GB2139398A - Signal Transmitting and/or Receiving Apparatus - Google Patents

Abstract

The apparatus comprises means for transmitting and/or receiving an optical electromagnetic signal of analogue form; means for transmitting and/or receiving such a signal of digital form; and means for switching between the digital and analogue signal transmitting/receiving means, and is primarily intended to demonstrate the properties and uses of optical fibres and signals, although it also has industrial application. The transmitter may comprise high and low impedance analogue inputs (3 and 4), digital inputs (11 to 13), a morse key (18) and an internal digital frequency generator, a digital or analogue signal being selected by a switch (I) and a signal input, or provided by, the transmitter being used to drive one or more LEDs (8 and 9). The output of an LED is passed down an optical fibre and input to receiving apparatus via a receiving diode. Apparatus is also provided comprising a source for directing optical electromagnetic radiation onto a flexible member, means for causing the latter to vibrate in response to an input signal, and optical fibre(s) for receiving radiation reflected from the flexible member so that a vibration- modulated signal directly proportional to the input signal is transmitted along the fibre(s). <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Signal Transmitting and/or Receiving Apparatus This invention relates to improvements in or relating to signal transmitting and/or receiving apparatus. The Apparatus finds particular application in the field of education in demonstrating the properties and uses of optical fibres and signals, but also has application in industry.

According to one aspect of the present invention, there is provided signal transmitting apparatus comprising: means for transmitting an optical electromagnetic signal of analogue form; means for transmitting an optical electromagnetic signal of digital form; and means for switching between the digital and analogue signal transmitting means.

According to another aspect of the present invention, there is provided signal receiving apparatus comprising: means for receiving an optical electromagnetic signal of analogue form; means for receiving an optical electromagnetic signal of digital form; and means for switching between the digital and analogue receiving means.

The invention also provides signal transmitting and receiving apparatus comprising: means for transmitting an optical electromagnetic signal of analogue form; means for transmitting an optical electromagnetic signal of digital form, means for switching between the digital and analogue signal transmitting means; means for receiving the analogue signal transmitted by the analogue signal transmitting means; means for receiving the digital signal transmitted by the digital signal transmitting means; and means for switching between the digital and analogue receiving means.

In one form of the invention, the signal transmitting apparatus comprises an output transducer for receiving an electrical input signal of analogue or digital form and delivering a corresponding optical electromagnetic radiation signal; a first means for accepting an analogue input signal and delivering an analogue electrical output signal suitable as an input signal for the output transducer; a second input means for accepting a digital input signal and delivering a digital electrical output signal suitable as an input signal for the output transducer; and signal switching means connected to the first and second input means and to the output transducer and selectively operable to supply either the analogue or digital electrical output signal to the output transducer.

In this form of the invention, the signal receiving apparatus comprises an input transducer for receiving an optical electromagnetic radiation signal of analogue or digital form and delivering a corresponding electrical input signal; a first output means for accepting an analogue electrical input signal and delivering a corresponding output signal suitable for operating an analogue device; a second output means for accepting a digital electrical input signal and delivering a corresponding output signal suitable for operating a digital device; and signal switching means connected to both the first and second output means and to the input transducer and selectively operable to supply the electrical input signal to either the first or second output means.

In a further aspect of the invention, there is provided signal transmitting apparatus comprising an electromagnetic radiation source for directing a beam of optical electromagnetic radiation onto a flexible member; means for causing the flexible member to vibrate in response to a signal input to the vibrating means; and one or more optical fibres for receiving radiation reflected from the flexible member so that, in use, an electromagnetic radiation signal modulated by the vibrations of the flexible member and directly proportional to the input signal is transmitted along the or each optical fibre.

For a better understanding of the present invention, and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic perspective view of signal transmitting apparatus embodying the invention; Figure 2 is a schematic perspective view of signal receiving apparatus embodying the invention; Figure 3 is a block diagram of the signal transmitting apparatus; Figure 4 is a block diagram of the signal receiving apparatus; Figure 5 shows a radiation signal source employing an earphone, a torch and an optical fibre cable.

