Broadband optical receiver with transformer coupled input circuit which is balanced to minimize stray capacitances

In an optical receiver an optical signal received from a glass fiber is fed to an optoelectrical converter. To reduce the influence of parasitic capacitances, the two connection terminals of the converter are connected to a common ground potential via respective impedance elements which are equal, and are also connected by a pair of tapped sub-transformers to the input terminals of a broadband amplifier of the receiver. As a result, a symmetrical signal voltage is developed across the converter and across the input terminals of the amplifier, while the influence of the parasitic capacitances is halved.

BACKGROUND OF THE INVENTION 
The invention relates to an optical transmission system comprising an 
optical transmitter coupled over an optical channel to an optical 
receiver, the optical receiver comprising an optoelectrical converter and 
an amplifier while a first connection terminal of the optoelectrical 
converter being coupled to a first input of the amplifier. 
The invention likewise relates to an optical receiver to be used in a 
system of this type. 
A transmission system as defined in the opening paragraph is known from the 
brochure "2610 B Broadband Photodiode Module", Ortel Corporation, May 
1991. 
Such a transmission system is used, for example, for TV signal distribution 
over large distances. This takes place, for example, in cable TV networks 
for TV signal transmission from a remote header station to a cable TV main 
distribution network. 
The cable TV signal to be transmitted, comprising a frequency multiplex of 
TV and radio signals, is used for amplitude modulation of a light source. 
The light source will generally comprise a semiconductor laser. The 
modulated light signal is sent over an optical channel, for example, a 
glass fibre, to the optical receiver. In the optical receiver the light 
signal is converted into an electric signal which can be further 
distributed in the cable network. 
The cable TV signal has a frequency range from .+-.80 MHz to 900 MHz. This 
means that the transmission system must to be suitable for broadband 
signals, meaning the aforesaid range. A requirement frequently made on 
such transmission systems is a passband which is flat within several 
tenths of dB's or increases with a certain slope with increasing 
frequency. 
Although the transfer characteristic of the prior-art optical transmission 
system is flat within 1 dB, a flatter transfer characteristic is desired. 
It is an object of the invention to provide an optical transmission system 
as defined in the opening paragraph in which the flatness of the transfer 
characteristic is improved. 
SUMMARY OF THE INVENTION 
For this purpose, the invention is characterized in that the first 
connection terminal of the optoelectrical converter is coupled to a 
reference potential via a first impedance element, and in that a second 
connection terminal of the optoelectrical converter is coupled to a second 
input of the amplifier and, across a second impedance element equal to the 
first impedance element, to the reference potential. 
By the measures according to the invention the electric signal is fully 
balanced with respect to the reference potential. The influence of 
parasitic capacitances between the connection terminals of the 
optoelectrical converter and the reference potential on the transfer 
characteristic is considerably reduced by this balancing. On the two 
connection terminals of the optoelectrical converter there are voltages 
available which have the same amplitude relative to the reference 
potential but are in phase-opposition relative thereto. As a result, the 
parasitic capacitances between the terminals of the electro-optical 
converter and the reference potential have an effect which is comparable 
to the effect of a capacitor connected between the two terminals of the 
optoelectrical converter, this capacitor having half the capacitance of 
the capacitor between either terminal of the optoelectrical converter and 
the reference potential. In the state-of-the-art transmission system in 
which the optoelectrical converter is connected unbalanced, said parasitic 
capacitance causes a resultant capacitance to occur between the terminals 
of the optoelectrical converter, which resultant capacitance is equal to 
this parasitic capacitance. 
The lower capacitance in the transmission system according to the invention 
reveals itself in a greater flatness of the transfer characteristic. In 
addition, the bandwidth of the transmission system according to the 
invention is considerably larger than the bandwidth of the prior-art 
system. 
An embodiment of the invention is characterized in that the optoelectrical 
converter is coupled to the amplifier via a transformer. 
A transformer used between the optoelectrical converter and the amplifier 
makes an optimum adaptation possible between the optoelectrical converter 
and the amplifier by appropriately selecting the transformation ratio. 
A further embodiment of the invention is characterized in that the 
transformer has primary and secondary windings which are partly common. 
The use of a common winding for both the primary winding and the secondary 
winding, results in a simple manufacture of the transformer. 
A further embodiment of the invention is characterized in that the 
transformer comprises two sub-transformers whose primary windings are 
connected in series and in which the first input of the amplifier is 
coupled to a tapping point of the primary winding of the first 
sub-transformer and the second input of the amplifier is connected to a 
tapping point of the primary winding of the second sub-transformer. 
By arranging the transformer as two identical sub-transformers whose 
primary windings are connected in series, in which the secondary winding 
of the transformers is located between two tapping points of the two 
sub-transformers, a fully symmetrical transformer is obtained in a simple 
manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the transmission system shown in FIG. 1 the signal to be transmitted is 
applied to an optical transmitter 2. An output of the optical transmitter 
2 is connected to an input of an optical receiver 6 over an optical 
channel in this case formed by a glass fibre 4. In the optical receiver 6 
the input is optically coupled to the optoelectrical converter in this 
case being a photodiode 8. 
A first connection terminal of the photodiode 8, i.e. the anode, is 
connected to a point of reference potential further to be referenced 
"earth". A second connection terminal of the photodiode 8, i.e. the 
cathode, is connected to a first connection terminal of a coil 10. A 
second connection terminal of the coil 10 is connected to a first 
connection terminal of a winding of a transformer 12. A second connection 
terminal of the winding of the transformer 12 is connected to earth. A 
parasitic capacitor 9 between the cathode of the photodiode 8 and earth is 
shown as a dotted line. 
