Abstract: A phase locked loop is frequency modulated by means of a modulation signal pplied to the loop filter thereof. The circuit is designed with two time constants, one of which determines the square wave modulation response thereof and the other the loop settling time. Each of these time constants is chosen to optimize the loop modulation response and the loop settling time.

Abstract: A clamp circuit in which, so long as the input voltage ranges between preselected voltages +E and -E, the output voltage tracks the input voltage. When the input voltage goes beyond +E or -E, the output voltage is clamped to +E or -E. When, thereafter, the input voltage is changed in the opposite sense by X volts, where .vertline.X.vertline..ltoreq. 2E below its maximum value the output voltage changes by X volts.The clamping circuit utilizes a differential amplifier and circuits providing two feedback paths between the output and inverting input of the amplifier, one or the other being operable dependent on the value of input signal to provide the clamping action described above.

Abstract: A rotor mounting a sensor to be inserted into the earth is rotatably mounted within and locked to a cylindrical housing. The latter may be dropped to the earth and may come to rest in any orientation. When at rest, the rotor is unlocked and a drive mechanism rotates the rotor with respect to the housing. Gravity causes a ball in a tortuous path in the rotor to traverse the path and to drop into one of a plurality of annularly spaced recesses in the housing when the rotor reaches a desired orientation. With the ball in place in a recess, a wedge surface on the rotor wedges against a portion of the ball extending from the recess and this stops the rotor--in the desired orientation.