Referring now to the drawings, Figures 1 to 4 show signal transmitting or receiving apparatus intended primarily for demonstrating the properties of optical fibres, in particular the use of optical fibres for carrying signals over relatively large distances.

As shown in Figures 1 and 3, the signal transmitting apparatus is able to transmit either analogue or digital signals, the type of signal required to be transmitted being selected by a switch 1. The receiving apparatus shown in Figures 2 and 4 is, of course, able to receive either digital or analogue signals and is provided with a switch 2 for selecting the type of signal to be received.

The transmitting apparatus is provided with both an a.c. coupled high impedance (20 Kiloohms) analogue input socket 3 and a low impedance (8) ohm) analogue input socket 4 in phase with the socket 3. As shown in Figure 3, the high impedance analogue input socket 3 is connected via a first amplifier 5 to a variable gain amplifier 6. The gain of the variable gain amplifier 6 is controlled by control knob 6a provided on a front panel 10 of the transmitting apparatus. The amplifier 6 has a range of 28 dB. The analogue voltage gain for a low impedance analogue input will be in the range 1 to 25 while the voltage gain for a high impedance analogue input will be in the range 4 to 100. The output of the variable gain amplifier 6 is connectable via the switch 1 to a variable current LED (light emitting diode) driver 7.For ease of understanding, the switch 1 is shown as a single pole double throw switch but is, in reality, a four-pole double throw switch. The LED driver 7 is arranged to drive a high radiance red LED 8, an infra-red LED 9 and an output indicator LED 22. The high radiance red LED 8 is mounted in a connector housing.

The signal transmitting apparatus also provides suitable interfaces for a number of different digital input signals. As shown in Figures 1 and 3, a TTL digital input socket or contact 11, a CMOS input socket or contact 12, and an RS232 input socket or contact 13 are provided. The appropriate input socket or contact 11, 12 or 13 is selected by connecting the output of a digital signal producing circuit between the selected input socket or contact 11, 12 or 13 and an earth socket or contact 14 and by selecting the correct interface by means of a selector knob 1 5 which is a single pole multi-position switch.The selector switch 1 5 also enables internal pseudo-random and square wave digital signal generators 1 6a and 1 6b (Figure 3) to be selected, the frequency of the signal being controlled by a control knob 17 provided on the front panel 10 to a desired value between 20 Hz and 4.5 kHz. A morse key 1 8 is also provided to enable a digital signal in the form of a morse code to be transmitted. To enable the morse key, the selector knob 1 5 must be turned to select the "TTL/contact/morse" position. The morse key overrides all other digital inputs.

The output of the selector switch 1 5 is input to a threshold detector and amplifier 1 9 which provides an input signal to the LED driver 7 via the switch 1 when the correct position of the switch is chosen.

The transmitting apparatus may be powered either by batteries or by an optional external DC supply. The transmitting apparatus requires a power supply voltage in the range 9 to 1 5V and typically draws 25 mA current. Thus, as shown in Figure 1, connections in the form of terminals or sockets 20'a and 20'b for connection to an external +9V to +1 5V d.c. supply and the supply ground, respectively, are provided. A 2.5 mm socket 20c' provides a connection for optional external power supply of +9V to + 1 5V D.C. which can be used as an alternative to the supply terminals 20'a and 20'b. As shown in Figure 1, the transmitting apparatus is also provided with a battery holder 20a for a PP3 type battery and a battery indicator 20b is provided to indicate the state of the battery.An on/off switch 20 is provided to select the battery power supply but does not control the external supply. The output power of the transmitter may be controlled by a control knob 21 which controls the current supplied by the LED driver 7.

