GB2161665A - Optical fibre modem - Google Patents

Abstract

An optical fibre modem comprises a light emitting diode transmitter (11) and a photodiode receiver (12) both energized by a power supply (13-20) arranged to draw required power solely from the electrical data transmit line. The photodiode in the receiver (12) is connected with substantially zero bias to minimize dark current flow and hence temperature dependent noise. A switch mode supply comprising a single stage regulator (17) driven by a CMOS oscillator (18) is used for the power supply. The modem described in the preferred embodiment meets the EIA RS 232-C and CCITT V24 standards. <IMAGE>

Description

SPECIFICATION Optical Fibre Modem This invention relates to optical fibre modems.

Optical fibre links are commonly used for computer communication and industrial control applications where communication by means of an electrical cable is unsuitable due to problems such as susceptibility to lightning strikes, creation of ground loops, and electrical interference. Known optical fibre modems, have a relatively high power consumption and one known type is energized by power supplied from an external source. In some applications it is desirable to provide an optical fibre modem which will plug directly into the output port of various apparatus and function without an external power supply. Previously optical fibre modems suitable for this application function without an external power supply by obtaining power from the outer port of the apparatus either via a special connection or by drawing power from control lines additional to the data transmit line.

These devices suffer from the disadvantage that not all apparatus to which optical fibre modems might be connected have suitable connections to provide power to energize the modem. In particular, some apparatus to which optical fibre modems can be connected have as little as three connections, these being a data transmit, a data receive and a ground connection. The prior art optical fibre modems are not suitable for use with such apparatus and are limited to certain apparatus which either have appropriate connections to supply power to energize the modem or have been suitably modified.

It is an object of the invention to provide an optical fibre modem which will overcome, or at least ameliorate, the above disadvantages.

This invention provides an optical fibre modem comprising transmitter means to convert a received electrical data signal to an optical signal; receiver means to convert a received optical data signal to an electrical signal; and power supply means adapted to energize said transmitter means and said receiver means by power drawn solely from said received electrical data signal.

Preferably, the power supply means comprises a switch mode supply. The switch mode supply is driven by a CMOS oscillator to provide a low power consumption.

Preferably the switch mode supply is a single stage regulator and in the preferred embodiment the power supply circuit comprises a switching transistor series connected with a transformer primary winding. The primary winding of the transformer is supplied with a full wave rectified voltage drawn from the received electrical data signal.

For preference, the receiver means comprises a photodiode connected wihh a substantially zero bias to minimize dark current flow in the photodiode. In other embodiments the receiver modems can comprise a photodiode supplied with from +0.05 volts forward bias to -1.0 volts reverse bias.

When the photodiode is connected with substantially zero bias the receiver means further comprises a fixed threshold detecting means to detect voltages generated by the photodiode when an optical signal is received.

One embodiment of the invention designed to meet the Electronics Industry Association (EIA) RS232C or International Telegraph and Telephone Communications Committee (CCITT) V24 standard, will now be described by way of example only, with reference to the accompanying drawings in which: Figure lisa circuit diagram of an optical fibre modem according to the invention.

Figure 2 is a circuit diagram of the power supply portion of the modem shown in Figure 1.

Figure 3 is a circuit diagram of the transmitter circuit of the modem shown in Figure 1 and Figure 4 is a circuit diagram of the receiver circuit of the modem shown in Figure 1.

As shown in Figure 1 the optical fibre modem of the preferred embodiment comprises a transmitter means 11 to convert a received electrical data signal to an optical signal and a receiver means 12 to convert a received optical data signal to an electrical signal. The transmitter means 11 and receiver means 12 are energised by a power supply means comprising substantially the remainder of Figure 1 and more clearly shown in Figure 2.

