Tunable upconversion mixers that have a controllable passband response and provide substantial image rejection, making costly post-mixing filtering at least optional, if not unnecessary for many applications. A single mixer is able to support several frequency bands (and/or be accurately tuned within one frequency band) by means of varying the capacitance in a tunable load circuit for the mixer circuit. The tunable mixer comprises a mixer circuit having inputs for receiving a signal to be upconverted and an oscillator signal to be mixed with the signal to be upconverted. The mixer generates a mixer output signal at a desired sideband frequency and an image signal at an image frequency. A tunable load circuit is coupled to the mixer circuit and responsive to a control signal to resonate and pass signals in a desired passband corresponding to the mixer output signal at the desired sideband frequency, and which attenuates signals in an attenuation band that includes the image signal at the undesired image frequency.