JPH04248727A - Transmission output level control circuit - Google Patents

Abstract

PURPOSE:To prevent production of an excess output level by using a transmission output level control circuit of a slave equipment so as to correct nonlinearity without use of an analog circuit. CONSTITUTION:An A/D converter 41 converts a reception signal level of a transmission output level control circuit of a slave equipment into a digital signal and the digital signal is used for an address input of a memory 42. A data of a transmission output level inversely proportional to the reception signal level stored in the memory 42 is inputted to a D/A converter 43, in which the data is DA-converted and the transmission level of the optical transmission circuit is decided by the analog output.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION The present invention is applied to a transmission circuit of a transmission device provided in a slave device of a 1:n point/multipoint type transmission system. The present invention relates to a circuit that controls the signal output level of the transmission device so that the level of the signal reaching the main device is constant in accordance with the received signal level.

0002

2. Description of the Related Art FIG. 2 shows an example of the configuration of a 1:n point multipoint transmission system in which one main device and a plurality of slave devices are connected through optical fiber transmission lines. In this example, n=
It is set at 3. Here, numeral 1 is a main device, numerals 2, 3 and 4 are slave devices, numerals 5 and 6 are star couplers, and numerals 7, 8, 9, 10 and 11 are optical fibers.

The main device 1 and each of the slave devices 2 to 4 perform bidirectional signal transmission using a time division frame consisting of a plurality of time slots. The downlink frame signal transmitted by the main device 1 is
Distributed to all slave devices 2-4. Each slave device 2 to 4 is
Signals within required time slots on the downlink frame signal are processed as valid information. The frame signals transmitted by slave devices 2 to 4 reach the main device 1 as a sum, so if multiple slave devices transmit signals at the same time in the same time slot, the signals will collide and normal transmission will not occur. . In order to avoid this, the slave devices 2 to 4 operate to transmit signals only in different time slots and not to transmit signals in other time slots.

[0004] The transmission loss between the slave device and the master device differs depending on the slave device. This transmission loss is calculated by the number of star couplers passing through,
It is determined by the coupling ratio of the star coupler, excess loss, optical fiber loss, etc. When the upstream and downstream transmission paths have the same configuration, the upstream and downstream transmission losses between one slave device and the main device are approximately equal. The difference between the two arises due to wavelength characteristics of optical fiber loss, for example, when uplink transmission and downlink transmission are realized by wavelength multiplexing. However, when the transmission distance is short and the number of branches is large, the ratio of the difference in optical fiber loss to the total transmission loss is small and can be ignored. Transmission distance 10K
m, 10 branches, 1.3 μm and 1.5 μm
In the case of wavelength division multiplexing transmission, the ratio is on the order of several percent at most.

FIG. 3 shows the state of signal attenuation between the master device and the slave device. Here, main device 1 and slave device 2
The transmission loss between the main device 1 and the slave device 3 was set to be large, the transmission loss between the main device 1 and the slave device 4 was set to be medium, and the transmission loss between the main device 1 and the slave device 4 was set to be small. In the case of uplink transmission, slave device 2 sends a signal to time slot A, slave device 3 sends a signal to time slot B, and slave device 4 sends a signal to time slot C. In the case of downlink transmission, the slave device is shown in Figure 3.
As shown in (a), signals each having a temporally constant amplitude are received. Slave devices 2 to 4 are AGC (automatic gain control A
Automatic Gain Controller)
The input signal is amplified to an amplitude suitable for identification. AG
C varies the amplification factor by the time average of the amplitude of the input signal. On the other hand, in the case of uplink transmission, the main device receives signals with temporally different amplitudes, as shown in FIG. 3(b). Therefore, the main device cannot amplify the signals of all time slots to optimal amplitudes using AGC. In this case, as a result of reducing the amplification factor so that the amplifier does not become saturated with respect to the received signal with the largest amplitude, a problem arises in which the received signal with a small amplitude is not amplified to a size suitable for identification. Variations in the amplitude of the received signal are determined by the length of the optical fiber, the number of optical star couplers inserted, the characteristics of the optical star couplers, etc. For example, if one 1:8 optical star coupler is inserted in the middle 9 dB if there are two slave devices inserted, other things being equal
An amplitude difference occurs. As a result, the identification sensitivity of the main device is approximately 9
dB deterioration.

