1.5.4 Further Testing
The following tests should be performed to get a good understanding of the use of the controls.
Test 1:
1. Increase/Decrease the value of the Bias Voltage: The DC offset of the modulated bias voltage will
be seen to increase and decrease as the Bias Voltage is changed. Change the Bias Voltage from 1V to
2V. If the Rate of the Bias Voltage change was set to 1V/s, the modulated voltage will take one second
to reflect the change of the bias voltage from 1V to 2V.
2. Change the amplitude of the modulation: The amplitude of the displayed current will immediately be
reflected.
Note
The 100 mV PEAK value of the bias voltage modulation produces a 1 nA PEAK value of
the tunnel current signal as displayed on the oscilloscope. This is because the 100mV bias
voltage generates a 1nA current in the TGS (I=E/R, or I=0.1V/10e8= 1nA).
3. Change the frequency of the oscillation: The frequency of the oscillation will be immediately
reflected by the change in the control.
4. The Channel 1 Drive output attenuator allows the output voltage to be scaled to a full scale value of
less than ten volts. This provides improved resolution when generating very small signals. If Bias
voltages of more than ten mV are typically used, and AC modulations of more than a few mV are used,
the output attenuator can be left at a value of 1.
Test 2:
1. Set the bias voltage to 10 mV.
2. Set the Oscillation Amplitude to 1 mV. The displayed signal on the Oscilloscope will show a sine
wave with a Peak value of 10 pA. To more clearly see the signal, change the display settings of the
Oscilloscope for a range of +/- 100 pA. Alternatively, the autoscale can be turned on.
3. At this point, the input circuitry may be limiting the quality of the signal. Move to the Channel 2 input
section and increase the input gain to a value of 8. An improvement in the signal quality will be seen.
Try different values of input gain to see the effect on the signal quality. When the value is set to too
high of a value, the amplified signal will saturate the ADC and the oscillation will no longer be
displayed.
4. Turn the ADC Input Gain back to 1 and enter a value of 8 into the Channel 1 Drive Attenuation. This
will reduce the full scale range of the Bias Voltage output from +/- 10V to +/- 1.25V with an increase in
output precision of eight times. The quality of the displayed sine wave will improve as the output gain is
increased. When the value of the output attenuation is set too high, the sine wave will clip and become
distorted.
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