The present invention relates to signal shaping.
In digital communication systems, digital data is sent from a transmitter to a receiver in the form of a bit stream over a transmission media such as a copper or fiber line. Typically, these systems are maximized for bandwidth and minimized for errors. A bit error is caused if a receiver of a digital communication system detects a logical zero in a received bit stream instead of a logical one and vice versa. A measure for bit errors in digital communication systems is the bit error ratio—BER.
Particularly, the BER depends on the quality of the received signal containing the digital data. Most receivers of digital communication systems are adapted to regenerate a bit clock from the received signal with a clock and data recovery—CDR—circuit. However, timing aberrations or jitter, respectively, of the received signal usually cause malfunctions of the CDR circuit which result in bad sampling of the digital data transmitted with the received signal causing bit errors.
Jitter is an important issue in high-speed digital communication systems since influences of a transmission channel on the transmitted signal are increasingly important with respect to the BER. Jitter may be distinguished in random jitter—RJ—and deterministic jitter—DJ. One sub-component of DJ is the data dependent jitter—DDJ—which is a function of bit patterns. DDJ is distinguished in Duty Cycle Distortion—DCD—and Inter-Symbol Interference—ISI. ISI is usually caused by long and short bit cycles and results from bandwidth limitations or from loss within transmission channels.
In order to test for jitter, testing signals may be shaped in order to simulate the transmission behavior of a channel.
It is an object of the present invention to provide an improved signal shaping. The object is solved by the independent claims. Further embodiments are shown by the dependent claims.
According to embodiments of the invention, a signal shaping circuit for shaping an input signal is provided which allows to generate an output signal with a predefined shape. For example, the generated output signal may have a shape like a signal with droop as it is generated by a typical transmission channel. Alternatively, the generated output signal may have a shape like a signal with a peak and a droop such that droop of a transmission channel may be compensated.
According to an embodiment of the invention, a signal shaping circuit for shaping an input signal comprises a first signal path with a first transmission behavior and adapted to receive an input signal, and at least one further signal path with a second transmission behavior and adapted to receive an input signal. The outputs of the signal paths are connected to a signal combiner adapted to combine the output signals of the signal paths to an output signal of the signal shaping circuit.
The transmission behavior of at least one signal path may be adjustable. Thus, it is possible to vary the shaping of the input signal and to simulate different lossy transmission lines or channels.
In a further embodiment, the first signal path comprises a first amplifier, and the at least one second signal path comprises a second amplifier and a low pass filter with an adjustable time constant for adjusting the transmission behavior of the at least one second signal path. The low pass filter implements a transmission behavior comparable to a typical transmission channel or line. This embodiment may be implemented at low cost and allows replacing special filters or boards required for the generation of an output signal of a particularly lossy transmission line or channel.
Furthermore, the amplification of at least one amplifier may be adjustable for adjusting the transmission behavior of a signal path. Thus, the range of adjustment of the shaping of the output signal is increased and the input signal may be shaped in order to generate a wider range of output signals of different particularly lossy transmission lines. Particularly, the amplifier allows amplifying an input signal such that it comprises peaks. Thus, losses of a transmission channel may be compensated.
In a further embodiment, the at least one amplifier is a differential amplifier. The differential amplifier may be implemented with bipolar transistors or field effect transistors (FET). It may comprise an adjustable current source for adjusting the output signal amplitude.
In a further embodiment, the low pass filter of a signal path may comprise a variable capacitor for adjusting the time constant. The variable capacitor may be connected between the output lines of the differential amplifier. This implementation is more symmetrical than an implementation of the low pass filter with two separate and variable capacitors.
In a further embodiment, a decoupling stage is connected between the low pass filter and the signal combiner. The decoupling stage decouples the low pass filter of a signal path from the other signal paths, which enables a more precise signal shaping.
In a further embodiment, the at least one second signal path comprises an input amplifier connected on its input to the input of the signal shaping circuit and on its output to a low pass filter with an adjustable time constant; the output of the low pass filter is connected to the input of a differential amplifier with adjustable amplification.
In a further embodiment, the signal combiner comprises two resistors both connected with one end to a variable voltage source and with the other end to the output lines of the signal paths. The variable voltage source serves to adjust the high level of the output signal.
Furthermore, at least one of the signal paths may comprise an attenuation circuit. The attenuation circuit allows to attenuate the signal amplitude, and thus to further influence the signal shaping.
The signal shaping circuit comprises according to a further embodiment two further signal paths with an adjustable transmission behavior.
A variable capacitor of the signal shaping circuit may be implemented by a circuit comprising two capacitance diodes, a resistor, and a variable voltage source, wherein the cathodes of the two capacitance diodes are connected to the variable voltage source over the resistor.
A variable capacitor of the signal shaping circuit may also be implemented by a circuit comprising several series connections of a switch and a capacitor, wherein the series connections are connected in parallel.