A known method for solving this problem is to control the output level of the transmission signal of the slave device.

The configuration shown in FIG. 4 is an example of the configuration of a slave device in this conventional method. Reference numeral 21 is a multiplexer/demultiplexer for optical signals between the main device, 22 is an optical receiving circuit, 23 is an optical transmitting circuit,
Reference numeral 24 is a transmission output level control circuit according to the prior art, and reference numeral 25 is a signal processing circuit.

[0008] Its operation will be explained. A signal from the main device 1 is sent to an optical receiving circuit 22 by a multiplexer/demultiplexer 21 . The optical receiving circuit 22 outputs the identified information to the signal processing circuit 25 and outputs the amplitude of the input signal to the transmission output level control circuit 24. The transmission output level control circuit 24 determines the amplitude of the transmission output signal based on the amplitude of the input signal. Optical transmitter circuit 2
3 transmits the information generated by the signal processing circuit 25 at an amplitude determined by the transmission output level control circuit 24. The signal is sent to the main device 1 via the multiplexing/demultiplexing circuit 21.

FIG. 5 shows an optical receiving circuit 22, an optical transmitting circuit 23,
It shows details of the transmission output level control circuit 24. The input signal is converted into an electric signal by the light receiving element 31, amplified by the preamplifier 32, sent to a circuit for signal identification, and inputted to the amplitude detector 33 in the transmission output level control circuit 24. The amplitude detector 33 is constituted by a low-pass filter, a charging circuit, etc., and detects the average amplitude, peak amplitude, etc. of the input signal. The signal voltage output from the amplitude detector 33 is input to an analog divider 34, and the analog divider 34 converts the reference voltage to the amplitude detector 3.
The voltage divided by the signal voltage from 3 is output, and a voltage inversely proportional to the amplitude of the input signal is output. The output signal of this analog divider 34 is input to an amplitude setter 35 that sets the optical transmission output level of the optical transmission circuit 23, and an optical signal of the set level is output from the light emitting element 36 by the output of this amplitude setter 35. be done.

By configuring the transmission output level control circuit of the slave device in this way, the light reception level at the main device can be kept constant regardless of optical loss to the slave device. That is, the state of the upstream transmission signal shown in FIG. 3(b) is as shown in FIG. 6. Since the input amplitude of slave device 2 is small, the output amplitude is large. Since the input amplitude of the slave device 3 is medium, the output amplitude is also medium. Since the input amplitude of slave device 4 is large, the output amplitude is small. Since the relationship between the input amplitude and the output amplitude is thus controlled to be inversely proportional, the amplitude of each time slot received by the main device 1 is constant. Therefore, the signals of all time slots can be amplified to the optimal amplitude using an amplifier with the required amplification factor.
Deterioration of identification sensitivity can be prevented.

[0011]

However, since the conventional signal output level control circuit is constituted by an analog divider that utilizes the logarithm characteristic of a diode and the addition/subtraction characteristic of an operational amplifier, it has the following problems. (1) The optical/electrical conversion characteristics of the light-receiving element and the light-emitting element are nonlinear. Therefore, even in the case of an element with large nonlinearity, in order to equalize the amplitude of each time slot, it is necessary to correct the nonlinearity with a transmission output level control circuit. However, since the division characteristics of the analog divider depend on the characteristics of the diode elements, it is extremely difficult to change the division characteristics. (2) When there is no optical signal from the main device to the slave device, such as immediately after power is turned on, there is a risk that an excessive optical output level will be set due to the logarithm characteristic of the diode, damaging the light emitting element of the slave device.

An object of the present invention is to easily correct the nonlinearity of the optical/electrical conversion characteristics of the light receiving element and light emitting element of the slave device, and to improve the transmission output without setting an excessive optical output level when there is no optical signal input. The object of the present invention is to provide a level control circuit.

