Measurements of the noise levels at the output of surface and bulk channel charge-coupled devices with three-phase overlapping polysilicon electrodes are presented. Pulser noise, correlated transfer noise, shot noise, dark current noise, and electrical insertion noise at the input have been measured and studied. The dependences of the electrical insertion noise and the transfer noise on charge packet size and clock frequency are discussed in detail and the latter related to interface state densities. New schemes and input circuits for low-noise electrical insertion of the signal charge are discussed. Our measurements indicate that the noise levels due to the intrinsic noise sources (transfer and storage noise) agree with our physical understanding of the device operation. The noise levels due to the extrinsic noise sources (pulser noise and electrical insertion noise) are above the expected theoretical values.