Patent Application
20060222115
The present application generally relates to receivers, and more particularly to receivers such as television receivers that use automatic gain control (AGC).
Contemporary television receivers are required to operate in complex environments. For example analog (NTSC/PAL/Secam) and digital (ATSC/DVB/ISDB) television (DTV) signals now coexist within a crowded signal spectrum. The analog and DTV signals have different characteristics and place different requirements on television receivers. For example, the signal level (normally expressed as signal-to-noise ratio or SNR) required to demodulate a tuned DTV signal is much lower than that required to demodulate a tuned analog signal. In addition television receivers are often required to discriminate weak signals from geographically distant transmitters, while in the presence of spectrally near channels such as adjacent channels from nearby transmitters. The different characteristics of the analog and digital signals and various signal levels make it difficult to design receivers that are capable of tuning all channels with acceptable results.
To ensure proper output signal levels, television receivers commonly use automatic gain control (AGC). AGC is a mechanism that automatically adjusts the gain of a circuit such as a tuner in response to differences in received signal level to ensure that acceptable levels are available for subsequent processing.
Known television receiver AGC circuits attenuate the power in an input radio frequency (RF) signal in proportion to the power detected in an intermediate frequency (IF) tuned signal. This AGC technique, however, fails to account for power in signal spectra that fall outside of any filtering that may occur prior to the power detector and thus may cause components such as amplifiers, mixers, and filters to distort. Thus known AGC techniques properly control the gain of the tuned signal but may fail to properly manage the gain due to other signals.
What is needed, then, is a receiver such as a television receiver that performs AGC more flexibly in today's complex spectral environments while keeping distortion low.
In one form a receiver includes a processing path, a plurality of power detectors, and an automatic gain control circuit. The processing path has an input for receiving an input signal, and an output for providing an output signal. Each of the plurality of power detectors has an input coupled to a different node of the processing path, and an output. The automatic gain control circuit has inputs coupled to respective outputs of each of the plurality of power detectors, and a first output adapted to be coupled to a first controllable gain element for controlling a gain thereof in response to the outputs of the plurality of power detectors.
In yet another form a receiver includes a processing path, a power detector, and an analog-to-digital converter. The processing path has an input for receiving an input signal, and an output for providing an output signal. The power detector has an input coupled to a node of said processing path, and an output. The analog-to-digital converter has an input coupled to the output of the power detector, and an output for providing a digital representation of the output of the power detector.
In still another form there is provided a method for performing automatic gain control. An input signal is received and processed to provide an output signal and intermediate signals are thereby formed. A power is detected in a plurality of the intermediate signals. A gain of at least one controllable gain element is adjusted in response to detecting the power in the plurality of the intermediate signals.
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:
The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.
Tuner 130 has an input terminal connected to the second terminal of attenuator 120, and an output terminal for providing a tuned IF output signal. Tuner 130 includes a low noise amplifier (LNA) 140, a mixing element 142, a bandpass filter 144, and an amplifier 146. LNA 140 has an input terminal connected to the second terminal of attenuator 120, and an output terminal. Mixing element 142 has a first input terminal connected to the output terminal of LNA 140, a second input terminal for receiving a local oscillator signal labeled “LO”, and an output terminal. Bandpass filter 144 has an input terminal connected to the output terminal of mixing element 142, and an output terminal. Amplifier 146 has an input terminal connected to the output terminal of bandpass filter 144, and an output terminal for providing the tuned IF signal.
SAW filter 160 has an input terminal connected to the output terminal of amplifier 146, and an output terminal. NTSC demodulator 170 has an input terminal connected to the output terminal of SAW filter 160, an output terminal connected to the control terminal of attenuator 120, and provides output signals to properly drive video and audio devices, not shown in
Receiver 100 provides a gain to the RF input signal by amplifying it in LNA 140. Amplifier 146 further amplifies the mixed signal. Moreover, mixer 142 and filter 144 may provide amplification as well, depending on their design. The AGC mechanism ensures that the tuned IF signal received at the input of NTSC demodulator 170 has an acceptable SNR. As part of this mechanism NTSC demodulator 170 includes a power detector 172 that measures the power in the tuned IF signal and then provides a control signal to cause attenuator 120 to properly attenuate the signal.
