Frequency to Voltage Conversion: how?

[BigBrother at mail.utexas.edu]

MAP and BP sensors apparently do not return voltage as output.  Instead
the oscillate at a frequency which corresponds to the absolute pressure.
What electronics are involved with frequency to voltage conversion?

This brings up another point: what output frequency corresponds to what
pressure level?  I'm sure we could test one to find out, but isn't there
already some book or manual that gives in depth data on sensor readings?
If we have to test every sensor, it is going to take a LONG time.  Surely
Ford, Chevy, and/or Bosch, publishes a manual for servicing EFI that
describes how each particular type of sensor responds.