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We present a simple method for widening the Intermediate frequency (IF) bandwidth of high frequency superconductor-insulator-superconductor (SIS) mixers. The method only requires a mixer with a single ~1 μ m2 tunnel junction and an IF transformer fabricated with standard printed circuit board technology, avoiding the complexity of requiring several very small tunnel junctions in series or the use of on-chip IF tuning circuits. The method is based on employing a multi-stage microstrip IF transformer to transform the dynamic impedance of the SIS mixer chip to 50 Ω output over a wide IF bandwidth. This method assumes moderately high capacitance junction (75 fF/μm2), and is particularly useful for high frequency SIS mixers. In this paper, we shall first describe the design methodology of the IF transformer using an existing 650 GHz SIS mixer chip design as an illustration and then present the measured IF performance of the mixer.