Abstract: A direct conversion circuit that not only down-converts the received modulated signal using a down-converting mixer into a baseband signal, but also, after performing a passive low pass filtering to remove higher-order components, performs up-conversion of the baseband signal using an up-converting mixer. Active elements such as highly sensitive amplifiers do not operate on the baseband signal itself, but on the up-converted version of that baseband signal, thereby reducing the 1/f noise introduced by those active elements. The downstream circuitry after the up-conversion may be coupled by intervening capacitors since the downstream circuitry is operating on a higher frequency signal. Accordingly, the DC offset introduced by the downstream active elements is reduced.

Abstract: An oscillation circuit including a resonating element such as a crystal, an inverting amplifier and a resistor that each span the resonating element terminals, and two capacitors that capacitively couple the resonating element terminals to ground. An AC current source such as a temperature compensated and properly trimmed ring oscillator generates a differential AC current when active. The differential AC current has a frequency that is within a tolerance of the resonant frequency of the resonant element for a given set of operating conditions. Two buffers connect the differential outputs of the AC current source to respective terminals of the resonating element to thereby shorten startup time. A control logic circuit carefully times the application of the differential AC current to the resonating element terminals such that the current is applied for a sufficient time such that startup would occur under any anticipated operating condition.