555 makes handy voltage-to-time converter: 02/01/01 EDN-Design Ideas / PDF contains multiple circuits - scroll to find this circuit.

Analog Tachometer for Gasoline Engines:

Circuit Converts Pulse Width to Voltage #1: 10/25/01 EDN-Design Ideas  / PDF contains multiple circuits - scroll to find this circuit.

Circuit Converts Pulse Width to Voltage #2: 10/28/99 EDN-Design Ideas  / The circuit is optimized for a positive-going pulse width of1 to2 msec, repeating at intervals of approximately17 msec. The output produces a voltage of0.95V for a1-msec pulse to2.25V for a2-msec pulse. 

Frequency to Voltage Converter uses Sample & Hold to Improve Response & Ripple: National Semiconductor Application Notes,28-Jun-1996 

LM2907 Tachometer / Speed Switch Building Block Applications: National Semiconductor Application Notes,04-Nov-1995 

New Phase Locked Loops:

Stereo Compressor:

V / F Converter ICs Handle Frequency to Voltage Needs: 09/28/01 EDN-Design Ideas  / PDF contains multiple circuits - scroll to find this circuit