Novel techniques for obtaining high isolations in practical microwave FET MMIC mixers

This paper presents novel techniques for obtaining high RF/LO to IF isolations in microwave mixers without the need for output filtering. Two techniques are described which essentially involve feeding some of the input LO and RF signals forward to cancel out with the LO and RF signals appearing at the output. The first technique involves using a single MESFET as the mixer element, and relies on the LO to RF leakage of ihe mixer to obtain the required signal for cancellation. The second technique involves a cascode connectioL of two MESFETs to mimic the dual-gate mixer, with the LO and RF signals tapped-off at the input separately, and then applied at the output by accordingly inverting and amplifying to allow cancellation of the RF and I, O. Due primarily to cost considerations, the first technique was chosen for implementation as this consumed the minimum circuit area. The MMIC demonstrator chip, which measures only 1.4mm × 1.3mm, has achieved measured results exhibiting over 30dB of RF/LO to IF isolations between 6-1 8GHz.