A tapping point of the winding of the transformer 12 is connected to a 
first connection terminal of a primary winding of a transformer 14. A 
second connection terminal of the primary winding of the transformer 14 is 
connected to earth. A first connection terminal of a secondary winding of 
the transformer 14 is connected to a first input of an amplifier 16, 
whereas a second connection terminal of the secondary winding of 
transformer 14 is connected to a second input of the amplifier 16. The 
output of the optical receiver 6 is formed by the output of the amplifier 
16. 
The signal to be transmitted, for example, a frequency multiplex CATV 
signal, is converted into an intensity-modulated light signal by an 
electro-optical converter, usually by a laser. This modulated light signal 
is carried to the optical receiver 6 through the glass fibre 4. 
The photodiode 8 reconverts the received light signal into an electric 
signal. This electric signal is transferred to the amplifier 16 via the 
transformer 12 and the transformer 14. The parasitic capacitor 9 in 
parallel with the photodiode 8 largely determines the high-frequency 
behaviour of the optical receiver. The inductance value of the coil 10 is 
selected such that the transfer function shows a desired flatness. The 
transformer 12 is included for matching the output impedance of the diode 
8 to a load impedance of 75 .OMEGA.. 
The transformer 14 converts the asymmetric signal at the output of the 
transformer 12 into a symmetric signal which can be amplified to the 
desired level by the amplifier 16. The amplifier 16 may, for example, be 
of the BGY685 or BGY787 type marketed by Philips Semiconductors. 
In the optical receiver as shown in FIG. 2 the cathode of the photodiode 8 
is connected to a first connection terminal of a resistor 24, a first 
connection terminal of a coil 10 and a first connection terminal of a 
(parasitic) capacitor 9. A second connection terminal of the resistor 24 
is connected to a first connection terminal of a resistor 20 and to a 
first connection terminal of a resistor 22. A second connection terminal 
of the resistor 20 is connected to earth, whereas the second connection 
terminal of the resistor 22 is connected to a positive terminal of a 
voltage source 26. A negative terminal of the voltage source 26 is 
connected to earth. 
The anode of the photodiode 8 is connected to a first connection terminal 
of the resistor 30, a first connection terminal of a primary winding of a 
transformer 28, and a first connection terminal of a (parasitic) capacitor 
11. A second connection terminal of the resistor 30 is connected to a 
first connection terminal of a resistor 32 and to a first connecting point 
of a resistor 34. A second connection terminal of the resistor 34 is 
connected to a test point. A second connecting point of the capacitors 9 
and 11 is also connected to earth. 
A second connection terminal of the coil 10 is connected to a second 
connection terminal of the primary winding of the transformer 28. The 
connecting terminals of the secondary winding of the transformer 28 are 
connected to inputs of the amplifier 16, which amplifier supplies at its 
output the output signal of the optical receiver. 
The voltage source 26 combined with the voltage divider formed by the 
resistors 20 and 22 applies an inhibit voltage to the cathode of 
photodiode 8. By equalizing the impedance to earth seen at the cathode of 
the photodiode 8 and the impedance to earth seen at the anode of the 
photodiode it, is achieved that at the occurrence of an optical input 
signal the current produced by the photodiode 8 is converted into a 
voltage which is symmetrical relative to earth. 
The parasitic capacitors 9 and 11 may be considered to be replaced by a 
single capacitor having half the capacitance, because these capacitors are 
connected in series for the anti-symmetrical behaviour. Since these 
parasitic capacitors have a considerable effect on the high-frequency 
properties of the optical receiver, the reduction of the effective 
parasitic capacitance will significantly improve the high-frequency 
properties of the receiver in that the input section of the optical 
receiver is rendered symmetrical relative to earth. 
The transformer 28 provides that the output impedance of the optoelectrical 
converter is matched to the input impedance of the amplifier 16. The coil 
10, which may be adjustable, is used for obtaining as fiat a transfer 
characteristic as possible. The amplifier 16 may be of the BGY685 or 
BGY787 type. On the outputs of the amplifier 16 the output signal of the 
optical receiver is available. 
In FIG. 3 the transformer 28 is formed by two series-arranged 
subtransformers 40 and 42. These sub-transformers each have a secondary 
winding S which comprises a part (half in this case) of the primary 
winding P. This structure of the transformer 28 is advantageous in that 
the transformer 28 is fully symmetrical. 
FIG. 4 represents the mechanical structure of one of the sub-transformers 
shown in FIG. 3. The sub-transformer is constituted by a ring core 50 on 
which a bifilar winding, i.e. the windings 51 and 52, is applied. The two 
windings 51 and 52 are connected in series and the central tap is formed 
by the node between the windings 51 and 52. 
In the transformer shown in FIG. 5 the two sub-transformers are 
accommodated on a single core as it were. The advantage of this is that 
only a single core is needed. Since the two sub-windings have opposite 
directions of winding, a direct current flowing through the two 
sub-windings does not cause a premagnetization of the core to occur. Such 
a direct current is often present as a bias current for the transistors 
included in the amplifier 16. FIG. 6 shows the transfer characteristic of 
a state-of-the-art optical receiver. FIG. 6 distinctly shows that the 
transfer characteristic presents variations of more than 1 dB in the 
transfer band, the maximum frequency (-1 dB) being about 735 MHz. 
FIG. 7 shows the transfer characteristic of an optical receiver according 
to the invention. From FIG. 7 it appears that the transfer characteristic 
is flat within 0.5 dB inside the passband, whereas the maximum frequency 
(-1 dB) is now about 1 GHz.