An apparatus and a method for performing an attenuation function on a signal using an attenuator in a cable broadcast receiver having a Low Noise Amplifier (LNA) that does not perform an attenuation function on a signal are provided. In the method, an input Radio Frequency (RF) signal is tuned, The RF signal is converted into an Intermediate Frequency (IF) signal. An intensity of the converted IF signal is compared to a threshold, and an RF Automatic Gain Control (AGC) value is generated based on a result of the comparison. When the generated RF AGC value is smaller than an RF AGC threshold, the attenuator is turned on to attenuate the input RF signal by a predefined amount. Therefore, a high SNR, such as in an analog-digital composite signal, may be obtained without using an LNA that performs an attenuation function on a signal.