Are you sure that the tachometer is really intended to give a DC voltage proportional to the rotation speed?

Having a DC generator on a motor wpiöd be a way to measure the speed of a motor.

But for a servo motor, it's more common to want to count the number of revolutions (or fractional revolutions), to not only be able to measure speed but how far the motor has rotated. In that case, you normally get out pulses. Either pulses which fixed length (which would look like a DC voltage proportional to the rotation speed) or pulses who also gets shorter as the frequency (RPM) increases.

A motor with a generator on the axis normally needs a low-pass filter to give a smooth DC voltage. If the signal is fed to a mechanical RPM instrument (basically a galvanometer but displaying RPM instead of volt) then there is a mechanical low-pass filtering done in the instrument.

A motor with a pulse-generating tachometer on the other hand informs about the speed as pulses - not as the amplitude of the pulses. So you would count number of pulses/second without caring about the absolute voltage values of the pulses.

Don't you have a link to a datasheet for your motor? You really are scared of giving us too much information. A reference to used motor, and a oscilloscope capture of the tachometer output and drive voltages in the first post would have been just so much faster to get people to know what you are doing, and what problem you are seeing.