Data access arrangement using a high frequency transformer for electrical isolation

Data access device using high-frequency transformer for electrical isolation

BACKGROUND OF THE INVENTION The present invention relates to isolation barriers for selectively isolating circuits from each other. This isolation barrier device is used in modems and other equipment, especially those that require electrical isolation barriers between the equipment and the public telephone network.

Part 68 of the Federal Communications Commission has required that the electrical connection of the public telephone network is between the circuit directly connected to the network (called the "line side" circuit) and the circuit such as a modem that is directly connected to residential electricity (called the "system side"). An isolation barrier device is provided between the circuit). This isolation barrier device must provide isolation so that a high-voltage voltage source of one thousand volts or one thousand five hundred volts of 50 Hz or 60 Hz rms applied between various points of the equipment causes a leakage current of no more than 10 milliamperes.

The theory of isolation barriers is known in the prior art. For example, US Patent No. 6,137,827 discloses the theory and background of isolation barriers in detail, and includes many patents showing isolation barriers and devices using isolation barriers by reference. Generally, electrical isolation is provided in the data access device (DAA) of the device. U.S. Patent No. 6,137,827 and the patents included by reference therein are all included in the content of this article by reference.

Traditional modems use voice band transformers in their DAA to provide electrical isolation barriers. This kind of transformer carries the sending signal and the receiving signal. A hybrid type circuit for coupling a four-wire circuit and a two-wire circuit is used to separate the two signals. Hybrid circuits are well known in the prior art. They have four sets of terminals arranged in two pairs, and are designed to produce high losses between the two sets of terminals in the other pair when one of the terminals is properly terminated. It is known that a modem using a voice band transformer DAA has high reliability. However, the voice band transformer must handle low frequency (LF) signals of about 100-4000 Hz.

The main source of distortion in the transformer is the nonlinearity that causes signal harmonics. For voice band signals, many harmonics fall within the same 100-4000 Hz frequency band. Signal harmonics are characterized by the presence of unnecessary energy at multiple desired signal frequencies. Therefore, the signal frequency components at 500 Hz will cause noises of 1000 Hz, 1500 Hz, 2000 Hz, 2500 Hz, 3000 Hz, 3500 Hz and 4000 Hz; all fall within the required signal frequency band. On the other hand, if the signal is modulated to a high frequency, such as 1MHz, then the required signal will be in the range of 0.996MHz to 1.004MHz. Now, the 500Hz component is at 0.9995MHz and 1.0005MHz. The lowest harmonic is at 1.9999MHz, which is just above the highest signal frequency at 1.004MHz. Therefore, a simple low-pass filter can be used to eliminate harmonics and will not cause distortion in the desired signal. The linearity of a transformer largely depends on the magnetic induction density of its magnetic core. The higher the magnetic induction density, the lower the linearity of the transformer, and the higher the energy of signal harmonics. Therefore, the voice transformer requires high linearity, which usually increases its size and cost.

In addition, the DAA of a modem that uses a voice band transformer generally uses a direct drive method. In this approach, the transmitted signal from the modem driver passes directly through the transformer without further amplification. This direct drive method requires a transformer to transmit high transmission power, which further increases the linearity requirements of the transformer. Because of these shortcomings, a satisfactory voice band transformer with this type of electrical isolation is bulky and expensive.

These are several ways to solve this problem. One approach is to use digital transformers or pulse transformers to replace voice band transformers. However, digital or pulse transformers are binary, so they cannot carry two signals at the same time-sending and receiving signals. In order to allow the sending signal and the receiving signal to be sent acceptably, either use two pulse transformers, one for sending the signal and the other for receiving the signal, or resorting to some kind of time division multiplexing method to carry alternately Send and receive signals. However, this time division multiplexing method will destroy the self-clock capability of the signal. Therefore, the clock signal must be carried by a different device, and another transformer is generally used, which increases the additional cost of the product. Another disadvantage of digital or pulse transformers is that they must operate in their saturated range, which requires more power than similar transformers operating in their non-saturated (or "linear") range.

Another way is to use capacitive coupling. This approach uses one or more high-voltage capacitors as electrical isolation barriers, because capacitors generally exhibit good linearity. Therefore, it is possible to use a hybrid circuit to separate the transmission signal and the reception signal. On the other hand, the LF voice band signal required in the modem requires a large capacitor. Such high-voltage large capacitors are expensive. Therefore, some devices are used to modulate the signal to a higher frequency to reduce the requirements on the capacitor.

