JPS58143646A - Automatic gain controller - Google Patents

Abstract

PURPOSE:To stabilize the output level of a TV signal, by applying a photoelectric conversion to an optical TV signal which is negatively modulated when this signal is received to detect the clamping level, and then performing a gain control to fix the clamping level. CONSTITUTION:A received optical TV signal (a) is converted into an electric signal by an avalanche photodiode APD1. An inverted waveform (c) at an output point A of preamplifier 2 and applied to a clamping level detecting part 3 and an output amplifying part 4. The part 3 defines a synchronizing signal level, for example, as a clamping level and detects this level to hole it until the next synchronizing signal. This detected signal is compared with the reference voltage Vr through an expanding amplifier 5. Then the base of a transistor TRQ1 of a bias control part is controlled to control the bias of the APD1, and the amplification factor is controlled so as to secure a fixed clamping level. The part 3 detects V'C and controls the bias voltage although the peak levelreaches P2. As a result, the potential at the point A forms a curve (c) to the TV signal of a curve (b).

Description

[Detailed Description of the Invention] Technical Fees of the Invention The present invention provides an optical television signal transmission system in which the level of a synchronization signal, etc. is kept constant (copped) on the transmitting side and transmits an optical television signal which is subjected to negative R#. On the receiving side, the received optical power f-
≦: This relates to an intransitive S control method that provides control to obtain a stable output level regardless of the situation.

PRIOR ART AND PROBLEMS Direct Modulation Optical television signal transmission system - A known method is to clamp the level of a synchronizing signal, perform direct current regeneration, and thereby drive a light emitting element with negative modulation. In this method, the optical signal level increases at the black level of the television signal and decreases at the white level, and the average picture level (AFL) s: Differential gain (DG) and differential digit M ( It has the advantage that there is no change in DF), and there is less influence from cut noise (2).However, in order to stabilize the output level against fluctuations in received optical power (1);
Optical power detection A is an inexpensive automatic gain control (AGC) method.
The GC method cannot be used. This is because the optical power of the transmitted optical television signal changes due to the adoption of the clamp method.

Therefore, in the conventional AGC method based on pilot detection C:
Peak-to-peak detection (AGC) methods have been proposed, but they have the disadvantage of becoming expensive as the configuration becomes complex.

OBJECTS OF THE INVENTION It is an object of the present invention to detect a clamp level and control the gain to obtain a stable output level with an inexpensive configuration regardless of fluctuations in received optical power.

Structure of the Invention The present invention photoelectrically converts a received optical television signal, detects its clamp level, and controls a C2 gain control element so that the clamp level is constant. Example 6 below
Two points will be explained in detail.

Embodiment 111 of the invention is a block diagram of an embodiment of the invention.
is an avalanche photodiode (APD) that photoelectrically converts the received optical television signal, 2 is a single-place amplifier that converts current to voltage, Rf is a return resistor, 6 is a clamp level detector, 4 is an output amplifier, and OUT is an output amplifier. 5 is an output terminal, 5 is an expansion amplifier, Ql and Q2 are transistors forming a bias control section, Vc is a bias voltage, and Vr is a reference voltage. Since the avalanche photodiode 1 has a multiplication factor controlled by bias control, it performs the functions of photoelectric conversion and gain control.

In addition, the clamp level detection section 3 is configured to detect the synchronization signal level and hold it until the next synchronization signal, for example, when the synchronization signal level is set to the clamp level, and the detection signal is sent to the expansion amplifier via a resistor. Added to 5. The expansion amplifier 5 compares the detection signal with the reference voltage vr, adds a control signal to the pace of the transistor Q1 of the bias control section, controls the bias of the avalanche photodiode 1, and controls the multiplication factor so that the clamp level is constant. It is controlled as follows.

By adding the output of the avalanche photodiode 1 to the negative input terminal of the preamplifier 2 and grounding the positive input terminal, the potential at point A of the output terminal of the preamplifier 2 has a waveform that is an inversion of the waveform of the received optical television signal. becomes. The output signal of this preamplifier 2 is sent to a clamp level detector 5 and an output amplifier 4.
42 added.

The TV signal amplified by the output amplifier 4 is, for example, 75g.
It is output from the output terminal OUT via the resistor.

When the received optical television signal input to the avalanche photodiode 1 is shown by the curve G in FIG. 2, the potential at point A of the output terminal of the preamplifier 2 is shown by the curve C. Therefore, if the synchronization signal level of the television signal is a clamp level, the clamp level detection section 6 detects the potential Vc at point A of the curve C.

When the synchronization signal level, that is, the peak level of the received optical television signal is detected as shown in song 111g (curve C when Pt), the Vc of the potential at point A is detected as the clamp level, the optical power of the received optical television signal decreases and the curve It will be as shown below,
Assuming that the peak level reaches P2, if no AGC operation is performed, the potential at point A will be as shown by the dotted curve d1. Therefore, the clamp level detection section 6 detects Vl, the expansion amplifier 5 compares the detection signal with the reference voltage Vr, applies a control signal to the base of the transistor Q1 of the bias control section, and sends it to the avalanche photodiode 1 via the transistor Q2. By increasing the applied bias voltage, the multiplication factor of the avalanche photodiode 1 is increased. Thereby, the multiplication factor of the avalanche photodiode 1 is controlled so that Vt' z Vc, even for the received optical television signal shown in curve -.

