RF Techniques at ISSCC 2014
February 10, 2014, ISSCC, San Francisco—This session focused on RF building blocks that can minimize or eliminate external component in radio systems in wireless systems. The first half of the session looked at the transmission side, mainly for cell phones, and the remainder offered receivers.
One interesting paper from David Murphy at Broadcom is a noise-canceling receiver that inverts the noise elements and adds the inverted signal to the inputs to cancel noise. The challenges are that the blocker signal harmonics get aliased into the input and are amplified by the front-end circuitry. The various clock signals also constructive and destructively combine to make a pseudo- local oscillator. And finally, the local oscillator harmonics get amplified in the front end.
To overcome these problems, one technique is to use a m-input, m-output operational trans-impedance amplifier, which effectively rejects the local oscillator harmonics. By combining the signals with various weights, you can also decrease the synthetic LO. Together, these techniques reduce the blocker harmonics and other noise sources whe the main and inverted signal paths are combined.
Another technique is to use multiple RF-Gm blocks to cancel the even harmonics. By combining multiple gain blocks, it is possible to eliminate other lower-order harmonics and improve small-signal head room while enhancing large-signal harmonic blocker tolerance.
Using both techniques results in an enhanced noise-canceling architecture. The test results show that this design achieves better noise figures and noise blocking than others in the literature.
In-Young Lee from KAIST described an integrated TV tuner front end circuit. This design overcomes many of the issues of TV tuner designs such as desensitization, harmonic rejection, high linearity even with high-power inputs, image rejection, low phase noise, and AGC. The tuner has a very wideband, low noise front end with a frequency response from 48-1000 MHz and a noise figure less than 4 dB.
The linearity is addressed with a feedback LNA with even-order harmonic canceling, and a 4th ordre active RC low pass filter. Instead of the normal Sallen-Key filter topology using a unity gain op-amp, this filter feeds back the output through a sub 1 ohm source follower. This topology also removes a zero in the response which further improves stop-band rejection at higher frequencies. The lower the output impedance, the higher the Q and noise figure, and the source follower configuration pushes poles ans zero further for better wideband response.
