A CMOS 80-400MHz fifth order Chebyshev Gm-C low pass filter with a unique auto tuning system is presented. Both Gm and C are tuned digitally leading to a 5X tuning range. A digital magnitude-locked loop liked tuning system is proposed. The tuning system uses a digital calibration scheme, broadening the tuning range dramatically. A fifth order Chebyshev Gm-C filter with the proposed auto tuning system was designed with TSMC 0.13µm RF CMOS process for verification. Measurement results show that the cut-off frequency of the filter can be tuned between 80-400MHz, with an average tuning error of no larger than 3.6%. The dynamic range of the filter for THD less than 1% is 60dB. The power dissipation is only 5.5mW with 1.2V supply voltage.