V with respect to the potential of interest
DELAY 2 PASS (see Note)
SCAN 2 STEP
E / I 3 -0.05 V with respect to the potential of interest
Control Mode CONTINUOUS
Note: 62.5 Hz rate results from setting both delays to PASS.
d. Establish the Cell ON state (CELL ON and CELL ENABLE). Then press [START] to
apply the square wave to the cell. The cell should be completely set up for the
measurements to be made. (Any change at the cell will probably change R .)
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e. Monitor the I MONITOR connector with an oscilloscope. The Model 273A should be set
to the same current range as will be used in the forthcoming measurement. If the
Current Range setting is changed, the actual value of IR compensation can change as
previously discussed. At the oscilloscope, use internal triggering and display the
waveform so that the waveform's risetime can be readily observed. A horizontal
deflection factor of 2 ms per division will generally give good results.
f. Press SET IR so that the set value of R (it should be "0") is displayed on the Model
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273A.
g. While observing the displayed waveform on the oscilloscope, hold in the Scan Setup [ ]
key. The R compensation will increase, causing the decay intervals in the displayed
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waveform to become shorter. The longer the key is held in, the greater will be the
applied compensation. The actual value can be directly read from the Model 273A
display.
Figure 6 illustrates the compensation's effect. For optimum stability, the rise time should be as
fast as possible without ringing. When the uncompensated resistance is about 90% to 95%
compensated, the waveform will show a smooth decay with perhaps just a bit of undershoot. This