Are you filtering in the frequency domain or the time domain? The Fourier transform process creates negative frequencies (mathematically). In the time domain, where the filter deisgn tools work, negative frequencies do not exist as Ravens Fan pointed out.
If you downconverted a signal with a carrier frequency of 1 MHz and amplitude modulation of 1 kHz, then what you started with had signal components from 999 kHz to 1001 kHz and the converted signal has a spectrum from -1 kHz to +1 kHz. If you filter the time domain downconverted signal with a low pass filter with a cutoff frequency of 1.1 kHz, you would retain all of the information in the original modulated signal (assuming an ideal filter). If you then upconvert the filtered signal back to 1 MHz, you would get the same signal you started with, less any broadband noise which the filter removed.
The negative frequencies in the spectrum cannot be ignored because phase information would be lost without them.
Lynn
