Cascaded AGC Circuit to Prevent TV Tuner Image Glitches
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Solution Overview
Problem
Continuous Automatic Gain Controls (AGCs) in wide band receivers for analog or digital TV cause severe image impairments when adjusted in steps, especially in multi-tuner applications, where stepwise gain adjustments can result in complete loss of the image or annoying glitches.
Innovation Solution
A cascade connection of two AGCs, where the first AGC adjusts gain in steps and the second AGC synchronizes and counters or speeds up the gain adjustments, allowing for combined impulsive gain variation that meets Digital Video Broadcasting-Terrestrial (DVB-T) requirements, enabling the use of step AGCs with gain steps greater than 1 dB without image artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a step AGC with gain steps of 1 dB or larger is used, then power consumption is reduced and design is simplified, but severe image impairments and glitches occur
Solution Approach 1:
The AGC system is divided into two separate AGC circuits operating in cascade. The first AGC provides coarse gain adjustment in large steps (e.g., 1 dB or larger) for power efficiency and simplicity, while the second AGC provides fine gain adjustment in small steps (e.g., 0.1 dB) to eliminate image artifacts. This segmentation allows each AGC to be optimized for its specific function, resolving the contradiction between simplicity and image quality.
2Ease of manufacture
If a step AGC with gain steps of 0.5 dB or larger is used in digital TV, then design is simplified, but complete loss of image can occur
Solution Approach 1:
The AGC system is segmented into two independent but synchronized AGC circuits. The first AGC handles coarse adjustment with larger steps for ease of manufacture, while the second AGC handles fine adjustment with smaller steps to ensure reliable image reception without complete loss. The segmentation allows the system to achieve both ease of manufacture and reliability.
3Speed
If gain adjustment is performed in steps during active image display, then response time is reduced, but annoying glitches appear in the image
Solution Approach 1:
The gain adjustment function is segmented between two AGC circuits. The first AGC performs fast coarse adjustment in steps, while the second AGC performs slower fine adjustment that smooths out the transitions. This segmentation allows the system to maintain fast response time while eliminating visual artifacts through the smoothing effect of the second AGC.
Solution Approach 2:
The second AGC is configured to counteract the gain steps produced by the first AGC. When the first AGC makes a discrete gain step change, the second AGC introduces an opposing adjustment that smooths the transition, preventing the abrupt changes that cause visual artifacts while maintaining the overall gain adjustment capability.
Data Source
AI summary
An automatic gain control (AGC) circuit based on a cascade of a step automatic gain control unit and a second automatic gain control unit connected to adjust their gain in a synchronized manner to result in a combined impulsive gain variation. Hereby it is possible, e.g. to use a step automatic gain control unit instead of a continuous automatic gain control unit in an RF part of a TV tuner without resulting in visual artifacts—this applies both for digital and analog TV tuners. According to a preferred embodiment, the synchronization between the two automatic gain control units may be achieved by the first automatic gain control unit generating a control signal upon gain adjustment, this control signal serving to speed up a loop bandwidth controlling gain change of the second automatic gain control unit. In another embodiment, the control signal generated by the first automatic gain control unit is used to control a gain step of the second automatic gain control unit. In addition, the invention provides a tuner with an automatic gain control unit as mentioned, and a device, e.g. a TV or a hard disc recorder with such tuner.