In a further embodiment, the signal shaping circuit is adapted to receive adjusting signals generated by a computer program for automatically adjusting the transmission behavior in accordance with predefined parameters. For example, the predefined parameters may be s parameters of a typical transmission channel, which are input to the computer program. The computer program processes the input parameters and generates adjusting signals for adjusting the transmission behavior in correspondence with the predefined parameters. Thus, a signal shaping circuit is provided which is flexible and may be used for different test applications.
According to a further embodiment of the invention, one second signal path of the signal shaping circuit may comprise a filter and the signal combiner is adapted to combine the inverted output signal of the one second signal path with the filter with the output signals of the other signal paths. For example, this embodiment may be applied in an automatic test equipment—ATE—system for shaping an output signal of a pin electronic of the ATE system in order to stimulate a device under test—DUT—with a signal similar to the output signal of a typical “real world” transmission channel.
According to a further embodiment, at least one signal path may comprise a series connection of a filter and a multiplier, wherein the multiplier is provided for multiplying an output signal of the filter with a control signal in order to adjust the transmission behavior of the signal path. This embodiment has the advantage that is may also be applied in an ATE system and allows to adjust the shaping of an output signal of a pin electronic of the ATE system in order to simulate different transmission channels.
According to a further embodiment of the invention, the control signal may be a digital signal with several predefined states. Thus, the signal shaping may be adjusted to certain predefined states as predetermined by the states of the digital control signal.
According to a further embodiment of the invention, the control signal may be an analog signal adjustable within a predefined amplitude and frequency range. This allows a very flexible adjusting of the signal shaping within the predefined amplitude and frequency range.
According to a further embodiment of the invention, the control signal may be provided by a programmable controller. For example, the controller may be part of an ATE system and provided for controlling the operation of the ATE system. The controller may be programmed by a test program which may comprise instructions for adjusting the control signal within the test program.
Furthermore, an embodiment of the invention relates to a test instrument comprising an signal shaping circuit according to an embodiment of the invention.
According to a further embodiment of the invention, the test instrument may be an automatic test equipment—ATE—system adapted for testing high speed serial data receivers.
According to a further embodiment of the invention, the test instrument may be a pulse data generator adapted for generating a high speed serial data test signal.
According to a further embodiment of the invention, the test instrument may be a bit error ratio—BERT—tester comprising a pulse data generation module which is adapted for generating a high speed serial data signal.
According to a further embodiment of the invention, a signal shaping method for shaping an input signal is provided wherein a first signal path with a first transmission behavior receives an input signal, and at least one second signal path with a second transmission behavior receives an input signal, wherein the outputs of the signal paths are connected to a signal combiner which combines the output signals of the signal paths to an output signal of the signal shaping circuit.
Embodiments of the invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. Software programs or routines are preferably applied to a computer implementing a signal shaping circuit according to embodiments of the invention.
Other objects and many of the attendant advantages of embodiments of the present invention will be readily appreciated and become better understood by reference to the following more detailed description of embodiments in connection with the accompanied drawing(s). Features that are substantially or functionally equal or similar will be referred to by the same reference signs.
Real transmission channels, particularly transmission lines such as copper or optical lines behave like a low or high pass filter with a non-linear phase characteristic. The filter effect of a real transmission channel causes a bandwidth limitation. Furthermore, loss in a transmission channel causes droop, which limits signal settling before the next signal transition occurs. Both the bandwidth limitations and the droop lead to a shortening of bits in a bit stream since usually a bit has not yet settled, i.e., reached about 100% of its amplitude before the next signal transition occur. Especially in high-speed digital communication systems with data rates in the range of 1 gigabit per second (GB/s) and short times of some picoseconds (ps) between two signal transitions, jitter such as DCD and ISI may essentially influence the opening of the eye in an eye diagram of a received signal containing digital data, or in other words may lead to a closing of the eye. Especially at high frequencies, the physical behavior of the material of a transmission channel limits the bandwidth, particularly if the channel is a long fiber line or a cheap PCB material. Thus, a test of components of high-speed digital communication system with signals containing jitter caused by a transmission channel is important with regard to the BER. In order to simulate an output signal of a typical transmission channel, a signal shaping circuit may be used.
In the following, several embodiments of signal shaping circuits according to the invention and of its components are described in detail. The embodiments of the signal shaping circuit may be part of a particularly integrated amplifier which is applied in a test instrument such as a bit error ratio tester—BERT—or a pulse data generator.
Each signal path SP1, SP2 to SPn of the circuit 10 contains an amplifier 12, 14, and 16 for amplifying an incoming input signal 18. The input signal 18 may be a typical bit stream of digital data as it is transmitted from a sender to a receiver of a high-speed digital communication system. For example, a bit stream of “1” and “0” may be simulated by a square-wave pulse signal. The input signal 18 is amplified in each of the signal paths SP1, SP2 to SPn by the amplifiers 12, 14, and 16. After amplification, the input signal is supplied to the low pass filters 20 and 22 in the second signal paths SP2 to SPn. The low pass filters 20 and 22 “introduce” a time constant in the input signal for simulating the behavior of a typical transmission line with a capacitance and resistance per unit length. The time constant can be adjusted with time constant adjusting signals 25 to 27.
