JPH08125456A - Balanced output driver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate noise generated inside a cable by transmitting balanced floating signals. SOLUTION: This balanced output driver 60 for generating floating balanced output signals from non floating balanced input signals is provided with an oscillator 100 for generating the balanced rectangular wave signals of a positive polarity and a negative polarity for which a duty cycle is about 50%, a transformer 110 for toroidal isolation provided with a primary winding for receiving the rectangular wave signals and a secondary winding for generating floating intermediate vibration signals for which the respective primary winding and secondary winding are formed as the bifilar windings of two wires and the end parts of the two wires are electrically connected together so as to form the center tap of the winding, a means for rectifying and smoothing the intermediate vibration signals so as to generate floating DC power supply signals and an output driver stage to be supplied with floating DC signals for receiving input signals and generating output signals.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to balanced output drivers.

[0002]

BACKGROUND OF THE INVENTION Electronic signals, such as analog audio signals, often must be transmitted over long lengths of cable. For example, audio signals may have to be transmitted along a cable between two buildings in a complex studio.

Furthermore, in order to reduce the effect of electromagnetically induced noise in the transmission cable, the local ground potentials at the two ends of the cable (for example between two separate buildings) may be different. In consideration of the above, it has been proposed so far to either install a 1: 1 isolation transformer in the signal path or transmit a balanced signal.

[0004]

Since the isolation transformer cannot make the frequency response uniform over the required frequency band, it may cause signal distortion. Moreover, the use of bulky coil components of this type in the signal path is not preferred.

When transmitting a balanced signal, the induced noise can be rejected at the signal receiver side by a differential amplifier input stage. However, a signal that is balanced with respect to the ground potential of the signal transmitter side may be received by an unbalanced input stage (for example, if the signal does not pass the correct route or if a balanced receiver is used). Induced noise cannot be separated from this signal (since it cannot).

This problem can be solved by transmitting a balanced floating signal for transmission. However, it is extremely difficult to generate such a signal without generating signal noise. For certain types of signals, such as high quality analog audio signals, it is extremely important to minimize the noise produced by the various stages in the signal transmission chain.

[0007]

SUMMARY OF THE INVENTION The present invention is a balanced output driver for generating a floating balanced output signal from a non-floating balanced input signal, the balanced positive and negative polarities having a duty cycle of approximately 50%. Oscillator for generating a rectangular wave signal, a primary winding for receiving the rectangular wave signal and a secondary winding for generating a floating intermediate vibration signal. Each is formed as a bifilar winding of two wires, with the ends of the two wires electrically connected together to form the center tap of the winding and a toroidal isolation transformer and a floating Means for rectifying and smoothing the intermediate vibration signal so as to generate a DC power supply signal, and said floating DC signal is fed.
A balanced output driver having an output driver stage for receiving an input signal and generating an output signal.

The present invention solves the above problems by providing a balanced output driver powered by a floating power supply. This means that there is no need for an isolating transformer in the signal path (instead it is in the feed path), and that a floating balanced signal can be generated with all of the above advantages. I mean. The use of a 50% duty cycle in square wave generation means that the power supply to the drive amplifier can be made particularly smooth, which is useful for noise sensitive applications, such as audio signal transmission.

In the technique of forming a bifilar winding (coil) and connecting both ends of two wires of the bifilar winding to form a center tap, two half windings on both sides of the center tap of the transformer are mutually connected. , And with the toroidal core and with the other winding of the transformer, in terms of inductive and capacitive coupling, they are very well matched to each other along the entire length. Such an effect is that the signals induced in each half-winding of the secondary winding are very well matched to each other and the phase is symmetrical (due to the symmetrical anti-phase drive of the corresponding primary winding). It is in reverse phase. This is
Except for a slight fluctuation at the time of switching, this means that the output of the rectifying means becomes a very smooth DC signal, and the center tap on the secondary winding side forms a very steady floating earth signal. This fact means that there is a signal of opposite phase at the end of the winding, so that noise can be largely removed.

