Wave Format - Keithley 148 Instruction Manual

MODEL 148 NANOVOLTMETER OPERATION
b. Improper shielding can cause the Model 148 to react in one or more of
the following ways:
1. Needle jitter or instability, from 10% to 20% of full scale.
2.
High offset (dc bias). Changing the power cord polarity or the
connection between the LO and GND terminals may affect the amount of offset.
3.
Slow response time, sluggish action and/or inconsistent readings
between ranges.
4. Amplifier saturation.
Observe the wave form with an oscilloscope
connected to the DEMODULATOR TEST Jack (Figure 3). With the input shorted,
it should approximate the wave form shown in Figure 6. If excessive pick-
up occurs, the wave form will resemble that of Figure 7. The circuit
will operate reasonably well as long as the wave formis not clipped, as
shown in Figure 8. At this point a dc offset is introduced.
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FIGURE 6.
Normal Wave Form at
Demodulator with Input Shorted.
Scale is 0.1 v/cm vertical and
10 msec/cm horizontal.
FIGURE 7. Wave Form at Demodulator
Shown with Some Pickup.
Measurements
are adequate;
there is no offset but
some noise. Scale is 0.5 v/cm vertical
and 10 msec/cm ho,rizontal.
FIGURE 8. Wave Form at Demodulator when
Amplifier is Saturated. DC offset is intro-
duced and there is greater noise. Note the
Nanovoltmeter still reacts to the input signal,
Scale is 5 v/cm vertical and 10 msecfcm horizontal.
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