Design and Implementation of Spur Reduction Technique for Carrier NCO in GPS Receiver

In the GPS chosen there is an immense scope in understanding and developing this technology for better post-correlation SNR. Most interesting part of this technology is to improve the post-correlation SNR in GPS receiver. The scope of this paper is to achieve improvement in the post-correlation SNR. In baseband Correlator, local carrier generator is designed using NCO. In order to achieve post-correlation SNR improvement Dither logic is added to the NCO. Adding Dither to the N-bit NCO, (N-3)-bit PRN generator is designed and modelled. The NCO-Dither block is integrated to the baseband Correlator. The post-correlation SNR during acquisition is evaluated for with and without Dither collecting the real time GPS receiver IF samples. The functional simulations were carried out for the sampling frequency of 16.368 MHz without Dither SNR of 11.99 dB and 13.03 dB with Dither, for 45.15 MHz without Dither SNR of 14.83 dB and 15.52 dB with Dither, and for 56 MHz without Dither SNR of 14.18 dB and 14.91 dB with Dither. It shows that incorporation of NCO-Dither mechanism improves SNR by 0.7 dB to 1 dB. The code chip and frequency bin estimate of the acquired signal obtained using NCO-Dither mechanism match with that obtained using the standard correlation approach.