Please be clear about what volatges are you refering to as a SWR meter is a stand alone instrument for measuring standing waves usually works without any external power. Always write your querry in detail so that its understood at first reading.
Usually is a detector, a diode with some RF characteristics ( low junction capacitance, etc ) that rectifies the coupled RF signal from the two lines and is driven to a voltmeter, in the simple ones an analog one, on the good ones a digital one.
I have seen one with the detectors of Analog devices ( AD 8307 ).