Zero IF filter

[Mas313]

Hi,
I want to filter out image frequencies ( positive and negative image frequencies) after downconversion to zero IF and need a band pass filter to pass frequencies between -50khz and 50 Khz. I have found fir filter in the library but it requires only one cutoff frequency. I want to implement dual channel receiver so I downconvert using nco and using following for filtering images but not achieving thr desires results as not knowing how to filter negative frequencies.

#define SAMPLERATE 1024000.0f
#define CUTOFF_HZ 50000.0f
#define NUM_SAMPLES 1024

float fc = CUTOFF_HZ/SAMPLERATE; // cutoff frequency
unsigned int h_len = 64; // filter length    
float As = 70.0f; // stop-band attenuation

float complex r[NUM_SAMPLES]; // zero if  signal nco output
float complex rf[NUM_SAMPLES]; // filtered signal

// design filter from prototype and scale to bandwidth
firfilt_crcf q = firfilt_crcf_create_kaiser(h_len, fc, As, 0.0f);
firfilt_crcf_set_scale(q, 2.0f*fc);

while (1) {
    ReadzeroIFsamples(r, NUM_SAMPLES); //
    for (int i=0; i<NUM_SAMPLES; i++)
        firfilt_crcf_push(q, r[i]);
        firfilt_crcf_execute(q, &rf[i]);
    }