Another way is to use high-voltage optocouplers. Still due to the typical highly non-linear characteristics of this type of optical device, separate couplers must be used for transmitting and receiving signals. In DAA that uses optical coupling as an electrical isolation barrier, a baseband approach or a passband approach can be used. In the case of the baseband route, the voice band signal is sent directly through the optocoupler. However, this has the disadvantage of requiring a good method to compensate for the nonlinearity of the optical coupler. In the case of the passband approach, some devices are used to modulate the signal to a higher frequency to reduce the influence of the nonlinearity of the optocoupler, thereby increasing the additional cost and complexity of the solution.

In addition to the sending and receiving voice band signals that must be sent by the above-mentioned types of modems, there are also control and status signals that also need to pass through electrical isolation barriers. The signals mentioned later are either carried through a separate isolation barrier, or multiplexed with the voice band signal and carried through the same isolation barrier.

Summary of the invention

We have observed that the linearity of a transformer is largely dependent on the magnetic induction density in its core. The magnetic induction density is directly proportional to the power delivered by the transformer, and inversely proportional to the volume of the magnetic core, the number of turns of the transformer, and the signal frequency. In other words, the smaller the signal power, the larger the magnetic core, the more coil turns, and the higher the signal frequency, the higher the linearity of the transformer. In other words, in order to achieve the same linearity, a smaller signal power and a higher frequency will result in a smaller magnetic core and a smaller number of turns, resulting in a smaller and less expensive transformer.

The present invention uses two approaches to obtain a low-cost and reliable electrical isolation barrier using a high frequency (HF) transformer. First of all, we can add an amplifier to the line end of the electrical isolation barrier to reduce the power requirements of the transformer.

Second, we use some devices to modulate the signal to a higher frequency. At higher frequencies, any harmonics produced by the non-linearity of the transformer are out of band. Then, a simple device can be used to eliminate any remaining distortion caused by the transformer nonlinearity.

Unlike digital or pulse transformers, HF transformers are not driven to their saturation range. Therefore, the power required to implement the present invention is lower than that required by conventional isolation devices using digital or pulse transformers. Similarly, by operating the HF transformer in its linear range, it is possible that one transformer can carry both the transmit and receive signals, while using an HF hybrid circuit to separate the two directions.

Description of the drawings

The present invention can be better understood with reference to the accompanying drawings, which only show examples of embodiments of the present invention and should not be considered as limiting the scope of the present invention. The present invention can tolerate other embodiments with equivalent effects without departing from its scope.

Figure 1 shows a DAA circuit based on a voice band transformer in the prior art, including a hybrid circuit for converting a four-wire circuit into a two-wire circuit.

Figure 2 shows a block diagram of a DAA isolation barrier using an HF transformer according to the principles of the present invention.

Figure 3 shows the DAA of a hybrid circuit that uses an HF transformer to isolate the barrier and convert a four-wire circuit into a two-wire circuit, where the CODEC is entirely located in the system-side circuit.

Figure 4 shows a DAA using two HF transformer isolation barriers, one for TX and one for RX, where the CODEC is entirely located in the system-side circuit.

Fig. 5 shows the connection between an HF transformer and two hybrid circuits used in Fig. 3 or Fig. 6.

Figure 6 shows a DAA of a hybrid circuit that uses an HF transformer to isolate the barrier and convert a four-wire circuit into a two-wire circuit, where CODECs are distributed between the circuit side and the system side circuit.

Figure 7 shows a DAA using two HF transformers to isolate the barrier, one for TX and one for RX, where CODEC is distributed between the line side and the system side circuit.

Figure 8 shows an example circuit for transferring power from the system-side circuit to the line-side circuit.