Preferably, the primary winding is arranged on the first part of the toroidal core and the secondary winding is arranged on the second part of the toroidal core, the first part and the second part being the primary winding. Both ends of the wire are
The first separation distance and the second separation distance are substantially equal to each other so that they are separated by the separation distance and the second separation distance. This also means that the capacitive coupling between the ends of the primary and secondary windings is equal so that the two inputs fed to the bridge rectifier are well matched in amplitude. Only the polarities differ to a large extent. Such a duty cycle is 50
A feature combined with the use of a% square wave signal is that the output of the bridge rectifier can be a very smooth DC signal, except for slight irregularities in switching time.

Preferably, the rectifying / smoothing means is one bridge rectifier having a pair of output terminals for rectifying the intermediate vibration signal, and one connected between each output terminal and the center tap of the secondary winding. It comprises the above capacitor and a bifilar wound noise removing choke connected in series to the output terminal of the bridge rectifier.

The above techniques for reducing noise in a balanced signal are particularly applicable to signals having very stringent low noise conditions, such as audio signals.

The driver as described above is particularly effectively used in a signal processing device.

When generating an input signal from a sampled digital input signal, the oscillator is preferably operable to generate a square wave signal in synchronization with the sampling frequency of the sampled digital input signal. This can prevent periodic interference between the rectangular wave and the sampled digital signal.

In a preferred embodiment, the device comprises a plurality of circuit cards interconnected by a digital communication bus backplane (motherboard) having a characteristic impedance, and a communication busback having an output impedance greater than the characteristic impedance of the backplane. And a bus driver for supplying a signal to the plane. Such measures work to reduce induced noise in the circuit card, and such measures are important in applications where noise is critical, such as audio processing.

The bus-to-ground capacitance is obtained solely by the intrinsic capacitance of the bus itself, but in some cases it is increased by providing a capacitor connected between the communication bus backplane and the signal ground potential. Can be advantageous.

In the preferred construction, the communications bus backplane has a characteristic impedance of about 75 ohms and the bus driver has an output impedance of about 300 ohms. However, in general, the bus driver output impedance (R) and the capacitance (C) between the bus backplane and the signal ground potential are expressed by the following formula with the maximum data transmission frequency (F) on the bus backplane. It is preferable to have a relationship.

[0018]

Preferably at least one of the cards
One comprises a differential amplifier whose input terminals are arranged along a line substantially parallel to the communication bus backplane. This takes advantage of the recognition that inductive noise travels along the card with wave heights nearly parallel to the bus backplane.

In another preferred embodiment, at least one of the cards comprises a differential amplifier, the device comprising means for coupling the noise components in the signal ground potential to each input of the differential amplifier approximately equally. Equipped with. The noise thus equally induced is almost canceled by the differential amplifier.

In view of the second aspect, the present invention provides an oscillator for generating balanced positive and negative polarity square wave signals having a duty cycle of approximately 50%, and an oscillator for receiving the square wave signals. It has a primary winding and a secondary winding for generating a floating intermediate vibration signal, each of the primary winding and the secondary winding being formed as a bifilar winding of two wires. A toroidal isolation transformer, the ends of which are electrically connected together to form the center tap of the winding, and means for rectifying and smoothing the intermediate oscillating signal to produce a floating DC power signal. And a floating power supply with.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings. In the figure, the same numbers indicate the same parts.

FIG. 1 is a schematic diagram of a balanced output drive stage in which an input signal 10 is provided to an output driver (amplifier) 20 and via an inverter 30 to a second output driver 40. Inverter 30 inverts input signal 10 so that the output of output driver 40 is the inverted version of the output of driver 20. The output signals of these two drivers form the version output.

FIG. 2 is a schematic diagram of a floating balanced output drive stage. Signal source 5 with built-in inverter 52
The balanced (not floating) input signal from 0 is
It is supplied to a driver circuit 60 consisting of two output drivers (amplifiers) fed by a positive line and a negative line. The two outputs of the output driver in driver circuit 60 form a balanced and floating output signal 80.

