We have designed and built a low‐cost current–voltage converter with a bandwidth larger than 200 MHz and a fixed ac insertion gain of 20 dB for use in multiple‐frequency phase fluorometry (MFPF). We describe its application with the Hamamatsu R928 photomultiplier (PMT) and discuss problems of radio‐frequency (rf) interference, signal‐to‐noise ratio (S/N), PMT nonlinearity, and overall frequency bandwidth. The separation of ac and dc signal paths introduces the possibility of direct determination of relative modulation in two simultaneous measurements. We demonstrate that systematic errors such as PMT nonlinearity can be completely eliminated in MFPF by an appropriate experimental procedure and report an accuracy of our dual‐beam instrument of 5 ps.