The potential at point A of the output terminal of preamplifier 2 is as shown by curve C. That is, the avalanche photodiode 1, which is a gain control element, is
The multiplication factor of is controlled, and a television signal with a stabilized output level is output from the output terminal OUT.

FIG. 5 is a block diagram of another embodiment of the present invention, 1
1 is an avalanche photodiode (APD) that charges and converts received optical television signals, 12 is a preamplifier that converts current to voltage, and Rf is a feedback resistor.

13 is a clamp level detection section, 14 is a buffer amplifier,
15 is an expansion amplifier, Qll and Ql2 are transistors forming a bias control section, Vg is a bias voltage, Vr is a reference voltage, and Vo is an offset voltage.

In this embodiment, by applying an offset voltage VO to the positive input terminal of the preamplifier 12, it is possible to obtain an output signal from which unnecessary DC components have been removed. Output. Further, the potential of the negative input terminal of the preamplifier 12 via the buffer amplifier 14 and the potential of the output terminal are detected by the clamp level detection section 13 (
Add two. Therefore, the clamp level detection section 15 detects and holds the synchronizing signal level of the difference between the negative input terminal potential and the output terminal potential of the preamplifier 12 and applies it to the expansion amplifier 15. This expansion amplifier 15 and transistor Q
11. The bias control section consisting of Q12 operates similarly to the embodiment shown in FIG.
A television signal with a constant synchronizing signal level is output from the output terminal OUT by controlling the multiplication factor of 1.

FIG. 4 is a block diagram of still another embodiment of the present invention, in which a PIN diode 21 is used as the light receiving element, 22 is a 1ft JI amplifier, an Rf
is a feedback resistor, 23 is a clamp level detection section, 24 is an output amplifier, 25 is an expansion amplifier, 26 is a field effect transistor as a gain control element, Va is a '8° can pressure,
Vr is a reference voltage, and OUT is an output terminal.

The PIN diode 21 is similar to an avalanche photodiode; since the multiplication factor cannot be controlled by controlling the bias pressure, it is provided with a gain control element controlled by the output C2 of the expansion amplifier 25, and the clamp level The detection unit 26 detects a clamp level such as a synchronization signal level, controls the field effect transistor 26 as a gain control element so that the clamp level is constant, and outputs the output level from the output terminal OUT via the output amplifier 24. A stabilized TV signal is output.

In each of the above-mentioned embodiments, there are two cases where the synchronizing signal level is set as a clamp level and the optical television signal is sent from the transmitting side. Therefore, the clamp level detecting section 5
, 15, and 25 have a so-called peak level detection function for detecting the synchronization signal level. For example, when the pedestal level is set as a clamp level on the transmitting side, the clamp level detection unit 3, 15.23 (2) The pedestal level can be detected by subtracting the amplitude level of the synchronization signal from the peak level detected by the peak level detection function described above.

DETAILED DESCRIPTION OF THE INVENTION As described in detail, the present invention provides an optical television signal transmission system that clamps the level of a synchronization signal or the like on the transmitting side, drives a light emitting element with negative modulation, and transmits a negatively modulated optical television signal. (=In order to ensure that the clamp level detected by the clamp mill detection unit on the receiving side is constant 1:), the gain control element is controlled, and the output level of the photoelectrically replaced TV signal is stabilized with a simple configuration. Furthermore, when an avalanche photodiode whose multiplication factor can be controlled is used as the gain control element, it can also be used as a light receiving element.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one embodiment of the present invention, FIG. 2 is an explanatory diagram of operation, and FIGS. 3 and 4 are block diagrams of other embodiments of the present invention. 1.11 is an avalanche photodiode, 2, 12.
22 is a preamplifier, 5, 13.25 is a clamp level detection section, 4.24 is an output amplifier, 5, 15.25 is an expansion amplifier, Ql, Q2. Qll and Ql2 are transistors constituting a bias control section, 14 is a buffer amplifier, 21 is a PIN diode, 26 is a field effect transistor as a gain control element, and OUT is an output terminal. Patent author: Fujitsu Ltd. Attorney: Gobe Tamamushi (3 others) Figure 2 Figure 3

Claims (1)

[Claims] Optical television signal transmission system C clamps the synchronization signal level and other levels on the transmitting side, negatively modulates the light emitting element, and transmits the modified optical television signal. ; A frang level detection unit and a gain +1!1111 element are provided to charge and convert the optical television signal and detect the clamp level of the received and converted television signal, and the clamp level detected by the clamp level detection *1= An automatic gain control system characterized in that the gain element is controlled to a constant l2 to 5.