[0013]

[Means for Solving the Problems] The present invention is directed to a slave device of a communication system in which a plurality of slave devices are connected to a main device, and signals are transmitted and received using a time division multiplexing method between the main device and the slave devices. a transmission output level control circuit for controlling the transmission output level of each slave device so that the signal level from the plurality of slave devices received by the master device is at a constant level; A transmission output level control circuit comprising an amplitude detection means for detecting the level of a received signal, and a means for outputting an output value inversely proportional to the level of the reception signal and determining a transmission output level value of the slave device. The means for determining the level value includes an A/D converter that converts the input level of the detected received signal into a digital signal, and a memory that stores the transmitted output level value and uses the output of the A/D converter as an address input. The device is characterized by comprising a D/A converter that converts the output of the memory into an analog signal and outputs the analog signal.

[0014]

Operation: The amplitude of the received signal is input to the A/D converter, and its digital signal output is input as the address signal of the memory storing the transmission output level value of the slave device. The transmission output level value read from memory is D/
The signal is input to the A converter, converted into an analog signal, and input to the amplitude setter of the optical transmitter circuit. Thereby, the optical output level of the optical transmission circuit is output at a level inversely proportional to the received signal amplitude, so that the level of the received signal from each slave device received by the main device is controlled to be a substantially constant value.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which a plurality of slave devices 2 to 4 are connected to a main device 1 via optical fibers 7 to 11 via star couplers 5 and 6. is,
The main device 1 is provided in the slave devices 2 to 4 of a 1:n point/multipoint communication system in which signals are transmitted and received using a time division multiplexing method between the main device 1 and the slave devices 2 to 4.
Each of the slave devices 2 to 4 adjusts the transmission output level from the optical transmitter circuit 23 based on the received signal level from the main device 1 so that the signal level received from the plurality of slave devices 2 to 4 becomes a constant level. Transmission output level control circuit 40 that controls
, an amplitude detector 33 as an amplitude detection means for detecting the level of the received signal coming from the main device 1, and an output value inversely proportional to the level of the received signal to determine the transmission output level value of the slave device. The transmission output level control circuit 40 includes an A/D converter 41 for converting the detected input level of the received signal into a digital signal, and a transmission output level control circuit 40 as a means for determining the transmission output level value of the slave device. A memory 42 that stores output level values and uses the output of the A/D converter 41 as an address input, and a D/D converter that converts the output of this memory 42 into an analog signal and outputs it.
A converter 43.

Next, the operation of this embodiment will be explained.

The main operation is the same as that of the transmission output level control circuit according to the prior art, but differs in the following points. That is, the transmission output level control circuit 40 of this embodiment inputs the output of the amplitude detector 33 to the A/D converter 41, and
The output of the /D converter 41 is input as an address signal to the memory 42. Then, the data stored at the address is input to the D/A converter 43, and the converted output is used as the amplitude of the signal to be transmitted. The memory 42 stores data of transmission output signal level values such that the input signal level and the output signal level are inversely proportional. Therefore, the level of the transmitted signal can be made inversely proportional to the amplitude of the received signal by the output read out and converted into an analog signal.

Specific examples of this data are shown in Tables 1 to 4 below.
This is explained by

Table 1 shows the relationship between the amount of propagation between the main device and the slave device, the input signal amplitude of the slave device, and the output value of the A/D converter 41 of the slave device. This relationship is linear. Table 2 shows the relationship between address values of the memory 42 and data. Their relationship is inversely proportional. Table 3 shows D
3 shows the relationship between the input value of the /A converter 43 and the transmission output signal amplitude of the slave device. Their relationship is linear. Table 4 shows the relationship between the resulting propagation amount between the main device and the slave device and the output signal amplitude of the main device. All signal amplitudes are shown as relative values with the output of the main device as "1".

