The KP preamp circuit is a differential measurement that, for best results, requires a balanced input. To do this, two variable capacitors are used: C2 for coarse adjustments (3-36 pF) and C14 for fine adjustments (1.6 - 5 pF).
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This setup is intended to perform a coarse calibration of your equipment. The goal is to display approximately 20 cycles of the output wave to prepare the equipment for the next calibration step. This will help eliminate difficulties associated with varying oscilloscope settings.
- DC Offset: 0
- Upper BP: 0 V
- Lower BP: -0.02 V
- Frequency: 300 Hz
- Amplitude: measure at Test Point (TP) Vcoil and adjust Amplitude to produce 0.5V p-t-p
- Scan steps: 0
- Set BP Source to "Variable BP filter": Above 16 KHz
- Back the sample away from the probe
Now, the test:
- Shield the probe and sample to reduce 60 Hz noise
- Amplitude: 0
- DA Delay: 50 msec
- Upper BP: +2 V
- Lower BP: -2 V
- Scan steps: 1
- Click 'START'
Set both C2 and C14 to midrange values (this can only be assured by looking at them.) Measure the preamp output on the oscilloscope using the "Output" BNC on the front pannel. Trigger the oscilloscope off Vbacking TP on the KP6500 board. Every time Vbacking changes between levels there is a corresponding transient which eventually decays away exponentially. Adjust C2 and C14 until the exponential switching transients are minimized. For most probes this will require C2 set near its midpoint. Adjust C2 first, and when you are close, home it on the proper adjustment whith C14.
To guide you, what follows is some typical data taken with the above settings. For this data, the first transient corresponds to Vbacking falling from +2 V to -2 V, and the second transient is due to the reverse transition. The vertical scale is in volts and the horizontal scale is in seconds.
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Figure 4 shows C2 + C14 optimized. Note the horizontal scale change. Note also that the initial spike seen in the transient changes polarity as you adjust from "C16 too small" to "C16 too large". The spike never completely goes away.
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If a different probe is to be used or if a new probe-to-sample distance is used, rebalance the circuit with the new configuration. If you have moved the probe-to-sample distance, the old distance can be obtained by adjusting the probe-to-sample distance until the exponential switching transient is minimized.