Summary: In this paper, a high-frequency resolution Digital Frequency Discriminator (DFD) IC using a Time to Digital Converter (TDC) and an edge counter for Instantaneous Frequency Measurement (IFM) is proposed. In the proposed DFD, the TDC measures the time of the maximum periods of divided RF short pulse signals, and the edge counter counts the maximum number of periods of the signal. By measuring the multiple periods with the TDC and the edge counter, the proposed DFD improves the frequency resolution compared with that of the measuring one period because it is proportional to reciprocal of the measurement time of TDC. The DFD was fabricated using 0.18-um SiGe-BiCMOS. Frequency accuracy below 0.39MHz and frequency precision below 1.58 MHz-RMS were achieved during 50 ns detection time in 0.3 GHz to 5.5 GHz band with the temperature range from -40 to 85 degrees.