JP2011009853A - Signal transmitter - Google Patents

Abstract

A bias reduction in the output of a transmission end is prevented.
First and second transmitters for inputting an input signal in common to a positive phase input terminal 1 and a negative phase input terminal 2 and outputting output signals having the same phase to a transmission line, and first and second Capacitive elements 14 and 15 that connect between the output ends of the transmitter section, and resistance elements 12 and 13 that impedance-match the transmission path and are connected as a DC load of the second transmitter section, The transmission unit connects the input resistors 21 and 22 of the input stage on the reception side via a transmission line as a DC load. Here, the first transmission unit includes NMOS transistors 8 and 9 serving as a differential pair, and a constant current source 6 that supplies current to the differential pair, and the second transmission unit includes a differential pair and NMOS transistors 10 and 11, and a constant current source 7 for supplying current to the differential pair.
[Selection] Figure 1

Description

  The present invention relates to a signal transmission device, and more particularly, to a termination technique for matching impedance with a transmission line.

  In recent years, in digital AV equipment, a digital video / audio interface has been used instead of an analog video / audio interface. The HDMI (High-Definition Multimedia Interface) standard that appeared in 2002 is a typical example. With the advent of Blu-ray discs and the remarkable progress of video processing technology in thin digital televisions, the transmission speed of digital video / audio interfaces has increased. For example, the transmission speed of the HDMI standard has been improved in 2006, and the speed has been increased from 1.65 Gbps to 3.4 Gbps per conventional transmission line set. When such an interface is increased in speed, a signal integrity (integrity, consistency) problem in which a transmission signal quality is deteriorated due to a waveform distortion or the like caused by impedance mismatching in the middle of a transmission path such as a connector has become apparent.

  Therefore, in order to absorb the reflected wave to the input end caused by the impedance mismatch, a transmission termination (Back termination) technique is known in which a termination resistor is connected so as to achieve impedance matching not only at the reception end but also at the transmission end. ing. In the prior art, a circuit having a configuration in which a terminating resistor is connected on the transmission side between differential transmission paths outside a semiconductor is disclosed (see Non-Patent Document 1).

  FIG. 3 is a diagram illustrating a circuit example of the signal transmission device described in Non-Patent Document 1. In FIG. 3, a transmitter 160 includes a differential pair including NMOS transistors 8 and 9, a constant current source 6 connected to the differential pair, a positive phase input terminal 1, a negative phase input terminal 2, and a positive phase input terminal 2. A phase output terminal 4 and a negative phase output terminal 5 are provided, and a transmission termination resistor 25 is connected between the positive phase output terminal 4 and the negative phase output terminal 5.

  The receiver 17 connected to the transmitter 160 includes a differential comparator 23, a positive phase input terminal 18, a negative phase input terminal 19, and a termination power supply terminal 20, and the positive phase input terminal 18 and the termination power supply terminal 20. A termination resistor 21 is connected between them, and a termination resistor 22 is connected between the negative phase input terminal 19 and the termination power supply terminal 20.

  The positive-phase output terminal 4 of the transmitter 160 and the positive-phase input terminal 18 of the receiver 17 are connected by DC coupling via the positive-phase transmission line of the differential transmission line, and the negative-phase output terminal 5 of the transmitter 160 and the reception are connected. The negative phase input terminal 19 of the device 17 is connected by DC coupling via the negative phase transmission line of the differential transmission line.

  In an actual apparatus having such a configuration, contacts such as connectors and plugs exist in the middle of the differential transmission path, where impedances are mismatched and signal reflection occurs. It is generally known that this reflected wave returns to the transmission end, is re-reflected at the transmission end, and is superimposed on the output signal, resulting in output waveform distortion and deterioration in signal integrity. Regarding re-reflection at the transmission end, the reflection coefficient ρ at the transmission end is expressed as follows: ρ = (Rt−Z0) /, where Z0 is the differential impedance of the differential transmission path and Rt is the resistance value of the transmission termination resistor 25. It is expressed as (Rt + Z0). Here, by selecting Rt = Z0, ρ = 0, and re-reflection at the transmitting end of the waveform reflected from the mismatched portion of the impedance to the transmitting end can be prevented, and signal integrity is kept good. Can do.

“Interface Connection with MAX3815 DVI / HDMI Cable Equalizer”, April 2008, [Search June 23, 2009], <URL: http://pdfserv.maxim-ic.com/en/AN3854J. pdf>

  The following analysis is given in the present invention.

  In FIG. 3, if the current value of the constant current source 6 is i0 and the voltage of the termination power supply terminal 20 is Vb, the output DC voltage Vh on the high level side is Vh = Vb−i0 × Rl × Rl / (2Rl + Rt). The output DC voltage Vl on the low level side is expressed as Vl = Vb-i0 * Rl * (Rl + Rt) / (2Rl + Rt). Therefore, due to the presence of the transmission termination resistor 25, Vh becomes an output voltage whose bias is reduced from Vb by i0 × Rl × Rl / (2Rl + Rt). When the bias is lowered, the eye opening ratio for discriminating signals on the receiving side is lowered, and there is a possibility that stable reception cannot be performed.