As shown in Figure 2 the power supply means is a switch mode supply and the circuit comprises a full wave bridge rectifier 13 which can be supplied with power from data transmit connection TxD to supply a voltage to a filter 14. It will be noted that various other connections to the input of filter 14 are shown in Figure 2. These connections allow power to be alternatively drawn from RTS (request to send), CTS (clear to send), DTR (data terminal ready), DSR (data set ready) connections or even a POWER connection provided on apparatus with which the modem is used. These conditions are provided to give additional flexibility of operation to the modem and do not form part of the invention.

Filter 14 supplies a voltage VS to the primary winding 15 of a transformer 16 and current flow in primary winding 15 is controlled by a single stage regulator comprising a series connected switching transistor 17 driven by an oscillator 18. The oscillator 18 comprises a CMOS chip and the oscillator frequency is 25kHz. The inductance of primary winding 15 is chosen so that no saturation occurs at 25Khz with a 8mA current and so that inductor voltage cannot fall to zero. An inductance of 30mH is used and the primary winding has 164 turns.

Transformer 16 has two secondary windings 19 and 20 which respectively provide power supply from transmitter 11 and receiver 12. The desired voltage across winding 19 is 2.2 volts which requires 57 turns while winding 20 is required to produce +/- 5.5 volts so that two segments each of 120 turns are required. A wire size of 0.13mm is used on a p18/11 size bobbin. The supply voltages produced in windings 19,20 are smoothed in a substantially conventional manner to produce a transmitter supply VT+ and a receiver supply V+, V-.

Figure 3 shows the circuit of the transmitter means 11. Optical signals are generated by light emitting diode (LED) 21 which is optically coupled with an optical fibre (not shown) in the known manner. The operation of LED 21 is controlled by a series connected driving transistor V2 which causes current to flow in LED 21 in response to electrical signals received via data transmit connection TxD.

The single stage driving transistor is sufficient to allow the modem to operate at the transmission rates set by the RS232-C standard.

Figure 4 shows the circuit configuration of receiver means 12. A photodiode 22 is optically coupled with an optical fibre (not shown) in the conventional manner to receive optical data signals.

Photodiode 22 is connected with zero bias so that there is no temperature dependent dark current flow. The use of zero orvery low bias photodiode 22 is possible because at the low transmission rates required by the RS232-C standard the rise time of photodiode is sufficient at zero bias or forward bias less than +0.05 volts. The dark current of photodiode 22 is sufficiently small to allow operation of the modem if it is connected with from +0.05 volts forward bias to -1.0 volts reverse bias.

The receiver 12 further comprises an amplifier 23 and a comparator 24. The operation of these components is substantially conventional and is further explained in the following example of choice of major circuit component values.

EXAMPLE The following is a calculation of the power consumption and exemplary major component values of the preferred embodiment as illustrated in Figures 1 to 4.

Assuming a +/- 12 V supply on the transmitting device, the following power distribution is required.

The modem supply is +1-8 Vat 8 mA. The transmit data line is first rectified to allow for all data patterns. Referring to Figure 2, the full-wave bridge 13 loses 1.4 V across the diodes, leaving 6.6 V (VS±VS-) at 8 mAto be converted to supply the different modem sections. Calculations of the bandwidth and gain required for the DC coupled receiver indicate that -37 dBm (pk) is the lowest optical power than can be received. Allowing a 5 dB margin and a 3 dBlkm loss over a 2 km fibre, -26 dBm (pk) has to be launched. For 50/125 um fibre, this requires 10 mA average at 1.5 V for high power LED 21, plus 0.2 Vforthe switching transistor V2.

Allowing 0.5 V loss for half wave rectification, 10 mA at 2.2 V is needed for the Transmitter shown in Figure 3.

Turning now to the receiver circuit of Figure 4, for a 1 mA drive back into the RS-232-C receive line RxD, 1 mA at +5 V or -5 V is required. In addition to this is 0.3 mA at +5 V and 0.3 mA at -5 V is required forthe op-amp power supply. The power requirement ofthe receiver is thus 0.3 mA at 11 V and 1 mA at 5.5 V. Again 0.5 V is lost for each half-wave rectifier.