Note that receiver 100 may be adapted for an Advanced Television Standards Committee (ATSC) system by replacing NTSC demodulator 170 with an ATSC demodulator. A typical ATSC demodulator does not include a power detector, so the ATSC receiver would require a separate power detector to measure the power at the output of amplifier 146 and a control circuit to adjust the gain of attenuator 120 in response to the measured power. Additionally an ATSC demodulator may require additional gain before or after SAW filter 160.
Television tuner IC 230 has several terminals for receiving power, ground, and various signal input and output terminals used in the operation thereof.
SAW filter 260 has an input terminal connected to the IF_SAW_1 terminal of television tuner IC 230, and an output terminal. ATSC demodulator 270 has an input terminal connected the output terminal of SAW filter 260, and various audio and video output terminals not shown in
More particularly television tuner IC 230 includes a signal processing path 240, an RF AGC circuit 250, a power detector 251, a power detector 252, a multiplexer 254, an analog-to-digital converter 256, and a data port with registers 258. Signal processing path 240 has an input for receiving RF_I and first and second output terminals for respectively providing IF_SAW_1 and IF_SAW_2. Signal processing path 240 includes a low noise amplifier (LNA) 241, a mixing element 242, a bandpass filter 243, an amplifier 244, and an amplifier 245. LNA 241 has a signal input terminal for receiving RF_I, an output terminal, and a control input terminal. Mixing element 242 has a first input connected to the output terminal of LNA 241, a second input terminal for receiving a local oscillator mixing signal labeled “LO”, and an output terminal. Bandpass filter 243 has an input terminal connected to the output terminal of mixing element 242, and an output terminal. Amplifier 244 has an input terminal connected to the output terminal of bandpass filter 243, and an output terminal for providing IF_SAW_1. Amplifier 245 has an input terminal connected to the output terminal of bandpass filter 243, and an output terminal for providing IF_SAW_2.
Power detector 251 has an input terminal connected to the output terminal of LNA 241, and an output terminal. Power detector 252 has an input terminal connected to the output terminal of filter 243, and an output terminal. RF AGC circuit 250 has a first input terminal connected to the output terminal of power detector 251, a second input terminal connected to the output terminal of power detector 252, a third input terminal for receiving RF_AGC, a data input terminal, a first AGC output terminal for providing D_AGC, a second AGC output terminal connected to the control input terminal of LNA 241, and a third output terminal for providing C_AGC. MUX 254 has a first input terminal for receiving GPIO_I, a second input terminal for receiving C_AGC, a third input terminal for receiving RF_AGC, a control input terminal, and an output terminal. ADC 256 has an input terminal connected to the output terminal of MUX 254, and an output terminal. Data port 258 has an input/output terminal for conducting signal SDA, a first input terminal for receiving signal SCL, a second input terminal connected to the output terminal of ADC 256, a first output terminal connected to the data input terminal of RF AGC circuit 250, and a second output terminal connected to the control input terminal of MUX 254.
In basic operation, television receiver 200 is part of a product such as a digital television. The heart of receiver 200 is television tuner IC 230, which integrates many of the components of television receiver 200. As will be described more fully below, television tuner IC 230 supports an improved AGC technique that allows receiver 200 to be well suited for processing both analog and digital television signals and for receiving channels with differing signal characteristics.
Antenna 210 receives a broadband RF television signal. For example in the case of an NTSC receiver, the band of interest is from 48 to 870 MHz, but other television signaling systems are possible. Note that other signal sources such as community access television (CATV) connections may provide the broadband RF television signal as well. Attenuator 220 controllably attenuates the broadband RF television signal to provide RF_I to television tuner IC 230.
Television tuner IC 230 includes a processing path 240 that tunes a selected channel by mixing RF_I to a fixed IF. LNA 241 amplifies RF_I by an amount that can be varied based on the control input thereof. Note that
Automatic Gain Control (AGC)
Television tuner IC 230 includes gain elements for both the RF and IF signals. Television tuner IC 230 performs AGC by altering the gain of attenuator 220 and/or LNA 241. Attenuator 220 typically provides about 40 decibels (dB) of gain range, whereas LNA 241 provides about 9 dB of gain range. Television tuner IC 240 alters the gain of attenuator 220, which may be in the form of a P-intrinsic-N (PIN) diode attenuator (π-shaped or T-shaped), a dual-gate MOSFET, or the like, through D_AGC.
