Abstract: A downhole signal generating mud pulser has a poppet and orifice signal valve to variably resist the drilling fluid stream. The poppet is upstream of the orifice and is moved by a piston in fluid communication with opposite flow related sides of the signal valve such that more pressure drop across the valve applies more piston force to open the valve. The poppet is spring biased toward the orifice and the piston force and spring balance when a preselected operating pressure exists. A servo valve controls flow in a by-pass loop to apply additional pressure to the piston to create a signal pulse when open. The servo valve is urged closed by flow in the servo loop and the closing is retarded by a dashpot to determine signal duration. The servo valve is spring biased to open and is automatically latched closed by a latch that responds to a solenoid actuated to release the servo valve.