In this paper, we propose a new approach to enforcing the passivity of a reduced system of general passive linear time invariant circuits. Instead of making the reduced models passive for infinite frequencies, which is difficult and inefficient using state-of-the-art optimization based methods for circuits with many terminals and operating in wideband frequency ranges, the new method works on the signal waveform driving reduced models. It slightly shapes the waveforms of the signal such that the resulting signal spectra are band limited to the frequency range in which the reduced system is passive. As a result, the reduced models only need to be band-limited passive (also called conditionally passive), which can be achieved much easier than traditional passivity for a reduced system, especially for ones with many terminals or requiring wide band accuracy (more poles). We propose to use spectrum truncation via FFT/IFFT and low-pass filter based approaches for transient waveform shaping processing. We analyze the delay and distortion effects caused by using low-pass filters and propose methods to mitigate the two effects. Experimental results on several interconnect circuits demonstrate the effectiveness of the proposed methods.