A High-Dynamic-Range Front End for an Upconversion Car-Radio Receiver

A monolithically integrated high-performance front end for an AM radio receiver is presented. Medium- and long-wave reception without any tuned-circuit alignments or band switches is made possible by using an intermediate frequency of 10.7 MHz, well above the maximum frequency to be received. Image- and oscillator-harmonics-related spurious responses are rejected by a simple low-pass filter. The upconversion concept puts high demands on the front-end circuits. A high intermodulation free dynamic range is realized by applying several unconventional and new circuit techniques. A low-noise low-distortion wide-band input amplifier with capacitive negative feedback realizes the required antenna match. The gain of this amplifier is automatically switched to a lower value in the vicinity of transmitters with high field strengths. A double-balanced mixer, driven by an oscillator employing a novel amplitude stabilization technique, provides the frequency conversion up to 10.7 MHz. Digital toning can be achieved very simply because only the local-oscillator frequency has to be varied. The dynamic range of this front end amounts to 130 dB. Field strengths up to about 20 and 200 mV/m can be handled without noticeable second- and third-order intermodulation, respectively.