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($a) MOTOROLA Technical Developments Volume 3 March 1983
ACCURATE MICROPROCESSOR CONTROLLED HIGH
VOLTAGE CURRENT GENERATOR
By Michael M. Stamler
In a typical radio installation the base station equipment is remotely located from the operator's console. Usually a telephone line is used to interface the console to the base upon which both audio and control signals are impressed. Base station control signalling is implemented with either DC cur- rents or tones. The controller described here is of the former type.
In a DC base station controller, it is required to generate accurate DC currents from 2 to 15 mA, in either polarity, at voltages as high as 180 volts. The currents must also be generated on the telephone lines without affecting balance, necessitating an isolated circuit independent of earth ground. Previous designs have utilized simple programmable current sources to provide a maximum of three currents of either polarity. Any currents that were of a different value than the standard three required additional engineering and factory effort.
One main factor that limited the number of unique currents was the need to provide isolated con- trol lines from the user panel on the earth ground side to the current generating circuitry on the isolated side. The interface means usually consisted of either a dedicated optoisolator or a hardwired input from a switch. The latter technique was possible because the actual user switch was close to the generating electronics thus allowing for easy access. However, the current system in which this new controller is being used does not provide for interfacing to the user's actual switch since all its control lines terminate at a remotely located microprocessor I/O port which does not provide for any hardware inputs. All control from the user panel to the remote electronics is performed with special inter- processor signalling techniques which are decoded at the remote electronics into control signals for the current generating electronics. Thus, all interfacing between these control lines and the isolated side must be done with dedicated isolating circuitry such as optoisolators.
CIRCUIT DESCRIPTION
The circuit block diagram as shown in FIG. 1 is divided into primary and secondary sides, the former interfacing to the resident microprocessor, and the latter to the telephone line. The isolating means for the high voltage power supply Is a transformer coupled DCDC converter while...