Fig. 9 shows a multiplexer used in the present invention.

detailed description

Figure 1 shows an isolation barrier used in a DAA circuit in the prior art. In this figure, the modem uses a voice band transformer 1 in its DAA to provide electrical isolation barriers. The transformer carries the sending signal and the receiving signal. The separation of these two signals is performed using a hybrid type circuit 2 that converts a four-wire circuit into a two-wire circuit. Hybrid circuits are known in the art. They have four sets of terminals arranged in two pairs and are designed to produce high losses between the two sets of terminals in the other pair when one of the terminals is properly terminated. The hybrid circuit shown in Figure 1 includes six impedance units Z1-Z6, and the impedance elements Z1-Z6 are usually implemented using resistors, capacitors, or some combination thereof. Other hybrid circuit structures can also be used in the traditional transformer DAA.

Figure 2 shows the basic principle of the present invention. In this figure, the HF transformer 24 provides isolation. The input signal 21 is connected to the modulator 22, and the input signal can be analog or digital. The analog output of the modulator 22 is connected to the input of the HF transformer 24 via a line 23. The output of the HF transformer 24 is connected to the input of the demodulator 26 via a line 25. The demodulator 26 produces an output signal 27, which can be analog or digital. Regardless of whether the input or output signal is digital or analog, in all embodiments of the present invention, the signal provided to the HF transformer 24 is analog. Simple amplitude modulation can be used in the modulator 22 to modulate the input signal 21. This can be done by multiplying the input signal 21 by the clock signal. Similarly, the demodulator 26 can multiply the signal from the HF transformer 24 by the clock signal. The clock signal used by the demodulator 26 can be derived from the signal, or can be provided through a separate isolation barrier, as shown in FIG. 8. A simple low-pass filter can be included in the demodulator 26 to eliminate the harmonic distortion caused by the HF transformer 24. The output signal 27 is basically the same as the input signal 21.

Figures 3 and 4 show the use of the present invention in DAA applications, where the system-side circuit includes a CODEC. First, the TX signal path in FIG. 3 will be described. The modem signals to be sent to the line are sent from the modem system to CODEC 33 in digital form, CODEC 33 converts them to analog form, and sends them to MUX 32. The control signal generator 31 sends a control signal in an analog form to the MUX 32. MUX 32 combines two analog signals into a "baseband" signal. In a preferred embodiment, the two signals are separated in frequency and can be combined by a direct summation circuit. The combined signal is sent to the modulator 36, which shifts the combined signal to a higher frequency, thereby producing a "passband" signal. The passband signal enters the high frequency mixing circuit 38, and the high frequency mixing circuit 38 connects the passband TX signal and the passband RX signal to the high frequency transformer 39. At the line end of the transformer, the passband TX signal passes through a second high frequency hybrid circuit 42. Next, the passband TX signal is sent to the clock recovery circuit 41 and the demodulator 43. The clock recovery circuit 41 locks the frequency of the line-side demodulator 43 to the frequency of the system-side modulator 36. The demodulator 43 shifts the signal frequency back to the original baseband. The baseband signal at the end of the line is sent to De-MUX 46, and De-MUX 46 separates the control signal from the TX line signal. In the preferred embodiment, this is a filter bank (filter bank). The control signal is sent to the control circuit 45, the TX line signal is sent to the LF mixing circuit 47, and the LF mixing circuit 47 connects the TX line signal and the RX line signal to the line interface 50. The control circuit 45 changes the characteristics of the line interface 50 based on the control signal. The line interface 50 is connected to the telephone network.

Now, the RX signal path in FIG. 3 will be described. The analog RX line signal from the telephone network enters the line interface 50. From here, it is sent to the LF hybrid circuit 47 where it is separated from the TX line signal. Next, the RX line signal is sent to MUX 48. The status signal generator 49 sends the status signal to the MUX 48 in an analog form. These signals are generated based on conditions in the line interface 50. MUX 48 combines two analog signals into one baseband signal. The combined signal is sent to the modulator 44, which shifts the combined signal to a higher frequency, thereby generating a passband signal. The frequency of the modulator 44 is locked to the frequency of the demodulator 43. The passband signal enters the high frequency mixing circuit 42, and the high frequency mixing circuit 42 connects the passband TX analog signal and the passband RX analog signal to the high frequency transformer 39. At the system end of the transformer, the passband RX signal passes through a second high frequency hybrid circuit 38. Next, the passband RX signal is sent to the demodulator 37, and the demodulator 37 shifts the signal frequency back to the baseband. The baseband signal is sent to De-MUX 34, De-MUX 34 separates the status signal from the RX line signal. The status signals are sent to the status circuit 35, which converts these signals into digital instructions for the modem system. For example, these digital indications may be in the form of ring indications or in the form of bits in a status register. The RX line signal is sent to CODEX 33, and CODEX 33 converts it into a digital signal for the modem system.

