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A wide dynamic range microwave frequency discriminator for cognitive radio

Microwave frequency discriminators based on six-port receiver architecture are becoming increasingly popular for a multitude of applications in sensing and identification of spectrum holes, e.g., for cognitive radio applications. One of the key limiting factors of a six-port receiver is its narrow dynamic range which is mainly limited by the use of square-law diode detectors in the primary detector circuit. In order to improve its dynamic range, this paper proposes to use logarithmic detectors for the detector stage of the six-port receiver. Such an approach helps in detecting signals over a wide dynamic range of 80 dB, thus making it almost comparable to that of the available heterodyne receivers. Additionally, the measured frequency error did not exceed ±51 MHz (3.5%) over the frequency range of 1 to 2 GHz using the equations for the power proportional voltages to estimate the phase angle and accordingly, the frequency, which can then be minimized using further digital post-processing. Also, since the basic operating principle of such a receiver was essentially based on square-law detector circuits so far, this paper also describes the new working principle that needs to be employed if using logarithmic detectors.