Allowing 1 mA to drive the oscillator for the switching supply, the following is evident:- Power In Power Dissipated 6.6x8 mA=52.8 mW 6.6x1 mA = 6.6mW 2.2x10 mA = 22.0 mW 11.0x0.3 mA= 3.3 mW 5.5x1 mA = 5.5mW Total Dissipation = 37.4 mW Allowing an 80% DC-DC power conversion efficiency at such small currents we have 52.8 mW-20% x 52.8 mW=42.24 mW for dissipation in the modem which is sufficient power for operation.

As shown in Figure 2 the supply current can be drawn off a number of lines, depending on their availability from the driving device. However, the above budget calculations are based on power from the TXD line only; any additional lines supplying power boost the transmitter optical power which can improve transmission performance.

It should be noted that if non-standard RS232 drivers without current-limited outputs are used, it is not advisable to use the transmit data line for the supply if any other of the lines are available, because the negative pulses of the TXD line will cause large currents to flow into it. This will cause no damage if the RS-232 drivers used have current limits.

A single stage switched regulator (DC-DC Converter) can provide the best efficiency at such small loads. A self-oscillating circuit requires too much current to saturate the core, so a driving oscillator of CMOS is used to limit the power dissipation.

The oscillator frequency is 25 kHz, which is above audio and low enough to prevent heavy core losses and losses due to CMOS switching. Recommended circuit values for the CMOS oscillator give R4 as 10xR5. This however causes larger current flow than if R4=1.5 R5. The hysteresis change causes faster transitions and limits the current flow. The transformer primary inductance is chosen so that no saturation occurs at 25 kHz and 8 mA and that the inductor voltage cannot fall to zero (i.e. the inductance is not too small).

The current in the primary, Ip=V/Lxt, where V is voltage on the primary winding 15, L is the inductance and t is the time the voltage is applied.

At 25 kHz, t=1 ,2x 1/25 kHz=20 microsec.

Thus with Ip maximum=8 mA at V=6.6 V, Lmin=6.6/8mAx20 us=16.5 mH To allow for larger voltages of up to 12 Vthe inductance is set to 30 mH. Using an H5A type pot core for low loss, the primary winding can comprise 164 turns.

Secondary winding 19 is 2.2 V thus requiring 57 turns.

Secondary winding 20 is +/- 5.5 V thus requiring two segments each of 120 turns.

The number of turns can be accommodated on a P 18/11 size bobbin using a wire size of 0.13 mm.

As discussed above the RS232-C standard requires a comparatively slow transmission speed (iess than 19.2 kbps). The transmitter Section is shown in Figure 3. Because of this low speed, and because such small currents are being switched, the driver for LED 21 need only be a single transistor stage V2. The power supply VT+ for the transmitter is set to a low voltage (2.2 V) so as to minimize power wastage in components other than the LED 21.

Referring to the receiver shown in Figure 4, a high impedance dual op-amp or dual op-amp/ comparator is required, with extremely low quiescent current. This minimises power consumption and also aids in reducing the physical size of the modem. Dual op-amps or dual op-amp/ comparators with characteristics similar to the LM442A fit this description. Again, because of the low transmission speeds, very low or no bias at all may be used for the photodiode as the rise-time is sufficient with no bias. Also, there is no temperature dependent dark current flow at zero bias keeping the DC and AC noise currents down so that small signals may be received over a wide temperature range. The non-inverting terminal of amplifier V1A is connected to centre tap (ground) giving a zero output for no input optical powder.

In order to achieve a rise time so that little distortion occurs with different input powers and a fixed decision threshold, the bandwidth of the input stage is chosen to be approximately 10 times that normally required for the maximum baud rate of 19.2 kbaud, (9.6 kHzx10=96 kHz).

