Using a polarity reversal technique, precise dc voltage measurements were made with both digital and analog voltmeters. The results were analyzed by calculating Allan variances and spectral densities for polarity-reversed measurements and comparing them with similar analyses of unreversed or unipolar measurements of the same processes. The Allan variances and spectra for the two measurement methods were found to agree quite well, particularly for sampling times corresponding to the 1/f noise regime for the measurement of Zener-diode voltages. The results show that polarity reversals do not affect 1/f noise of the Zeners. In polarity-reversed measurements, a rectangular voltage waveform is presented to the detector. Since this waveform can be synthesized by ac waveforms, the results also demonstrate that ac detection does not remove 1/f noise in the voltage source being measured. We also estimated the spectral density and Allan variance of some thermal (Johnson) noise processes and found that the results agree with the Nyquist expression to within 4%. This provides direct validation of our measurement and analysis methods.