Frequency conversion forms an integral block of the electronic circuits used in various applications including energy harvesting, communications and signal processing. These frequency conversion units however require external power sources and occupy a large device footprint making it difficult to be integrated in micro-circuits. Here we demonstrate that nanoscale magnetic tunnel junctions can act as frequency converters without an external power supply or DC bias source. The device directly mixes an external microwave signal with the internal spin precession oscillations to create new frequencies tunable by an external magnetic field in a single device with a small device footprint. We observe up-conversion and down-conversion of the input signal for excitation frequencies between 2 GHz and 6 GHz. We also show that the device acts as a zero-bias rectifier that can generate voltages exceeding 12 mV when the excitation frequency matches the natural oscillations mode of the device.