The operation of the DAA shown in Figure 4 is very similar to that of Figure 3. In this case, there are two HF transformers, one for the TX signal path. One is for the RX signal path. Because of this separation, no high-frequency hybrid circuit is required.

Figures 6 and 7 show the use of the present invention in DAA applications, where CODEC circuits are distributed between the line end and the system end. The analog part of the circuit is located at the line end, and the digital part is located at the system end. First, the TX signal path in FIG. 6 will be described. The modem signal to be sent to the line is sent in digital form to the Σ-Δ (Sigma Delta) CODEC digital part 63, and the Σ-Δ CODEC digital part 63 converts it into an over-sampled 1-bit digital form, And send it to MUX 62. The control signal generator 61 sends the control signal in digital form to the MUX 62. MUX 62 combines two digital signals into one digital signal. MUX 62 contains a framing circuit to facilitate synchronization in the line-side circuit. In the preferred embodiment, the generated digital signal is twice the bit rate of the oversampled 1-bit digital signal from the sigma-delta CODEC digital section 63. The combined digital signal is sent to the modulator 66, which uses it to modulate the high frequency analog signal, thereby generating the passband signal. For example, this can be done by multiplying the digital signal by the analog clock signal. The generated signal enters the high frequency mixing circuit 68, and the high frequency mixing circuit 68 connects the passband TX and passband RX signals to the high frequency transformer 69. At the line end of the transformer, the passband TX signal passes through a second high frequency hybrid circuit 72. Next, the passband TX signal is sent to the clock recovery circuit 71 and the demodulator 73. The clock recovery circuit 71 locks the frequency of the line-side demodulator 73 to the frequency of the system-side modulator 66. The demodulator 73 recovers the bits of the combined digital signal. These bits are sent to De-MUX 76, which separates the digital control signal from the oversampled 1-bit digital TX line signal. De-MUX 76 contains frame detection means for aligning the data correctly. The digital control signal is sent to the control circuit 75, and the oversampled 1-bit digital TX line signal is sent to the Σ-ΔCODEC analog part and the LF hybrid circuit 77, and the Σ-ΔCODEC analog part and the LF hybrid circuit 77 oversample the 1-bit The digital signal is converted into an analog form, and the TX line signal and the RX line signal are connected to the line interface 80. The control circuit 75 changes the characteristics of the line interface 80 based on the control signal. The line interface 80 is connected to the telephone network.

Now, the RX signal path in FIG. 6 will be described. The analog RX line signal from the telephone network enters the line interface 80. From here, it is sent to the sigma-delta CODEC analog part and LF hybrid circuit 77, where it is separated from the TX line signal and converted into an oversampled 1-bit digital form. Next, the over-sampled 1-bit digital signal is sent to MUX 78. The status signal generator 79 sends the digital status signal to the MUX 78. These signals are generated based on the conditions in the line interface 80. MUX 78 combines these two digital signals into one digital signal. MUX78 includes a framing circuit to facilitate synchronization in the system-side circuit. The combined signal is sent to a modulator 74, which uses the digital signal to modulate a high-frequency analog signal, thereby generating a passband signal. The frequency of the modulator 74 is locked to the frequency of the demodulator 73. The passband signal enters the high frequency mixing circuit 72, and the high frequency mixing circuit 72 connects the passband TX and passband RX signals to the high frequency transformer 69. At the system end of the transformer, the passband TX signal passes through a second high frequency hybrid circuit 68. Next, the passband RX signal is sent to the demodulator 67, and the demodulator 67 recovers the bits of the combined digital signal. The digital signal is sent to De-MUX 64, and De-MUX 64 separates the status bits from the oversampled 1-bit digital RX line signal. De-MUX 76 contains a frame detection device to locate the data correctly. The status signals are sent to the status circuit 65, which converts these signals into digital instructions for the modem system. For example, these digital indications can be in the form of ringing indications or in the form of bits in the status register. The oversampled 1-bit digital RX line signal is sent to the sigma-delta CODEC digital section 63, which converts it into a digital signal of the modem system.

