Abstract: A four terminal ohmmeter apparatus includes circuitry for blanking the meter read-out in the event one or more connections to a device or structure under test have been improperly made. The voltage drop between current terminals is determined and compared with a threshold value. The voltage drop between potential terminals or between potential and current terminals, as caused by a separate a.c. source, is also determined and compared with a standard. Individual indicators inform the operator of the validity of each connection in order that corrective action can be taken if necessary. Voltage inputs are also utilized for ascertaining whether a correct meter range is being used and for changing the range if it is improper.

Abstract: A four terminal ohmmeter apparatus includes circuitry for blanking the meter read-out in the event one or more connections to a device or structure under test have been improperly made. The voltage drop between current terminals is determined and compared with a threshold value. The voltage drop between potential terminals or between potential and current terminals, as caused by a separate a.c. source, is also determined and compared with a standard. Individual indicators inform the operator of the validity of each connection in order that corrective action can be taken if necessary. Voltage inputs are also utilized for ascertaining whether a correct meter range is being used and for changing the range if it is improper.

Abstract: The phase detector of this phase lock loop includes a transistor switch shaping the output of an Exclusive-OR gate which combines a data signal with a square wave local clock signal from a VCO for producing a digital error signal that is integrated to produce an error voltage that is applied to one side of a differential amplifier in an active lowpass filter. Other means integrates the local clock signal to produce a reference voltage that is applied to the other side of the amplifier, the output of the latter being a control voltage which drives the VCO. In a preferred embodiment, the reference voltage means and the path between the gate and the one side of the amplifier both include a switching transistor (the two being on a common substrate) and resistors of the same type and value for causing variations in the two input lines to the amplifier to track as a function of time and temperature.