[Table 1]

[Table 2]

[Table 3]

[Table 4]

When the amount of propagation between the main device and the slave device is 1/15, the amplitude of the input signal to the slave device is 1/15, and A
The output of the /D converter 41 is "0001". The data corresponding to that address is "1111", and the amplitude of the transmission output signal of the slave device is "15". Since the amount of propagation between the main device and the slave device is equal in both directions, the input amplitude of the main device is 15/15=1. Other propagation quantities are calculated in the same way, and as a result, the input amplitude of the main device becomes equal regardless of the propagation quantity. If the difference in propagation amount is 15 times, then 15 times without control.
Although an amplitude difference of twice as much occurs, in this embodiment, it is reduced to about twice. By increasing the data length of the memory 42, the amplitude difference can be made to the required precision.

As described above, in this embodiment, since the relationship between the input amplitude and the output amplitude is stored in the memory, it is easy to change the relationship. Furthermore, since the nonlinearity of the optical/electrical conversion characteristics of the light-receiving element and the light-emitting element is determined by the selection of the element, the nonlinearity of the element can be equalized by storing data obtained by converting the characteristics in advance in a memory.

Further, by setting the memory data corresponding to no input to "0", it is possible to avoid setting an excessive optical output level without an additional circuit. Further, as the memory 42, ROM, RAM, or associative memory can be used.

Note that the present invention is applicable not only to communications in point/multipoint transmission systems using optical fibers, but also to wireless transmission lines or wired transmission lines.

[0025]

[Effects of the Invention] As explained above, since the relationship between the input amplitude and the output amplitude is stored in the memory, it is possible to store the converted data of the characteristics of the optical/electrical conversion element in the memory in advance. , the nonlinearity of the element can be equalized. Further, by setting the memory data corresponding to no input to "0", it is possible to avoid setting an excessive optical output level without an additional circuit.

Depending on the number of slave devices that can be accommodated, the main device may be of two types: a small output/low sensitivity type and a high output/high sensitivity type. The former uses an LED as a light emitting element and a PIN-PD as a light receiving element, and the latter uses an LD as a light emitting element and an APD as a light receiving element. In the latter case, since the input level is high, the slave device operates to reduce the output level. That is, the slave device to which the present invention is applied has the effect of automatically adapting to the type of the master device. LE is used as a slave device due to its low cost and ease of burst transmission.
When D and PIN-PD are adopted, the main device of the former is 10
The latter main device can accommodate about 100 slave devices. This is the case for single-mode optical fiber, star wiring, and a signal speed of 150 Mb/s.

[Brief explanation of the drawing]

FIG. 1 shows a transmission output level control circuit according to an embodiment of the present invention.

[Figure 2] Example of point/multipoint type transmission system configuration.

FIG. 3 is a diagram illustrating the flow of signals in a point/multipoint transmission system.

FIG. 4: An example of the configuration of a slave device in a conventional example.

FIG. 5 is an example of a conventional transmission output level control circuit configuration.

FIG. 6 is a diagram illustrating the flow of signals in a point/multipoint transmission system when signal output level control is performed.

[Explanation of symbols]

1 Main device 2, 3, 4 Slave device 5, 6 Star coupler 7, 8, 9, 10, 11 Optical fiber 21 Multiplexer/demultiplexer 22 Optical receiving circuit 23 Optical transmitting circuit 24, 40 Transmitting output level control circuit 25 Signal processing Circuit 31 Light receiving element 32 Preamplifier 33 Amplitude detector 34 Analog divider 35 Amplitude setter 36 Light emitting element 41 A/D converter 42 Memory 43 D/A converter

Claims (1)

[Claims] Claim 1: Provided in a slave device of a communication system in which a plurality of slave devices are connected to a main device, and signals are transmitted and received using a time division multiplexing method between the main device and the slave devices, the signal is received by the main device. A transmission output level control circuit that controls the transmission output level of each slave device so that the signal level from the plurality of slave devices becomes a constant level, and an amplitude control circuit that detects the level of the received signal arriving from the main device. In a transmission output level control circuit comprising a detection means and a means for outputting an output value inversely proportional to the level of a received signal and determining a transmission output level value of the slave device, the means for determining the transmission output level value comprises: An A/D converter that converts the input level of the detected received signal into a digital signal, a memory that stores the transmitted output level value and uses the output of the A/D converter as an address input, and converts the output of this memory into an analog signal. A transmission output level control circuit comprising a D/A converter that converts and outputs the data.