Receiver 200 selectively uses three power detectors (251, 252, and 292) to perform AGC. Power detectors 251 and 252 are implemented on television tuner IC 230, whereas power detector 292 is part of NTSC demodulator 290 and is external to television tuner IC 230. A bit field in a register in data port 258 is used to set the threshold (i.e. the trip point) of the signal at the input to mixing element 242. Another bit field in the register is used to set the threshold of the signal at the input to amplifiers 244 and 245. In the illustrated embodiment these bit fields are each three bits in length with one encoding indicating that the power detector is disabled, as described further below. Note that power detectors 251 and 252 can be designed to measure peak power or average power, but in the illustrated embodiment measure a hybrid of peak and average power.
RF AGC circuit 250 receives the three detected power inputs, and selectively adjusts the gains of attenuator 220 and/or LNA 241 in response. In general it is desirable for receiver 200 to operate at maximum gain without distorting the signal at any point in the processing path. RF AGC circuit 250 accomplishes gain reduction by allowing maximum gain unless any power detector indicates a signal level above its threshold. When excessive gain is detected by any available power detector, RF AGC circuit 250 reduces the gain by first reducing the gain of LNA 241, and then by reducing the level of RF_I through attenuator 220. In this manner television tuner IC 230 ensures that none of the circuits in signal processing path 240 distorts the signal.
Television tuner IC 230 includes several programmable features to provide a high degree of flexibility. As described above, the register in block 258 receives and stores thresholds for each power detector. When the power detected by a particular power detector exceeds its corresponding programmed threshold, then RF AGC circuit 250 attenuates RF_I in a selected one of attenuator 220 and LNA 241. Moreover a field in a register in data port 258, such as the threshold field, can be set to effectively disable the power detector such that it does not participate in the AGC function. Thus programming this field appropriately is analogous to opening a switch between the power detector and RF AGC circuit 250.
Unlike a conventional receiver with AGC, receiver 200 includes multiple power detectors that measure the power at multiple points in the signal processing stream and can be used to control the gain in a manner more appropriate to the signal type. Since it is difficult to discriminate wanted from unwanted signal power in a broadband signal, power detectors are placed after each filter. Accordingly power detector 252 is placed after bandpass filter 243, and power detector 292 measures the power after the signal has been filtered in SAW filter 280. Placing a power detector after each filter enables receiver 200 to more accurately determine the power of the desired signal.
Moreover, using multiple power detectors allows external AGC control to be distributed across multiple gain elements to prevent clipping and thus distortion may be reduced while maintaining an acceptable signal level.
Known television tuner ICs include an ADC for general-purpose use such as detecting the AFT (Automatic Fine Tune) pin on an analog demodulator. However according to another feature of the present invention, television tuner IC 230 includes an ADC 256 which is selectively connected through MUX 254 to the input of ADC 256 to also measure the AGC control voltage from power detector 292 or the AGC control voltage from power detectors 252 and 251, which may be the value of the voltage on the time constant capacitor. Television tuner IC 230 makes these values available to an external microcontroller connected to data port 258, which can use this information to adjust the threshold for any power detector during operation.
According to another aspect of the present invention, AGC parameters could be set on a per-channel basis. An algorithm to implement this feature is as follows. The receiver at initial power-on would be programmed to scan the spectrum for all available channels. Weak channels and strong channels could be detected, intermodulation distortion interference spectra could be calculated for each channel and thresholds set appropriately for each channel. The microcontroller attached to receiver 200 could store the thresholds in a table, such as in nonvolatile memory, that could be used to re-configure television tuner IC 230 through data port 258 each time a new channel is selected with the thresholds determined to be optimum for that channel.
Note that a receiver with improved AGC could be used for a variety of applications including digital televisions, set top boxes, and the like. Also the technique is applicable to a variety of existing analog and digital television standards. Furthermore the AGC technique is applicable to receivers tuning other types of signals from broadcast spectra, such as broadcast radio, satellite radio, and the like. The disclosed receiver used inputs from three power detectors to control the gain through attenuation in two controllable elements, but different numbers of such elements are also possible.
The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments that fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.
