Patent Application
20090135895
Not Applicable
Not Applicable
The non-ideal effects associated with channel loss in connection with broadband data communication systems such as high-definition television (HDTV) impact signal quality increasingly as the bit rate increases. In particular, non-ideal effects such as skin effect loss and dielectric loss in the channel, e.g., cable, printed circuit board trace, and the like, attenuate data more significantly at higher frequencies. Indeed, data attenuation can be represented by the following transfer function, L(f):
L(ƒ)=e−1(ks√{square root over (iƒ)}+k
where f is the frequency, I is the channel length, and ks and kd are the skin effect loss constant and dielectric loss constant of the channel, respectively.
One means for avoiding bit errors and inter-symbol interference (ISI) that results from the interference between adjacent pulses and for receiving a high quality data signal is “equalization”. Equalization counteracts channel loss to compensate for transmission loss and to recover the distorted signal using an inverse or reciprocal frequency transfer function as the channel loss, i.e., 1/L(f).
Because the exact characteristics of the channel are unknown, adaptive equalization is preferable to fixed equalization. Adaptive equalization refers to the ability of the system to adapt to find the proper compensation level for a specific channel.
One such adaptive equalization system known to the relevant art is shown in
In operation, output from the equalizer 15 is sent to both the LPF 12 and HPF 14, which extract signal energy within the respective frequency bands. Filter outputs are rectified by the pair of rectifiers 17 and 19 and then compared by the integrating circuit 13, which determines the difference between the signal energies of the LPF 12 and the HPF 14.
Based on that difference, the integrating circuit 13 outputs a control signal 11, e.g., a control voltage, to the equalizer 15. The equalizer 15 then uses the control signal 11 to adjust the high-frequency gain of the equalizer 15. The equalizer feedback loop 18 continues to adjust the control signal until the signal energy levels of the LPF 12 and HPF 14 are equal, which is to say that the difference between the signal energies is zero.
The ratio between the signal energies of the LPF 12 and the HPF 14 is preset and fixed, e.g., the ratio of high-pass-to-low-pass filter signal energy can be preset and fixed at 1:1. However, in practice, the adapted operating point is not fixed so the high-pass-to-low-pass filter signal energy, typically, is not 1:1. The high-pass-to-low-pass filter ratio is variable due to, for example, the channeling medium, the transmitted data, process, supply voltage, temperature, and the like. Accordingly, the control signal 11 of the adaptive equalizer 15 may be imperfect, resulting in a correspondingly incorrect or non-ideal equalizer gain setting.
In either instance, over-equalizing or under-equalizing an attenuated input signal causes jitter. Consequently, it would be desirable to provide a self-calibrating adaptive equalization system to improve jitter performance.
A self-calibrating, adaptive equalization system for generating an ideal digital signal is disclosed. The adaptive equalization system includes an equalizer and a high-gain buffer or “slicer”, that is adapted to provide a sinc2(x) spectrum. The equalizer includes a first equalizer (feedback) loop that feeds-back a control signal to the equalizer. The high-gain buffer includes a second equalizer (feedback) loop that provides a high-pass-to-low-pass filter ratio signal, which is fed-back to the first equalizer loop to adjust the control signal.
Each of the first and second equalizer loops has a high-pass and a low-pass filter, rectifying circuits for each of the filters, and an integrating circuit that compares signal energy output from the rectifiers. The adaptive equalization system generates an ideal digital signal.
The invention will be more fully understood by reference to the following Detailed Description of the invention in conjunction with the Drawings, of which:
An illustrative block diagram of an adaptive equalization system in accordance with the present invention is shown in
As previously described, the adaptive equalizer 15 receives and processes an attenuated input signal that can be affected by the transmitted data, the process, the temperature, the supply voltage, and the like. Output from the adaptive equalizer 15 is then fed to the adaptive equalizer loop 18. The LPF 12 and HPF 14 of the adaptive equalizer loop 18 extract signal energy within the respective frequency bands of the equalizer output. The outputs from the filters 12 and 14 are then rectified by their respective rectifiers 17 and 19 and the rectified signals are compared by the integrating circuit 13.
The integrating circuit 13 determines the difference between the signal energies of the LPF 12 and the HPF 14. As will be discussed in greater detail below, to account for changes in the high-pass-to-low-pass filter ratio (HPF/LPF) due to, for example, the transmitted data, the process, supply voltage, temperature, and the like, an “alpha” correction signal will be used to account for the changes and, as a result, to adjust the control signal 11 to the adaptive equalizer 15.
In the frequency domain, however, the high-gain buffer 25 outputs an ideal digital signal having sinc2(x) spectrum characteristics, which is to say that the edges of the waveform have been squared-up. Squaring-up the edges of the waveform translate into faster transition times and shorter rise and fall times. As a result, the frequency spectrum, or frequency content, is representative of the desired output after the adaptive equalization stage.
To take advantage of this, the high-gain buffer 25 is used to generate a corrective, “alpha” signal as a reference signal for use by the adaptive equalizer 15 in generating an ideal digital signal. For example, the high-gain buffer 25 output is fed through each of a LPF 22 and a HPF 24 in a second equalizer loop 28. The LPF 22 and HPF 24 of the second equalizer loop 28 also extract signal energy within the respective frequency bands of the high-gain buffer 25 output signal. The signal energies from the filters 22 and 24 are then rectified by their respective rectifiers 27 and 29. The rectified signals are compared by the integrating circuit 23.
The integrating circuit 23 determines the difference between the signal energies of the LPF 22 and the HPF 24. Based on the signal energy difference, the integrating circuit 23 outputs a corrective, “alpha” signal 21. See
The corrective, “alpha” signal 21 establishes the target HPF/LPF ratio gain, ensuring that the signal energy differences between the LPF 12 and HPF 14 are driven to zero (and the HPF/LPF ratio is driven to unity). After taking into account the HPF/LPF ratio, the first equalizer loop 18 generates the ideal control signal 11.
Output from the high-gain buffer 25 continues to pass through the LPF 22 and through the HPF 24 where the signal energy is adjusted as a function of the corrective, “alpha” signal 21. This corrective process continues until the signal energy levels between the LPF 22 and HPF 24 are equal, which is to say that the difference between the signal energies is zero. The resulting corrective, “alpha” signal 21 associated with the second equalizer loop 28 is representative of the HPF/LPF ratio required to make the output of the entire system 20 “ideal”.
Using a SPICE simulation tool, a fixed “alpha” equalization system 10 and a self-calibrating equalization system 20 were simulated. Table I summarizes the jitter results for a High-Definition Multimedia Interface Channel 5M cable (2.5 Gbps) for each system. NNN refers to a nominal process, a nominal supply voltage, and a nominal, i.e., room, temperature. HLH refers to a strong process corner, a low supply voltage, and a high temperature. LHL refers to a weak process corner, a high supply voltage, and a low temperature.
As shown in Table I, jitter of the self-calibrating adaptive equalization system 20 is improved (reduced) by about one-third over the fixed energy-ratio system for the HLH and LHL simulations.
In summary, the high-gain buffer 25 and second equalizer loop 28 are adapted to determine an optimum HPF/LPF ratio that is self-calibrating and, moreover, independent of process, supply voltage, and temperature conditions.
It will be apparent to those skilled in the art that modifications to and variations of the disclosed method and system are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.