The operation of the DAA shown in FIG. 7 is very similar to the operation shown in FIG. 6. In this case, there are two HF transformers, one for the TX signal path and one for the RX signal path. Because of this separation, there is no need for a high-frequency hybrid circuit.

Many integrated DAA control logic circuits use integrated control signals for hook control, line impedance control, and so on. The state of these integrated control signals is usually programmed into the line-side circuit by the system-side circuit and saved by the line-side circuit. However, since the circuit at the end of the line is usually powered by the line voltage, for example, in the case of a reverse connection of the line voltage, the interruption of the line voltage can destroy the state of the control signal. The traditional way to prevent such damage is to freeze the control signal state during this interruption, or to supply power at least partially from the system side. In the present invention, a new approach is used to save the control state information on the system side, and the control signal is frequently updated on the line side. Since the line end electricity only follows the control information of the system end, this way can logically form a separate control circuit. The line-side circuit only maintains the control state until the next update. The time between updates is variable, generally on the order of microseconds. During the interruption, the control state of the line end may be temporarily destroyed. However, subsequent updates will restore the control state to the correct state. In the case that the update is stopped, for example, if the system side is suddenly powered off, the line side circuit will restore its control signal to the default state. This will prevent such a defect in the programmable scheme, that is, when the system side is interrupted, the line side circuit remains in an error state.

Multiplexing of control/status signals and line signals In order to reduce the number of HF transformer isolation barriers in DAA using the present invention, the preferred embodiment uses a multiplexing scheme to combine TX and RX line signals with control and/ Or status signal. Generally, the control signal is multiplexed with the TX line signal, and the status signal is multiplexed with the RX line signal. In the DAA shown in Figures 3 and 4, an analog multiplexing scheme is used, and generally, a simple summation is performed on signals with different frequency content. Demultiplexing is done using filter banks. In the embodiment of FIG. 3 that uses an HF transformer through an HF hybrid circuit, different frequencies are used for control and status, so that the echo from one end does not cause confusion on the other end.

In DAA as shown in Figures 6 and 7, the preferred embodiment uses a digital multiplexing scheme. The digital multiplexing scheme can provide two functions: 1) a framing mechanism and 2) a sigma-delta clock recovery mechanism. In order to simplify the clock recovery, the bit rate of the multiplexed data is an integer multiple of the sigma-delta clock frequency. In a preferred embodiment, the bit rate of the multiplexing is twice the sigma-delta bit rate. This means that half of the bits in the multiplexed data stream can be used for control/status and framing. A simple way to allocate bits is to use every other bit for the digital signal, and allocate the remaining bits to framing and control/state.

Assume that a 1-bit digital signal is randomly distributed 1s and 0s without framing. Therefore, only the framing bits are needed to synchronize the status/control information. One framing method that can be used is to divide the status/control information into N groups of n bits. For framing, 0 is added before each group of n bits, and (n+1) 1s are added before N groups. The synchronization mechanism can find (n+1) 1s followed by 0s to detect the frame. This mode can only be used for a positioning of the frame; for status/control information, there is no chance to imitate this mode. It is possible for 1-bit digital signals to imitate this pattern, but due to the random nature of those signals, subsequent frames will not imitate the same pattern. Only the real frame synchronization bits will consistently match the pattern. The framing efficiency of this scheme can be calculated as n*N/[(n+1)*(N+1)].

To explain the concept of multiplexing and framing, it is assumed that the frame is based on 32 bits. Choose n=3 and N=3, so the frame is composed of 16-bit digital signal, 7-bit framing, and 9-bit status/control. The resulting bit stream is as follows:

DD=1 bit digital data stream [F0F1F2F3F4F5F6]=1111000Sn=nth state/control bit For the embodiment of the present invention according to FIG. 6, it is necessary to select different frame synchronization modes for the TX and RX directions, making it impossible for the echo signal Any kind of pattern appears in. Since the high frequency hybrid circuit combines the TX and RX signals, the demodulators 67 and 73 will see the signal echoes from the modulators 66 and 74, respectively. The hybrid circuit reduces the echo level, but cannot completely eliminate the echo. For example, in Figure 6, De-MUX 64 should not be locked to the combined digital signal from MUX 62; it should only be able to lock to the combined digital signal from MUX 78. It is also necessary to select the frame synchronization mode so that the binary inverse of the synchronization mode in one direction cannot appear in the other direction. This is because the polarity of the echo signal is uncertain. There are many possible frame synchronization mode pairs that meet these requirements. An example is shown below:

DD = 1 bit digital data stream For one direction, [F0F1F2F3F4F5F6] = 0000011 For the other direction, [F0F1F2F3F4F5F6] = 0100011Sn = nth status/control bit For these frame synchronization modes, the synchronization mechanism must check all 7 frame bits in 32 Correct positioning within the bit frame.