In the first signal path SP1, the amplified input signal is directly supplied to an attenuator 32 with adjustable attenuation, which adjusts the level of the supplied signal as set by a control signal 33 for adjusting the attenuation. In the second signal paths SP2 to SPn, the amplified input signal 18 is filtered by the low pass filters 20 and 22 and then attenuated by adjustable attenuators 34 and 36 the attenuation of which is adjusted by control signals 35 and 37, respectively. The output signal 24 of the first signal path SP1 and the output signals 26 and 28 of the second signal paths SP2 to SPn are combined by a signal combiner 23, which is an adder circuit, in order to generate an output signal 30 which reproduces an output signal of a real transmission channel. The signal combiner may also be a subtractor for subtracting received signals. It may also be a universal signal combiner, which adds the output signal of some signal paths and subtracts output signals of other signal paths.
The shape of the output signal 30 may be varied by the adjustment of the time constants of the low pass filters 20 and 22 with the time constant adjusting signals 25 and 27 and the attenuation of the attenuators 32, 34, and 36 with the attenuation adjusting signals 33, 35, and 37. Thus, the transition time and droop of different transmission channels may be simulated by setting the time constants of the low pass filters 20 and 22 and the attenuation of the attenuators 32, 34, and 36. Particularly, the adjustment of the time constants and attenuations may be adapted such that the course of the output signal 30 essentially matches real measurements of a transmission channel. It has been shown that two time constants appear to be sufficient to simulate the real behavior of a transmission channel. In order to simulate typical transmission channels for signals with data rates in the range of GB/s, the two time constants should be variable from 20 ps to 250 ps and from 300 ps to 3 ns.
The adjustment of the time constants of the low pass filters 20 and 22 with the time constant adjusting signals 25 and 27 and the attenuation of the attenuators 32, 34, and 36 with the attenuation adjusting signals 33, 35, and 37 may be controlled by a computer program. For example, the signal shaping circuit 10 may be integrated in a test instrument (as it is shown in
By inverting the slower path, an output signal 30 may be generated which may be input to a real transmission channel such as a copper line and generate an output signal of the transmission channel which reproduces the ideal output signal of a signal generator. Therefore, the influence of a real transmission channel on a signal may be compensated. This may be relevant if a device under test—DUT—is tested with the output signal 30 but connected to the signal shaping circuit by a long transmission line. The second signal path SP2 further comprises a low pass filter 42 with an adjustable time constant. The amplifier 40 and the filter 42 of the second signal path may be integrated as single unit 44, embodiments of which are shown in FIGS. 3 to 5. It should be noted that unit 44 may also be applied in the circuit shown in
In the following, embodiments of the unit 44 of
A third embodiment of the unit 44 of
As mentioned above, the transmission behavior of a transmission channel becomes more and more important with increasing data rates. Particularly, high speed serial data transmissions with data rates of 1 GBit/s or more increasingly face the problem of a frequency-dependent signal attenuation due to the signal path itself. These attenuations may limit the achievable data rate because the timing window for a data bit at a receiver of the transmitted data shrinks much faster than the nominal bit period given by the data rate. This causes a closing of the data eye measured at the receiver side and makes it much more difficult to reconstruct data from a received serial bit stream. In order to alleviate this problem, receivers or transmitters often utilize equalization techniques with which higher frequencies contained in a transmitted or received signal, respectively, are more amplified than lower frequencies in order to compensate a low pass characteristic of a typical transmission channel. It is more popular to implement such equalization techniques in receivers since techniques and algorithms can be applied in such devices which are independent of the length of a transmission channel (path length). Preferably, an equalization is performed in the digital domain with digital signal processing techniques. However, testing of such receiver devices with a commercial ATE (Automatic Test Equipment) system is difficult since most available ATE systems are designed to deliver “clean” signals to a DUT. A “clean” signal means a signal with a nearly ideal signal shape such as the signal “input” or “output 1” shown in
The signal shaping circuit according to the invention is well suited to be applied to ATE systems in order to simulate a transmission channel or transmission channel behavior and to generate a signal which approximates an output signal of a “real world” transmission channel.
Finally,
In the following, the influence of a simple digital control signal 76 with three different states it is briefly explained, as an exemplary embodiment. However, it should be noted that the following description merely relates to an exemplary control signal 76 and the transmission behavior of the second signal path 70 by the control signal 76 may be adjusted in a much wider range depending on the selection of the control signal 76 and its range of adjustment. If the control signal has a value of +1, the transmission behavior of the second signal path corresponds to the transmission behavior of the second signal path of the signal shaping circuit as shown in
Number | Date | Country | Kind |
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05 105 075.5 | Jun 2005 | EM | regional |
06 100 624.3 | Jan 2006 | EM | regional |