FIG. 3 is a schematic diagram of a floating balanced output drive stage with a floating power supply. The circuit of FIG. 3 utilizes a driver circuit 60 of the type described above with reference to FIGS.

A balanced (not floating) audio signal is supplied from the digital audio processing circuit 50 'to the driver circuit 60. The digital audio processing circuit 50 'comprises a digital-analog converter and an inverter (not shown), and is adapted to generate a balanced audio signal from a sampled digital signal. The digital audio processing circuit 50 ', eg a digital audio mixing console, ie a digital audio recording / playback device, operates by controlling the sampling clock signal. For example, the sampling clock signal can be a 44.1 kilohertz (kHz) or 48 kHz clock signal.

This sampling clock is generated by the AC generator 1.
00, the AC generator 100 is connected to the power supply ground 10
6 produces a fixed 50% duty cycle square wave signal with a frequency (3F) three times higher than the sampling clock signals of both positive polarity 102 and inverted polarity 104.
This duty cycle should be very precisely fixed at 50% within a tolerance of less than ± 0.1%.

This AC generator operates in synchronization with the sampling clock of the digital audio processing circuit 50 '. The output of the AC generator 100 is supplied to the primary winding part of the toroidal isolation transformer 110. In particular, a square wave signal of positive polarity and reverse polarity is supplied to the end tap of the primary winding of the transformer, and the ground 106 of the power supply is connected to the center tap of the primary winding.

The isolation transformer 110 will now be described in detail with reference to FIGS. 4a and 4b.

The secondary winding of the transformer 110 is connected to a bridge rectifier 120, which rectifies the AC output of the transformer secondary winding to form a DC power source. The center tap of the secondary winding of transformer 110 develops a floating ground potential midway between the positive and negative outputs of bridge rectifier 120.

The output of the bridge rectifier 120 is smoothed by the capacitor 130. The primary winding of the transformer 110 is supplied with a positive rectangular wave and an inverted rectangular wave having a duty cycle of 50%, and therefore, except for some fluctuations that may occur in the switching time of the rectangular wave, it is possible to obtain a steady-state potential. There is a property that becomes. The nature of such a rectifier output is
The capacitor 130 has a relatively small capacitance, for example, 10
It means that 0 nanofarad is enough.

The smoothed output of rectifier 120 is then
It is supplied to the driver circuit 60 of the type described above through a choke 140 for removing the common mode signal which is wound by a bifilar. The choke 140 has a high impedance for the common mode output on the output of the smoothed rectifier and a low impedance for the differential mode signal at the smoothed output.

The circuit of FIG. 3 is characterized by the AC generator 1
The rectangular wave output of 00 is a digital audio processing device 50 '.
It is possible to synchronize with the sampling clock of.
Such synchronization prevents beat effects that occur between the output of the AC generator 100 and the digital audio samples being processed within the device 50 '. This means that distortion caused by noise induced by the oscillation of the AC generator 100 can be reduced.

4a and 4b are schematic representations of toroidal transformer 110. In particular, FIG. 4a is a simplified schematic diagram showing the relative positions of the primary and secondary windings of the transformer, and FIG. 4b is a simplified schematic diagram showing the method of forming the windings.

Referring to FIG. 4 a, the transformer is wound on a ferrite toroidal core 180, and the primary winding 182 and the secondary winding 184 are arranged on both sides of the ferrite toroidal core 180. The windings are spaced so that the spacings I ₁ and I ₂ at adjacent ends of the windings are equal. This means that the capacitive coupling between the ends of the windings is equal, so that the two outputs fed to the bridge rectifier match well in amplitude and differ only in polarity to a considerable extent. There is. This feature, combined with the use of a 50 duty cycle square wave signal generated by the AC generator 100, makes the output of the bridge rectifier a very smooth DC signal, except for some variations in switching. .