  A signal transmission apparatus according to one aspect of the present invention includes a first transmission unit, a second transmission unit that inputs an input signal in common, and outputs an output signal having the same phase to each other, and a first transmission unit and a second transmission unit. A capacitive element that connects between the output ends of the transmission unit, and a resistance element that impedance-matches with the transmission line and is connected as a DC load of the second transmission unit. The first transmission unit includes a transmission line Connect the input resistance of the input stage on the receiving side as a DC load.

  According to the present invention, it is possible to prevent a decrease in bias at the output of the transmission end and reduce rereflection at the transmission end.

1 is a circuit diagram of a signal transmission device according to a first embodiment of the present invention. It is a circuit diagram of the signal transmission apparatus which concerns on 2nd Example of this invention. It is a circuit diagram of the conventional signal transmission apparatus.

  The signal transmission device according to the embodiment of the present invention includes a first transmission unit that inputs an input signal in common, and outputs an in-phase output signal to the transmission path, and a first transmission unit and a second transmission unit. A capacitive element that connects between the output terminals, and a resistance element that is impedance-matched with the transmission line and connected as a DC load of the second transmission unit, the first transmission unit receiving via the transmission line Connect the input resistance of the input stage as a DC load.

  In the signal transmission device, it is preferable that the first and second transmission units each have an output stage formed of a differential pair, and that two capacitance elements and two resistance elements respectively correspond to the differential pair.

  The signal transmission device preferably includes first and second current sources that supply currents to the differential pairs in the first and second transmission units, respectively.

  In the signal transmission device, the product of the current value of the second current source and the resistance value of the resistance element is added to the product of the current value of the first current source and the resistance value of the input resistance of the input stage on the receiving side. You may make it further provide the control part which sets the electric current value of the 1st and 2nd electric current source so that a value may become fixed.

  You may comprise as a semiconductor device provided with said signal transmission apparatus.

  According to the signal transmission apparatus as described above, the first transmission unit and the second transmission unit are connected by capacitive coupling, and the DC output bias of the transmitter is generated only by the first transmission unit. Moreover, the re-reflection at the transmission end is reduced by using a terminal resistor that matches the impedance of the transmission line as the resistance element connected as the DC load of the second transmission unit. Therefore, the termination resistor is terminated by capacitive coupling with respect to the transmission line, and does not cause a decrease in bias at the transmission end. As a result, it is possible to prevent a decrease in the eye opening ratio on the receiving side caused by a decrease in the bias and to perform stable reception.

  Hereinafter, it will be described in detail with reference to the drawings in accordance with embodiments.

  FIG. 1 is a circuit diagram of a signal transmission apparatus according to a first embodiment of the present invention. 1, the same reference numerals as those in FIG. 3 represent the same items, and the description thereof is omitted.

  The transmitter 16 includes a positive phase input terminal 1, a negative phase input terminal 2, a termination power supply terminal 3, a positive phase output terminal 4, a negative phase output terminal 5, constant current sources 6 and 7, NMOS transistors 8, 9, 10 and 11. Terminal resistors 12 and 13 and capacitors 14 and 15. The first transmission unit includes a first differential pair including NMOS transistors 8 and 9 and a constant current source 6 connected to the first differential pair. The second transmission unit includes a second differential pair composed of NMOS transistors 10 and 11, a constant current source 7 connected to the second differential pair, and a drain between the NMOS transistor 10 and the termination power supply terminal 3. And a termination resistor 13 connected between the drain of the NMOS transistor 11 and the termination power supply terminal 3.

  The gate of the NMOS transistor 8 and the gate of the NMOS transistor 10 are connected to the negative phase input terminal 2, and the gate of the NMOS transistor 9 and the gate of the NMOS transistor 11 are connected to the positive phase input terminal 1. The drain of the NMOS transistor 10 is connected to the positive phase output terminal 4 via the capacitor 14, and the drain of the NMOS transistor 11 is connected to the negative phase output terminal 5 via the capacitor 15. Note that the first and second transmission units may be formed on the same die.

  The receiver 17 connected to the transmitter 16 includes a comparator 23, a positive phase input terminal 18, a negative phase input terminal 19, a termination power supply terminal 20, and termination resistors 21 and 22. A termination resistor 21 is connected between the positive phase input terminal 18 and the termination power source terminal 20, and a termination resistor 22 is connected between the negative phase input terminal 19 and the termination power source terminal 20. The comparator 23 compares the voltages of the positive phase input terminal 18 and the negative phase input terminal 19 and outputs a comparison result.

  The positive phase output terminal 4 of the transmitter 16 and the positive phase input terminal 18 of the receiver 17 are connected by DC coupling via the positive phase transmission line of the differential transmission line, and the reverse phase output terminal 5 of the transmitter 16 is connected. The negative-phase input terminal 19 of the receiver 17 is connected by DC coupling via a negative-phase transmission line of a differential transmission line.

