Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
2001-11-13
2003-05-27
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S026000, C326S027000, C326S029000
Reexamination Certificate
active
06570406
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates to high speed data communications. More particularly, the present invention relates to method and circuit configured for providing pre-emphasis equalization during high speed data communications.
BACKGROUND OF THE INVENTION
As the speed of high performance microprocessors increases, consistent with CMOS transistor feature size reductions, the required power supply voltage continues to shrink. For example, in high speed data communications, lower power consumption is being demanded without a loss in data transmission speed. Moreover, greater flexibility and adaptability of the data communications systems to the various communication interconnects is also being demanded.
FIG. 1
shows a block diagram of a typical high speed digital communication system
100
, such as may be used to interconnect between integrated circuit (IC) chips across single or multiple Printed Circuit Boards (PCB), backplanes, units, and equipment racks.
Digital communication system
100
includes a transmitter
102
and receiver
104
, with transmitter
102
comprising an encoder/serializer
106
and an output buffer
110
, and with receiver
104
comprising an input buffer
122
and a decoder/deserializer
126
. Data is typically provided to transmitter
102
by a digital subsystem in a parallel format through a data input signal
118
along with a clock input signal
120
, both of which are received in encoder/serializer
106
which generates encoded data
108
. While clock input signal
120
can be externally supplied, typically, timing information for clock input signal
120
must be extracted through clock recovery on receiver
104
, such as from clock output signal
130
.
Encoded data
108
is transmitted by output buffer
110
comprising a driver for the transmitter, which generates an output signal
112
conforming to the established signaling requirements for this interface. Output signal
112
is transmitted along a communication channel
114
comprising a transmission medium such as traces on a printer circuit board (PCB), coaxial cable or any other like means for communication links.
Receiver
104
receives a transmitted signal
116
from transmission channel
114
which is degraded due to various limitations of the transmission channel
114
, including increased attenuation at high frequencies, and which are typically caused by the skin effect of copper transmission lines and various dielectric losses. For example, the skin effect of copper is the tendency of current to concentrate flow on the outer surfaces of the copper conductor, rather than the entire conductor, resulting in a higher effective resistance. Dielectric losses can occur since the dielectric within the transmission lines and PCB traces is not a perfect lossless material, e.g., at high frequencies, some energy gets dissipated in the dielectric, resulting in a degraded signal. Input buffer
122
comprises a pre-amplifier configured to receive and amplify degraded signal
116
such that an amplified signal
124
has sufficient amplitude to drive decoder/deserializer
126
, which is configured to recover the data output signal
128
and clock output signal
130
.
FIG. 2
illustrates timing diagrams
202
and
204
demonstrating the effect of a bandwidth limited transmission medium on the transmitted and received waveform that is realized from communication system
100
. A transmitted waveform
202
, such as that of output signal
112
provided to transmission channel
114
, frequently uses non-return-to-zero (NRZ) signaling, which represents a logical zero by a lower value and a logical one by a higher value. Detection of the high or low value typically includes setting a threshold halfway between those two values and making a comparison of the received value against the threshold.
A received waveform
204
, such as that of transmitted signal
116
from transmission channel
114
, shows the effect of bandwidth limiting on transmitted waveform
202
. The bandwidth limiting is due to the frequency dependent loss in the transmission medium, which is caused by factors such as the skin effect and dielectric losses discussed above. These factors typically result in losses which are relatively greater at higher frequencies. i.e., the transmitted signal gets severely attenuated at high frequencies, thus making the channel behave, in effect, like a low pass filter.
The effect of this bandwidth limiting can be seen in that waveform
204
does not reach full amplitude in a single bit period, so the value reached depends on the number of consecutive bits that are alike. For example, a lower amplitude occurs at a high peak
206
, which corresponds to a high bit after a long string of low bits, whereas a high amplitude occurs at a high peak
208
, which corresponds to a long string of high bits. The difference in amplitude at peaks
206
and
208
makes it difficult for receiver
104
to distinguish the logic low and high signals, i.e., the “0”s, and “1”s. Similarly, low peaks
210
and
212
both correspond to logic “0”, but there is a significant difference in the amplitude, depending on the string of previous bits. This effect is typically referred to as inter-symbol interference (ISI). In this manner, the maximum data rate that can be reliably transmitted in the channel
114
is very limited.
In order to address the above limitations, particularly at high frequencies, data communication systems include equalization techniques to adjust or correct the frequency characteristics of an electronic signal by restoring to the original level high frequencies of the electronic signal that have been attenuated. Equalizers can be implemented within the transmission channel, before the channel, e.g., within the transmitter, and/or after the channel, e.g., within the receiver.
FIG. 3A
illustrates a block diagram showing a high speed digital communication system
300
utilizing equalization to overcome the bandwidth limitation of the transmission channel and extend the maximum rate of operation for the communication link. Similar to
FIG. 1
, a transmitter
302
generates a transmit signal
312
. In this case, an output buffer
310
is cascaded with a transmit equalizer
313
, also known as a pre-emphasis equalizer, having desirable frequency characteristics. A receiver
304
is configured to accept a degraded signal
316
similar to that of FIG.
1
. Receiver
304
is cascaded with a receive equalizer
317
, also known as an adaptive equalizer that adapts to the transmission channel losses. The net effect is that the combined frequency response of equalizers
313
and
317
and transmission channel
314
can be shaped to overcome the bandwidth limitation in transmission channel
314
, resulting in higher overall bandwidth. In general, this shaping configuration requires equalizers
313
and
317
to provide additional gain at higher frequencies, or alternatively, to provide additional loss at lower frequencies while maintaining the high frequency gain. For the various linear buffers, amplifiers, and equalizers within communication system
300
, the particular order of cascading is not important, but for practical implementations, typically one order is preferred over the other.
It is also often desirable to combine the equalization function with the buffer or amplifier function in a single element. For example, with reference to
FIG. 3B
, transmitter
302
can be configured with an output buffer
315
which incorporates pre-emphasis or pulse shaping equalization. Likewise, receiver
304
can be configured with an input buffer or preamplifier
323
which incorporates an equalizer filter.
The differences in performance between non-equalized and equalized transmission signals can be realized with reference to data eye diagrams of the communication systems of
FIG. 1
(non-equalization) and
FIG. 3
(with equalization) For example, with reference to
FIG. 4
, the data eye of the non-equalized received signal with significant ISI is illustrated. The data eye comprises a time domain waveform showin
Pierson Richard C.
Tang Benjamim
Primarion, Inc.
Snell & Wilmer
Tokar Michael
Tran Anh Q.
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