Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-05-08
2001-06-05
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C330S308000, C327S514000
Reexamination Certificate
active
06242732
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to an optical receiver. More specifically, the invention relates to a control loop to be used in an optical receiver, wherewith the dynamic range of the input amplifier is widened.
BACKGROUND OF THE INVENTION
An optical transmission system comprises an optical transmitter converting an electrical signal to be transmitted into optical form, (b) an optical fibre serving as a conductor for the optical signal, and (c) an optical receiver detecting the transmitted optical signal and converting it into electrical form.
A typical optical receiver comprises a photo detector connected to the input of a transimpedance amplifier. The photo detector converts the optical signal it has received into electric current that is supplied to the transimpedance amplifier. The latter generates at its output a voltage proportional to the incoming current, and thus a voltage proportional to the current of the photo detector is obtained from the output of the amplifier. The photo detector is usually either an avalanche photo diode (APD) or an optical PIN diode. Avalanche photo diodes are typically used at shorter wavelengths and optical PIN diodes at longer wavelengths, at which avalanche photo diodes generate a considerable quantity of noise. Transimpedance amplifiers are used for example for the reason that they allow comparatively good sensitivity properties to be achieved with a relatively simple construction.
Notwithstanding transimpedance amplifiers, one problem in optical receiver solutions lies in their poor dynamics: good sensitivity often entails a poor power tolerance and a good power tolerance again poor sensitivity. Poor dynamics for their part impair the operational flexibility of the receiver; for example when beginning to use a shorter fibre, an extra attenuator is necessary between the transmitter and the receiver.
Since the power level of the optical signal arriving at the receiver can in practice vary a great deal (depending on how long fibre is used), automatic gain control (AGC) is typically used in connection with the transimpedance amplifier to keep the amplifier's output voltage essentially at a constant value, when the incoming signal is higher than a predetermined threshold value.
There are in practice two different principles for widening the dynamic range of the receiver.
The first way of increasing dynamics is to adjust the resistance of the feedback resistor of the transimpedance amplifier on the basis of the amplitude of the incoming signal. The drawback of this method is that the adjustment generates large parasitics at the input of the amplifier, which will again impair the sensitivity of the receiver. The impairment is particularly marked when discrete components are used. When good sensitivity is aimed at, the stray capacitances on the input point of the amplifier are significant; even a small capacitance will impair the sensitivity of the receiver. Hence, it is essential that the parasitics on the input of the amplifier can be minimized.
Another way to widen the dynamic range is to use an adjustable resistive element in front of the amplifier. The resistance of the element is adjusted in response to the strength of the signal arriving at the amplifier in such a way that at higher levels the resistance is diminished, as a result of which the current coupled to the input of the amplifier will diminish (part of the current passes through the resistive element) and the amplifier is not saturated. This basic solution is known in several different variations. The main drawbacks of these solutions include complex implementation and stray capacitances generated by the requisite additional components, which will impair the sensitivity of the receiver.
SUMMARY OF THE INVENTION
It is an object of the invention to eliminate the drawbacks described above and to provide a solution wherewith the dynamic range of the receiver can be widened in such a way that the circuit solution will remain as simple as possible and furthermore in such a way that impairment of receiver sensitivity is minimal.
This object is achieved with the solution defined in the independent claims.
The idea of the invention is to use a differential input stage in the transimpedance receiver and to reflect the direct voltage dependent on the optical signal input power onto the input of said input stage (instead of its output), thus enabling the linear dynamic range of the output of the transimpedance amplifier to be increased correspondingly.
The solution in accordance with the invention offers a very simple way of increasing the dynamics without any appreciable impairment in the sensitivity of the receiver.
An additional advantage of the solution in accordance with the invention is that conventional inexpensive components can be used in the circuitry, as the solution does not require any peak-value detecting circuits, for example, whose implementation at high frequencies requires special components.
REFERENCES:
patent: 4415803 (1983-11-01), Muoi
patent: 5030925 (1991-07-01), Taylor
patent: 5606277 (1997-02-01), Feliz
patent: 42 12 934 (1993-10-01), None
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patent: 0 451 748 (1991-10-01), None
patent: 0 514 592 (1992-11-01), None
patent: 0 666 657 (1995-08-01), None
patent: 2 247 798 (1992-03-01), None
May 1999, International Search Report for PCT/FI98/00890.
Altera Law Group LLC
Lee John R.
Nokia Networks Oy
Pyo Kevin
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