Due to the wideband nature of broadcast signals, local-oscillator (LO) mixing problems arise with odd harmonic frequencies. Such issues are not considered in the design of conventional narrowband receivers. Yet this odd harmonic mixing poses a serious problem, as the undesired channels are aliasing into the desired channel. As a result, the receiver's signal-to-noise ratio (SNR) is degraded. For universal tuners, a wideband, highly linear, low-noise CMOS RF front end has been proposed by Donggu Im and Kwyro Lee from the Korea Advanced Institute of Science and Technology together with Hongteuk Kim from Seoul's LG Electronics Institute of Technology.
Their front end comprises an inductor-less, wideband low-noise amplifier (LNA), integrated passive tunable filter, harmonic rejection mixer (HRM), and loop-through amplifier. This LNA shows a gain control range beyond 55 dB with a gain step of 0.5 dB or better while achieving higher linearity and a lower noise figure. For its part, the tunable filter covers the very-high-frequency (VHF) bands entirely without dividing the frequency range by multiple filters. Because the tunable filter and HRM work together, an overall harmonic rejection ratio (HRR) above -65 dBc is reached.
The researchers proposed an active-feedback loop-through amplifier (LTA) through a complementary source follower (CSF). The proposed RF front end achieves voltage gain to 42 dB with a minimum noise figure of 4.7 dB. It achieves a high second-order and third-order input intercept point with low power consumption. See "A Broadband CMOS RF Front-End for Universal Tuners Supporting Multi-Standard Terrestrial and Cable Broadcasts," IEEE Journal Of Solid-State Circuits, Feb. 2012.