  In the signal transmission apparatus configured as described above, the input signal supplied between the positive phase input terminal 1 and the negative phase input terminal 2 is inverted and amplified by the first and second transmission units, and the positive phase output terminal 4 The current signal is output from the negative phase output terminal 5. At this time, the direct current component is output only by the first transmission unit, and the alternating current component is output by the first and second transmission units. Signals output as current signals from the positive phase output terminal 4 and the negative phase output terminal 5 flow to the termination resistors 21 and 22 in the receiver 17 via the positive phase input terminal 18 and the negative phase input terminal 19, respectively. The comparator 23 obtains a received signal by comparing the voltage between the positive phase input terminal 18 and the negative phase input terminal 19.

  Here, the resistance value of the termination resistors 12 and 13 is Ra, and the capacitance value Ca of the capacitors 14 and 15 is selected so as to satisfy Ra >> 1 / (2π × f × Ca) at the frequency f of the output signal. The impedance due to the capacitors 14 and 15 can be regarded as being sufficiently small.

  Since the second transmitter is connected to the positive-phase output terminal 4 and the negative-phase output terminal 5 via the capacitors 14 and 15, respectively, the DC component is cut by the capacitors 14 and 15, and the DC bias of the output terminal is set. There is no effect. Therefore, the output DC voltage Vh is expressed as Vh = Vb, and the output DC voltage Vl is expressed as Vl = Vb−i0 × Rl. Since Vh = Vb, it is possible to prevent a decrease in bias, which was a problem in the prior art.

  The AC amplitude Vswing is the sum of the output amplitudes of the first transmitter and the second transmitter, and is expressed as Vswing = i0 × R1 + i1 × Ra.

  Here, when the differential impedance of the transmission line is Z0, the reflection coefficient ρ at the transmission end is expressed as ρ = (Ra−Z0) / (Ra + Z0). By setting Ra = Z0, ρ = 0, and re-reflection at the transmitting end of the waveform reflected from the impedance mismatched portion to the transmitting side can be prevented as in the conventional technique, and signal integrity is improved. It becomes possible to keep. For example, by setting Ra = Rl = Z0 and i0 = i1 = i / 2, Vh = Vb, Vl = Vb−i / 2 × Rl, Vswing = i × Rl, ρ = 0, and the bias decreases. Signal integrity can be maintained without any problem.

  FIG. 2 is a circuit diagram of a signal transmission apparatus according to the second embodiment of the present invention. 2, the same reference numerals as those in FIG. 1 represent the same items, and the description thereof is omitted. The transmitter 16a of FIG. 2 includes control means 24 connected to the constant current sources 6a and 7a as compared with FIG. The control unit 24 performs control so that the current value i0 of the constant current source 6a and the current value i1 of the constant current source 7a are variable.

  As described in the first embodiment, the AC amplitude Vswing is expressed as Vswing = i0 × R1 + i1 × Ra, the output DC voltage Vh is expressed as Vh = Vb, and the output DC voltage V1 is Vl = Vb−i0 ×. Represented as Rl. Therefore, the control unit 24 changes i0 and i1 under the condition that i0 × R1 + i1 × Ra is constant, so that the transmitter 16a maintains the output DC voltage of Vl while keeping the AC amplitude Vswing constant. It can be changed.

  It should be noted that the disclosures of the above-mentioned non-patent documents are incorporated herein by reference. Within the scope of the entire disclosure (including claims) of the present invention, the embodiments and examples can be changed and adjusted based on the basic technical concept. Various combinations and selections of various disclosed elements are possible within the scope of the claims of the present invention. That is, the present invention of course includes various variations and modifications that could be made by those skilled in the art according to the entire disclosure including the claims and the technical idea.

DESCRIPTION OF SYMBOLS 1, 18 Positive phase input terminal 2, 19 Reverse phase input terminal 3, 20 Termination power supply terminal 4 Positive phase output terminal 5 Reverse phase output terminal 6, 6a, 7, 7a Constant current source 8, 9, 10, 11 NMOS transistor 12 , 13, 21, 22 Termination resistor 14, 15 Capacitor 16, 16a Transmitter 17 Receiver 23 Comparator 24 Control means

Claims (5)

A first and a second transmitter for inputting an input signal in common and outputting an in-phase output signal to the transmission line;
A capacitive element for connecting between the output ends of the first and second transmission units;
A resistance element that is impedance matched with the transmission line and connected as a DC load of the second transmission unit,
With
The first transmission unit connects an input resistance of an input stage on the receiving side via the transmission line as a DC load. Each of the first and second transmission units has an output stage composed of a differential pair,
The signal transmission device according to claim 1, wherein two capacitance elements and two resistance elements are provided corresponding to the differential pair.   The signal transmission device according to claim 2, further comprising first and second current sources that supply current to the differential pairs in the first and second transmission units, respectively.   A value obtained by adding the product of the current value of the second current source and the resistance value of the resistance element to the product of the current value of the first current source and the resistance value of the input resistance of the input stage on the receiving side The signal transmission device according to claim 3, further comprising a control unit configured to set current values of the first and second current sources so as to be constant.   A semiconductor device comprising the signal transmission device according to claim 1.

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