FIG. 4b shows how to wind each winding (primary winding and secondary winding) of the transformer 110. Only one winding is shown in FIG. 4b for the sake of clarity.

This winding is first formed by forming a bifilar winding. In other words, each time the wire is wound around the ferrite toroidal core 180, the two wires come very close to each other, so that the ferrite toroidal core 18 is
Wrap two wires around 0.

Once the bifilar windings have been formed and positioned on the toroidal core, these windings are linked to form the center tap 186, as described with reference to FIG. 4a. A center tap is formed by connecting one end of one of the two wires of the bifilar winding to the other end of the other wire of the bifilar winding. This means that
This means connecting the lines of the book in series with the center tap. The winding starts at one end tap (ie, tap 188 on the left side in FIG. 4b) and progresses along the toroidal core from left to right in the figure and then back to the left end of the toroidal core by the center tap connection. Linked. The winding is then the other (right) end tap 18
Continue from left to right toward 9.

The bifilar winding is formed in this way,
According to the technique of connecting the two wires of the bifilar winding to form the center tap as described above, the center tap 186
Of the two half windings on either side of (i.e., the half winding between the tap 188 and the center tap 186 and the half winding between the center tap 186 and the end tap 189) and one of the ferrite toroidal cores. And in terms of inductive and capacitive coupling with the other winding of the transformer, it means that they are very well matched to each other along the entire length. Such an effect is that the signals induced in each half-coil on the secondary winding side match each other very well (because of the symmetrical opposite phase drive of the corresponding primary winding).
It has a symmetrical opposite phase. This means that the output of the rectifying means becomes a very smooth DC signal, and the center tap on the secondary winding side forms a very steady floating earth signal, except for a slight fluctuation during switching. . This fact means that there is a signal of opposite phase at the end of the coil, so that noise can be largely removed.

In this embodiment, at least one of a plurality of circuit boards, that is, cards on which digital and analog signal processing parts are mounted and which is connected to a common digital communication bus backplane (motherboard) is floated as described above. Power supplies and balanced output drive stages can be provided. Next, various measures for performing the low noise operation of the communication bus backplane will be described with reference to FIGS.

FIG. 5 is a schematic diagram of a previously proposed communication bus backplane 200 with multiple circuit boards or cards 210 connected thereto. The signals are driven by the driver 220 on the communication bus backplane. The backplane 200 has a characteristic impedance of 75.
Since it is ohms, the circuit shown in FIG. 5 is driven by a driver with an output impedance of 75 ohms (in this example shown by a discrete resistor 230 at the output of the driver with a very low output impedance). , 75 ohm termination resistor 240. The capacitance of the bus backplane is schematically illustrated by the capacitor 250.

FIG. 6 is a schematic diagram of a second type of previously proposed communication bus backplane. The circuit of FIG.
5 except that the termination resistor 240 is omitted.
Is very similar to the circuit. By omitting the resistors in this way, the resistors 230 and 24 of FIG.
The damping caused by the voltage divider formed by 0 is reduced.

A problem that can occur with backplane constructions such as those shown in FIGS. 5 and 6 is that high frequency noise is introduced into the circuitry on card 210. This noise can be at frequencies higher than the data transmission rate along the bus backplane 200. These high frequency noise components are caused by the sharp rise time of the pulses transmitted on the backplane. For example, in a system where the data rate along the backplane 200 is about 4 megahertz (MHz), high frequency noise up to about 100 MHz is induced in the circuitry on the card 210. If the card 210 carries analog components, or components that convert analog and digital signals, such induced noise can cause a significant degradation in the noise performance of the circuit.

