The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb&sngbnd;Nb point‐contact junctions at 4.2 °K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1, 0.3, and 9.2 GHz. The point contact is in a nonresonant structure and fed by a constant current source: neither a strong rf coupling of the contact with a cavity nor a stabilizing shunt resistance was proved necessary. The direct observation of frequency conversion was performed with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1600 GHz. The experiments with intermediate frequenciesfiof 0.1 and 0.3 GHz indicate that conventional frequency conversion does not occur but that the observed signals are related to noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator‐mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. The results show that low‐frequency current components are induced in the junction, which can give large spurious signals when small i.f. are used.