The receiving apparatus is, as mentioned above, provided with a switch 2 for enabling it to receive either digital or analogue signals. The switch 2 is, for simplicity, shown as a one-pole double throw switch but is, in fact, a four-pole double throw switch. The receiving apparatus is, of course, provided with outputs corresponding to the inputs to the transmitting apparatus. Thus, both a low impedance (less than 1 ohm) output socket or contact 23 and a high impedance (1 kilo-ohm) output socket or contact 24 are provided for analogue output signals while for digital signals a TTL socket or contact 25, a CMOS socket or contact 26 and an RS232 socket or contact 27 are provided. The appropriate interface is selected by connecting the inputs of a digital circuit across the appropriate socket or contact and an earth socket or contact 28.

A signal transmitted by the LED's 8 and 9 is received by a transimpedance amplifier 29 connectable via the switch 2 to either a variable gain analogue amplifier 30 or a variable sensitivity digital comparator 31. When the receiver is set to receive digital signals, the circuitry is d.c. coupled, allowing detection of signals down to d.c. frequency. The gain of the amplifier 30 is controlled in the 30 dB range by operation of an analogue gain control knob 32 while the sensitivity of the .comparator 31 is controlled in the 32.4 dB range by operation of a digital threshold sensitivity control knob 33.

Closing of the contacts of the switch 2 to enable digital signal reception will cause the output of a digital signal indicator LED 34 to light corresponding to the state of the digital signal, while closing of the contacts enabling analogue signal reception will cause an analogue indicator LED 35 to light and to fluctuate in proportion to the input signal.

The low impedance analogue output may be connected to a loudspeaker 37 which is selected by a switch 39 while the output of the digital comparator 31 may also be input to a buzzer (not shown) by means of a switch 36. The switch 39 ensures that the loudspeaker 37 is disconnected when a connection is made to the low impedance socket 23.

The signal receiving apparatus is either battery powered or is powered by an external DC supply.

The receiving apparatus requires a voltage supply in the range 9 to 1 5V and typically draws a 25mA current. Thus, as shown in Figure 2, the receiving apparatus is provided with a battery holder 46 for a PP3 type battery, a battery condition indicator 47 and an ON/OFF switch 48 for the battery. The receiving apparatus also includes a ground supply terminal 49 and a connection 50 to an external supply of +9V to +1 5V D.C. A 2.5 mm socket 51 may be provided as an alternative to the positive supply terminal. A terminal 52 for voltage supply in the range to -1 5V drawing approximately 5mA current is provided for the RS232 interface.

A supply of optical fibre cables are provided with the signal transmitting and receiving apparatus. Each optical fibre cable is provided at one end with a connector for connection to a socket 8a of the transmitting apparatus to enable radiation from the high radiance red LED 8 to be input to the cable. The other end of each cable has a connector for connection to a receiving diode socket 29a of the receiver to ensure that the radiation output of the said other end is input to the amplifier 29. The apparatus may also be used without the optical fibres to demonstrate free space transmission of light waves, in which case the major part of the transmitted radiation emanates from the infra-red LED 9 which is mounted behind opening 9a.

The apparatus is primarily intended for educational purposes but it also has applications in industrial and operational environments. A number of examples of the use of the signal transmitting and receiving apparatus will now be given.

Analogue Signal Transmission When the transmitting apparatus is used in the analogue mode by setting the switch 1 to the analogue setting, the intensity of the radiation output from the LED's 8, 9 and 22 will be directly proportional to a voltage signal input to either the high or low impedance input 3 or 4.

To demonstrate the transmission of light waves in free space, first switch on both the transmitting and the receiving apparatus and select the analogue setting on the switches 1 and 2. The transmitter's output indicator 22 and red LED 8 and the receiver's analogue indicator 35 should now be lit. Then using a suitable FM radio (which may be supplied with the apparatus) tune in to a clear signal, preferably on the VHF band.

