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A compact mushroom balanced-unbalanced (balun) filter which is based on negative and zero-th order resonances is proposed in this paper. The balun filter consists of two pairs of mushroom cells. Differently from the conventional mushroom cells, each pair of the proposed mushroom cells are connected by a coplanar line in the bottom ground. With this coplanar line, not only a filter with balun function is realized but also the corresponding negative and zeroth-resonant frequencies are significantly reduced compared to conventional mushroom cells. This means a large reduction in size for a given resonant frequency. The balun filter is designed, fabricated and measured with a center frequency of 2 GHz and fractional bandwidth of 5%. The output amplitude imbalance is less than 0.6 dB, and differential phase is between 180° ± 0.7° in the pass band. In addition, the minimum insertion loss of the balun filter in the passband is 0.9 dB and with a size smaller than 20 mm*20 mm.