Assuming 0.4pF capacitance for the resistor (R1), 1 pF capacitance for an equalization stabilising capacitor (C1), and a frequency of 96 kHz as the 3dB bandwidth, the resistor value required is approximately 1.2 Mohm. The worst case offset and bias noise currents for the op-amp are approximately 0.15 nA DC at temperatures up to 50 degrees C. The minimum signal current should be at least 10 times this value for good Signal to DC Noise Ratio (SNR).

The 1.2 M ohm resistor gives a gain of 1.2 mV per nA. The sensitivity of the photodiode is 0.45 A/W.

The gain of the stage is thus:- 1.2 mV/nAxO.45 nA/nW=0.54 mV per nW input power.

The output voltage due to DC noise currents is thus: 1.2 mV/nAxO.lS nA=0.18 mV.

The following stage is a comparator. The input signal to the comparator should be 10 times the noise voltages at the input for good SNR. The input offset voltage is 1.25 mV maximum to 50 degrees C.

This offset noise voltage dominates the 0.18 mV DC noise associated with the first stage of the op-amp.

The minimum signal voltages should therefore be 12.5 mV which results from a 23.1 nW optical input power.

Thus the minimum input power required is 23.1 nW for proper reception of the signal. However, in order to limit circuitry, it is desirable that the second stage drive the RS-232-C receive line TXD, requiring a +/- 5 V signal. If the 2nd stage is an op-amp, the op-amp gain is typically limited to 100 so that the minimum input voltage to the comparator has to be 10/100=100 mV (pk). This corresponds to an input power of 185 nW or -37.3 dBm (pk). Improved receiver sensitivity is possible by using a dual op-amp/comparator with the comparator as the 2nd stage. In this case the comparator has much greater gain and as a result the receiver sensitivity is than limited only by the DC offset voltages and noise.

This corresponds to 23.1 nW optical input power as described above.

Allowing 11 dB margin for the input power requires that -26.3 dBm (pk) be launched from the LED. This requires a current of 20 mA (pk) which is acceptable (10 mA continuous load at the transformer secondary=20 mA pulsed at a 50-50 duty cycle).

The threshold voltage for the second stage is set at -75 mV to allow switching at the minimum power input. At the maximum power input, the voltage into the comparator stage is 1.27 V.

The specifications of the exemplary modem are: Power Requirements TXD line only +/-8 V&commat;8 mA (minimum) RTS or DTR +8 Vg8mA (minimum) External Supply +8to =+15 V DC (8 mA minimum) Using TXD line only from a 1488 (RS-232-C driver chip): Maximum transmitter -26 dBm (pk) power Current drawn off TXD 8 mA Minimum receiver power -37 dBm (pk) Fibre Size 50 micrometre core Date rate 0--19.2 kbps (transparent to all data patterns) Temperature Range 0--60 deg. C.

Claims (8)

1. An optical fibre modem comprising transmitter means to convert a received electrical data signal to an optical signal; receiver means to convert a received optical data signal to an electrical signal; and power supply means adapted to energize said transmitter means and said receiver means by power drawn solely from said received electrical data signal.

2. An optical fibre modem as claimed in claim 1 wherein said power supply means comprises a switch mode supply driven by a CMOS oscillator.

3. An optical fibre modem as claimed in claim 2 wherein said switch mode supply comprises a single stage regulator having a switching transistor connected with its conduction path in series with a transformer primary winding.

4. An optical fibre modem as claimed in claim 3 wherein said switch mode supply further comprises a full wave rectifier to rectify voltage drawn from said received electrical data signal.

5. An optical fibre modem as claimed in any one of claims 1 to 4 wherein said receiver means comprises a photodiode supplied with from +0.05 volts forward bias to -1.0 volts reverse bias.

6. An optical fibre modem as claimed in claim 5 wherein said diode is supplied with substantially zero bias.

7. An optical fibre modem as claimed in claim 6 wherein said receiver means further comprises fixed threshold detecting means to detect voltages generated by said photodiode in response to said received optical signal.

8. An optical fibre modem substantially as herein described with reference to the accompanying drawings.