The above-mentioned digital multiplexing scheme can also be understood with reference to FIG. 9. In this figure, the multiplexer 91 has two inputs: one for the control or status bit 92 and one for the 1-bit digital signal 93. The first number of control or status bits are input to the framer 94, which combines them in a fixed framing pattern with a second number of bits to form a framer output 95. The framing mode is selected so that for all combinations of status or control bits, it uniquely indicates the correct framing position of the framing output bit. The framer output 95 and the 1-bit digital signal 93 are input to the interleaver 96. The interleaver 96 takes a third number of bits from the framer output 95, the third number being equal to the sum of the first and second numbers. The interleaver 96 interleaves the third number of bits with the corresponding proportional number of bits taken from the 1-bit digital signal input 93 to form a 1-bit digital output 97 having the fourth number.

In a preferred embodiment, the DAA shown in FIG. 6 uses two multiplexers, each of which has the function shown in FIG. 9. Those skilled in the art will understand that the HF hybrid circuit in this DAA will allow the demodulator at the same end of the HF transformer to see the echo output by the modulator. In normal operation, the signal from the modulator at the other end of the HF transformer is stronger than the echo signal from the modulator at the same end of the HF transformer, so the demodulator decodes the correct bits. However, if the circuit at the other end of the HF transformer is not energized, the demodulator will only see the echo signal from the modulator at the same end of the HF transformer. In this case, it is possible for the demultiplexer to see the bits from the multiplexer at the same end of the HF transformer instead of the desired bits from the other end. For this reason, the preferred embodiment uses a different framing mode for each multiplexer, so that for any combination of control or status bits, the framing mode on each end can prevent the multiplexer output from generating the other end. Frame mode. In addition, since the echo signal may be inverted, each framing mode can prevent the multiplexer output from generating a logical inversion of the framing mode at the other end.

The hybrid circuit in FIG. 5 shows the high frequency hybrid circuit that can be used in FIGS. 3 and 6. FIG. 5 shows two high frequency hybrid circuits 52 and 54, each of which can be at either end of the HF transformer 51. The structure of the hybrid circuit is the same as the structure of the low-frequency hybrid circuit in the DAA based on the traditional voice band transformer as shown in FIG. 1. The main difference is that the values of the impedance elements Z7-Z12 and Z13-Z18 used in the high-frequency hybrid circuit are generally different from the values of the impedance elements Z1-Z6 used in the low-frequency hybrid circuit. The design of the hybrid circuit is well known in the prior art.

Power supply for the line-side circuit In a preferred embodiment, the line-side circuit obtains its power source entirely from the telephone network. However, the present invention can also be used in a structure in which the line end does not have a power source, such as in the case of ADSL DAA, or the power source is insufficient. In these cases, additional power can be added to power the line-side circuit. For example, the circuit shown in Figure 8 can be used to supply power from the system side to the line side. In this circuit, the clock/power signal generator 81 on the system side generates a periodic power signal on the line 82, which is connected to the transformer 83. At the line end, the output signal of the transformer on line 84 can be capacitively coupled to the clock input of the modulator and/or demodulator. The output signal of the transformer is also adjusted by the half-wave rectifier circuit to provide stable power on the output line 87. The half-wave rectifier circuit is composed of a diode 85 and a capacitor 86. The output line 87 can be connected to the power supply (Vcc ) And ground (GND) input. The power supply circuit shown in FIG. 8 is a well-known content in the prior art.

Although the above content has been shown and described with reference to specific embodiments of the present invention, those skilled in the art should understand that various changes can be made to these embodiments without departing from the principle and essence of the present invention. The scope of the invention is defined by the appended claims.