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This option adds a circuit board with an isolated input that allows an external DC control voltage to set the beam voltage in the Model 11-020 electronics. A user-supplied digital to analog converter (DAC) can then be used to program the beam voltage, despite the fact that the beam voltage control circuitry is not referenced to chassis ground. The board is mounted vertically inside the chassis on the left side. A BNC connector on the rear of the 11-020 is provided for the DAC connection, and accepts scaled inputs from zero to +10 volts, corresponding to a beam voltage of zero to -1000 volts. A toggle switch located beside the BNC connector allows either Local (front panel control) or Remote (DAC or programmable power supply) operation. With the switch in Local position, all controls function as described in the 11-020 manual.
With Remote operation, if the beam voltage switch and crank knob are not in the positions described in the preceding section, the beam voltage will not accurately reflect the DAC input.
When calibrated, the beam voltage deviation from nominal DAC input is ±.5 volts over the range of about 5 to 1000 volts. Below five volts the error increases somewhat. These numbers assume a "perfect" DAC, and so do not include errors from the external digital to analog conversion.
The front panel digital meter (DPM) reads the sum of the beam voltage and the retard voltage. Therefore, when the retard voltage knob is in any position other than full counterclockwise, the DPM reading will differ from the DAC input by whatever retarding voltage is being used. The actual beam voltage is not affected.
In addition, the DPM is inherently less accurate than the external programmed voltage. In short, with DAC control, consider the DPM reading as only a rough indication of beam voltage.
The absolute maximum voltages at the BNC are +15 volts and -5 volts.
Before switching back to Local control, set the beam voltage switch and crank knob to zero volts. Then switch to Local and use the controls to dial in the beam voltage.
The rear panel MHV jacks should connected as described in the 11-020 manual, including the jumper between J13 and J14. The "remote programming" feature using J13 and J14 has nothing to do with the DAC control input described here.
There are two trim pots on the isolation board, Scale Trim (VR1) and Offset (VR2). The latter nulls DC errors in the circuitry, and will not need adjustment unless components on the board are replaced. Scale Trim (VR1) can be used to calibrate the beam at a specific voltage, and is normally set to give 500.0 beam volts for 5.000 volts input. VR1 is the trim pot closest to the front panel.
Warning: This procedure requires removing the top cover of the 11-020, which exposes components carrying life-threatening high voltages whenever AC power is applied to the unit. Only qualified personnel should attempt to service or adjust HV power supplies. Consult the 11-020 manual for additional precautions and warnings.
A stable DC source and two high-accuracy digital meters are required. The second DVM is used to monitor the DC input at the BNC. Before changing the setting, the 11-020 should be warmed up for 20 minutes with the beam voltage set to about 500 volts.
At the factory, the Scale Trim adjustment was set by viewing the output from a 10 Meg ohm input impedance 6½ digit DVM, connected at a join between the F1 and F2 outputs on the 11-020 rear panel. Before joining the outputs make sure that AC power is off and that the front panel filament knob is set full counterclockwise.