7 and 8 are schematic diagrams of a communication bus backplane and bus driver circuit according to an embodiment of the present invention. In the embodiment of FIGS. 7 and 8, bus driver 300
The output impedance of is increased from 75 ohms (in previously proposed structures as above) to larger values such as 300 ohms. This means that the time constant of the resistor-capacitor (RC) circuit formed by the driver output impedance and the bus capacitance 310 increases over a longer period of time than the corresponding time constants of FIGS. 5 and 6. ing. For example, in FIG.
The time constant of the RC circuit formed by the 300 ohm output impedance 320 and the bus capacitance 310 is selected to be approximately 200 nanoseconds (nS). Such a large rise time raises the signals applied to the bus, increases their rise time, and reduces the maximum frequency of HF noise to about 10 MHz. Bus backplane 35 driven in this way
Eliminating inductive noise with the maximum frequency component limited to about 10 MHz from the circuit on card 340 connected to 0 is simpler.

Generally, the bus driver output impedance (R) and the capacitance (C) between the bus backplane and the signal ground potential (C) are determined by the maximum data transmission frequency on the bus backplane (F) and the following equation. There is a relationship shown.

[0046]

In FIG. 7, the output impedance of the bus driver 300 is the low output impedance driver 30.
Shown as a discrete resistor 320 connected to the 0 output. However, it will be appreciated that the bus driver 300 can be manufactured to have the required output impedance in its own right. Bus capacitance 310
Is schematically illustrated by the capacitance shown in dashed lines. However, as shown in FIG. 8, a discrete capacitor 325 is placed across the bus (to signal ground) to change the total capacitance in the RC circuit.
Can be connected.

Bus backplane 350 of FIGS. 7 and 8.
Has a characteristic impedance of 75 ohms. Similar to FIG. 6, in FIGS. 7 and 8 the bus backplane 3
The remote end of 50 is not terminated.

FIG. 9 is a schematic diagram showing the rise time of a signal on the communication bus backplane of FIGS. 5 and 6.
Since this signal has a very sharp rise time, a high frequency component that is induced in the circuit on the card 210 is generated.

FIG. 10 is a schematic diagram showing the rise time of a signal on the communication bus backplane of FIGS. 7 and 8.
In this case, the rise time is longer than that shown in FIG. 9, so the maximum frequency component of the noise induced on the card 340 is lower for the same data rate.

FIG. 11 is a schematic diagram of a circuit card 340 connected to the backplane 350 of FIG. 7 or 8.
Wave height 40 of noise induced from the backplane 350
The 0 is recognized to be approximately parallel to the backplane 350. Therefore, the input terminal 405, which is a component susceptible to noise such as the differential amplifier 410, is placed on a line parallel to the backplane 350.
Therefore, such a measure means that the noise induced at each input terminal is always the same, since the input terminals are located along the common wave height.

FIG. 12 is a schematic diagram of an input noise rejection choke installed at the input of the differential amplifier 410 on one of the cards 340. This choke 430 includes three coupled windings, one of which is component 410.
Connected in series with the other input, the other one to ground potential 440
And shield blade 450 (if used)
Is connected between and. The inductance of the two windings connected in series with the input terminal helps eliminate the noise induced at the input of the component,
A third winding connected to 0 couples the noise induced in the signal ground equally into both input terminals. The noise equally induced at the two input terminals is
Removed by zero.

On each card 340, the noise removal choke multi-stage method can be used. For example, in a card that supports a microphone amplifier connected to an analog-to-digital converter, different noise-removing chokes can be selected to match the noise-removing characteristics of each component. For example, a noise removing choke having an inductance of about 300 microhenries can be used at the input end of the microphone amplifier, and a choke having an inductance of 4 millihenries can be used at the input end to the analog-digital converter.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a balanced output drive stage.

FIG. 2 is a schematic diagram of a floating balanced output drive stage.

FIG. 3 is a schematic diagram of a floating balanced output drive stage and a floating power supply.

FIG. 4 is a schematic diagram of a transformer for a toroidal isolate. b Schematic representation of a toroidal isolation transformer.

FIG. 5 is a schematic diagram of a communication bus backplane and a bus driver circuit.

FIG. 6 is a schematic diagram of a communication bus backplane and a bus driver circuit.

FIG. 7 is a schematic diagram of a communication bus backplane and a bus driver circuit.

