Telecommunications radios (including those in the microwave domain), employ several methods to obtain accurate power levels throughout the gain stages of the radio in both the transmit and receive directions. The radio may be calibrated in the factory to accurately set a single power level at which the radio will operate. Alternately, the radio may be dynamically calibrated by a digital system which utilizes tables of calibration data to maintain power levels at various detectors in the data path.
Factory calibrated radios generally use an analog feedback loop to control power levels in the data path. These radios provide a single output power due to the use of analog feedback via one or more feedback loops in the signal path. The analog feedback is able to closely control the power level throughout temperature fluctuations. Analog feedback is desirable for the remote modem receiving the signal because the feedback mechanism can be tuned to provide continuously ramping feedback, thereby allowing the remote modem to adapt as the signal strength changes. The drawback of factory-tuned single power radios is that they can provide only a single power level. However, signal level control from the user or software may be achieved through the use of a digitally controlled attenuator which provides accurate attenuation without calibration. Use of such attenuators, however, often results in a momentary loss of data on the signal path when attenuation is changed.
Digital systems utilize software to control power levels in the signal path via one or more detectors and attenuators. Gain stages are often static as application of attenuation is more exact than control of amplification. The radios often feature only a limited range of closed-loop operation due to the drastic requirements placed on high-power, high-frequency power detectors. Operation outside of this limited range often occurs in an open-loop manner. Applying attenuation under software control introduces inherent difficulties due to the discrete nature of digital control systems. The software of the digital system must be adapted to provide signal path power level control in such a manner that the remote modem receiving the signal does not experience a sudden change in power level which may not be correctable (i.e. applying 1 dB of attenuation to the signal path will cause a sharp edge which may not be distinguished from the edges in a modulated signal). Additionally, the dynamic calibration process itself restricts the detectors and attenuators that may be used in the radio, consumes a significant mount of electronic memory and necessitates potentially complex software to utilize the calibration data.
A prior art wide band automatic gain controller with limiting amplifiers is shown in U.S. Pat. No. 4,665,560. Although this patent appears to be an analog automatic gain control feedback loop controllable via dynamic range bias control, this bias control is not effected by microprocessor/digital control as in the present subject matter.
Prior art U.S. Pat. No. 6,104,919 is directed to an inter-modulation distortion reduction circuit utilizing variable attenuation in a dual mode FM/CDMA environment. When variable attenuator circuit 22 is enabled, it is controlled via a temperature compensation circuit 32 through automatic gain control circuit 30, as shown in FIG. 1. Although the figure discloses a feedback loop having an automatic gain control that controls a variable attenuator in the signal path, it does not disclose a digitally controlled attenuator in the feedback path, nor the advantages enumerated hereafter.
U.S. Pat. No. 6,122,331 discloses a system providing digital automatic gain control. This patent attempts to overcome some of the problems described herein by utilizing a delay generator 28 and a transition region detector 25, as shown in FIG. 2. However the digitally controlled automatic gain controller acts on the main signal line and not the feedback loop, and thus is still subject to disruptions introduced by the digital attenuator.
U.S. Pat. No. 6,836,647 discloses a system and method for estimating the power-level of a signal received at a mobile-station receiver operating in a wireless network according to a CDMA standard. The patent discloses using digital automatic gain controllers in closed feedback loops. However, the feedback loops are digital and retain the problems as described above.
U.S. Pat. No. 6,324,230 is directed to a multimode fast attack automatic gain control loop for narrow band receivers. The disclosure describes switching between an analog automatic control loop and a digital automatic control loop. The patent does not disclose the digitally controlled analog feedback loop of the present invention.
The present subject matter implements a radio design combining the continuously ramping feedback and calibration-free operation of an analog feedback system with the accurate power level control across a range of power levels of a digital feedback system. Additionally, the present subject matter offers closed-loop operation across a broad range of power levels, a feature not offered by any of the prior art design types described above.
The present disclosure describes a control system which utilizes a traditional analog feedback loop combined with software control applied to the feedback loop by means of a digitally controlled attenuator. The result is a calibration-free radio design with accurate selectable power levels across any desired power level range.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.