An automatic gain control circuit having a coarse analog gain adjustment section producing discrete increments of db gain and a fine digital gain adjustment section. The digital section provides for adjustment of gain through a final increment of db gain to achieve precise gain setting. The output of the digital section is squared and compared to a reference signal to derive an error signal whose value is fed to an apparatus which iteratively determines the precise coarse increment and fine digital settings to achieve the final desired gain setting.
The present invention is a method and apparatus for implementing a two stage AGC circuit. In the preferred embodiment, the present invention is used as part of a receive channel in a modem. The first stage of the AGC is a "coarse" AGC and is used to track large signal transients of an input signal. The coarse AGC locks on to transient signals without excessive settling time. In operation, the coarse AGC acquires a new signal by using a nonlinear clipped feedback loop technique supported by a linearized feedback loop. The coarse AGC stage uses an error signal derived from the noncoherent power fluctuations of the incoming signal. The second stage of the AGC circuit is a "fine" AGC using a decision-directed coherent amplitude error signal and a quick linear feedback loop to correct for finer signal level fluctuations. The fine AGC has a high pass characteristic which decouples its response from that of the equalizer for stability reasons. The present invention avoids performance and response limitations of prior art AGC's in that the coarse stage is not required to have wide band response and associated noisy tracking response. Further, when the input signal is in quadrature amplitude modulation (QAM) the coarse stage tracks by using a nonlinear power detection algorithm which removes the effect of data power modulation. The fine AGC stage utilizes a decision-directed (coherent) error signal and a linear feedback loop with zero excess delay so that wideband response can be achieved without introducing amplitude modulation error in the signal path. As a result, the two stage scheme of the present invention can track rapid gain changes and restore correct data detection within a few baud, limiting error corruption to only one data block.
An amplifier device has a gain programmable between 2.sup.n and 2.sup.n-1 in steps of 2.sup.p where p<n and n is the number of gain control bits. The device comprises a plurality of stages having two gain values selectable by a control bit and at least two identical stages having a different control bit. Each stage has a maximum gain less than 2.sup.n-1.
A method of automatic gain control in both analog and digital domain is performed by receiving an incoming analog signal, determining an overall gain factor, determining a coarse analog gain control value and a fine digital gain control value, each of which, when taken together substantially equals the already determined overall gain factor, modifying the incoming analog signal using the coarse analog gain control value to form a coarsely adjusted digital signal, digitizing the coarsely adjusted digital signal, and using the fine digital gain control value to process the coarsely adjusted digital signal to form an outgoing digital signal, wherein the outgoing digital signal has been modified in both the analog domain and subsequently in the digital domain to achieve an appropriate signal to noise ratio.
To provide a programmable-gain amplifier in which sufficiently fine and linear gain control according to the logic levels of control signals can be realized with a simple circuit configuration, a programmable-gain amplifier includes a cascade connection of differential amplifiers (E.sub.1 to E.sub.n) and a current controller (100) for controlling a product of corresponding emitter currents supplied to each of the differential amplifiers (E.sub.1 to E.sub.n) according to the logic levels of binary control signals. The current controller (100) has a current mirror circuit in which the emitter of each bi-polar transistor (Q.sub.0 to Q.sub.n, Q.sub.c1 to Q.sub.cm) is connected to the ground through a MOS (M.sub.0 to M.sub.n, M.sub.c1 to M.sub.cm) transistor having a relative on-resistance designed to be in inverse proportion to the relative emitter size of the bi-polar transistor, so that the emitter current can be exactly controlled.
An apparatus for use in transmitting compressed digital voice data includes an analyzer for compressing digitized voice data according to an algorithm which produces a root mean square of the voice data. An analog to digital converter is used to convert analog voice signals to digital data as an input to the analyzer. An AGC circuit controls to analog voice signal to keep it within predetermined limits for digitization, with the AGC setting being determined by the root mean square of the digitized data. The digitized data includes data representative of the root mean square adjusted by the inverse of the change induced by the AGC. A speech synthesizer expands the digitized data at a remote location to convert it back to analog speech signals.