FIG. 8 is a schematic diagram of a communication bus backplane and a bus driver circuit.

9 is a schematic diagram showing the rise time of a signal on the communication bus backplane of FIG. 5 or FIG.

10 is a schematic diagram showing the rise time of a signal on the communication bus backplane of FIG. 7 or FIG.

FIG. 11 is a schematic diagram of a circuit card connected to a communication bus backplane.

FIG. 12 is a schematic diagram of a choke for removing input noise.

[Explanation of symbols]

 50 'Digital audio processing circuit 60 Balanced output driver 100 AC generators 102, 104 Primary end tap 106 Center tap 110 Isolation transformer

Claims (13)

[Claims] 1. A balanced output driver for generating a floating balanced output signal from a non-floating balanced input signal, which produces balanced positive and negative polarity square wave signals having a duty cycle of approximately 50%. And a secondary winding for generating a floating intermediate oscillating signal, each of the primary and secondary windings having a two-wire bypass. Formed as a filer winding, the ends of the two wires are electrically connected together to form the center tap of the winding, and a toroidal isolation transformer to generate a floating DC power signal. Means for rectifying and smoothing the intermediate vibration signal, for receiving the input signal and generating an output signal, which is fed with the floating DC signal Balanced output driver and an output driver stage. 2. The primary winding is arranged on a first part of the toroidal core, the secondary winding is arranged on a second part of the toroidal core, and the first part and the second part are primary windings. Ends of the secondary winding are separated from the respective ends of the secondary winding by a first separation distance and a second separation distance so as not to overlap, and the first separation distance and the second separation distance are substantially equal to each other. The driver according to claim 1, wherein 3. The rectifying / smoothing means includes one or more bridge rectifiers for rectifying an intermediate vibration signal having a pair of output terminals, and one or more connected between each output terminal and a center tap of the secondary winding. Driver according to claim 1 or 2, comprising a capacitor according to claim 1 and a bifilar wound noise elimination choke connected in series to the output terminal of the bridge rectifier. 4. The driver according to claim 1, wherein the input signal is an audio signal. 5. A signal processing device comprising the balanced output driver according to claim 1. 6. A means for generating an input signal from a sampled digital input signal, wherein the oscillator is operable to generate a square wave signal in synchronization with the sampling frequency of the sampled digital input signal. Item 5. The apparatus according to item 5. 7. A plurality of circuit cards interconnected by a digital communication bus backplane having a characteristic impedance, and a bus driver for supplying a signal to the communication bus backplane having an output impedance larger than the backplane characteristic impedance. Claim 5 or 6 provided with
The described device. 8. The apparatus of claim 7, comprising a capacitor connected between the communication bus backplane and a signal ground potential. 9. The apparatus of claim 7 or 8 wherein the communications bus backplane has a characteristic impedance of about 75 ohms and the bus driver has an output impedance of about 300 ohms. 10. The bus driver output impedance (R) and the capacitance (C) between the bus backplane and the signal ground potential are shown in the following equation with the maximum data transmission frequency (F) on the bus backplane. In a relationship, The device according to any one of claims 7 to 9. 11. At least one of the cards comprises a differential amplifier, the input terminals of said differential amplifier being arranged along a line substantially parallel to the communication bus backplane. The device according to any of the above. 12. At least one of the cards comprises a differential amplifier, and the device comprises at each input of the differential amplifier means for approximately equally coupling the noise component in the signal ground potential. The apparatus according to any one of 7 to 11. 13. An oscillator for producing balanced positive and negative polarity square wave signals with a duty cycle of approximately 50%, and a primary winding and floating intermediate oscillator signal for receiving the square wave signals. Has a secondary winding for winding, each of the primary winding and the secondary winding is formed as a bifilar winding of two wires, and both ends of the two wires form a center tap of the winding. A floating power supply comprising a toroidal isolation transformer electrically connected together to form and means for rectifying and smoothing the intermediate vibration signal to generate a floating DC power supply signal.