Connect the earphone output socket of the radio to the low impedance socket 4 and set the transmitter analogue gain control 6 to minimum by turning the knob 6a fully anti-clockwise.

Next, adjust the radio volume control until the output of the transmitter indicator LED 22 begins to vary in intensity, and then reduce the radio volume to the point where the intensity of the output from the LED 22 just becomes constant.

(This procedure ensures that the transmitter is giving out a signal with very little distortion).

Put the transmitter output power onto maximum, by turning the output power control knob 21 fully clockwise, and then turn the loudspeaker indicator switch 39 to the "on" position.

Place the receiver so that the receiver diode socket 29 is facing the emitting diode socket 8a or 9a of the transmitter, and adjust the analogue gain of the receiver using the control knob 32 until an adequate output is heard from the loudspeaker 37. The receiver and transmitter may be separated by a distance of a few metres while still maintaining transmission.

In this demonstration, the signal being transmitted comes mainly from the infrared light emitting diode. Thus can be demonstrated by placing a finger over the high radiance red diode and noting that the level of the received signal is hardly affected.

To show that infra-red radiation behaves in a very similar way to visible light, place the transmitter and receiver at right-angles, and position a mirror to reflect the radiation from the transmitter into the receiving diode so that sound will again be produced from the loudspeaker.

It is possible to transmit the optical signal over many hundreds of metres using a lens system. This is done by positioning a converging lens one focal length away from the transmitting diode (and thereby producing a parallel light beam), and accurately positioning another converging lens some distance along the light beam, and focussing the signal down onto the receiving diode (which is similarly one focal length away from the lens).

The above demonstration may be carried out using the output from the digital square wave generator as the transmitted signal.

In order to demonstrate the transmission of light waves along an optical fibre cable, connect the radio to the transmitter, and set up the signal levels as described above. Take a length of optical cable, and push fit the end connectors of the cable into the sockets at both the transmitter and receiver. Reduce the output power of the transmitter by turning the output power knob 21 anti-clockwise until the received signal sounds clear and undistorted. (This operation is carried out to ensure that the intensity of the ligh coming out of the fibre is not high enough to overload the receiver). Then adjust the analogue gain of the receiver using the control knob 32 for the most suitable loudspeaker sound level.

A good demonstration of the fact that the signal is really passing down the fibre can be given, using the set-up described above to turn both the transmitter output power and the receiver analogue gain to maximum by turning the knobs 21 and 32 fully clockwise, by connecting a short optical cable to the transmitter, and a longer one to the receiver, and positioning the free cable ends close to each other. The intensity of the loudspeaker output will vary with the positioning of the two free ends.

The output indicator LED 22 emits the same signal as the main LEDs 8 and 9, although at lower intensity. This can be demonstrated by setting up the transmitter and receiver for analogue transmission of the radio signal as described above and turning both the transmitter output power to maximum and the receiver analogue gain to maximum by turning both the control knob 21 and the control knob 32 full clockwise. A shorter length of optical fibre is then connected to the receiver, and the other end of the fibre positioned close to the transmitter output indicator. The radio signal will be heard from the loudspeaker 37.

The radio used above may be replaced by a microphone connected to the high impedance input socket 3.

In order to prevent any high-pitched whistle caused by feedback from the loudspeaker to the microphone, the loudspeaker and microphone should be separated as far as the optical and electrical cables permit, and the analogue gain of the receiver reduced until the whistling stops. The transmitter analogue gain should be at maximum.

Any analogue signal in the bandwidth of 25Hz to 25kHz may be transmitted. There are choices of high and low impedance sockets at both the transmitter and the receiver. Care must always be taken when transmitting analogue signals to ensure that: 1. The analogue gain in the transmitter is adjusted to ensure that the signal at the emitting diode 8 or 9 is not sufficiently high to distort the optical output through 'clipping'. If the signal is too high the output of the transmitter indicator LED 22 will fluctuate in intensity. The optimum gain of the transmitter is at a position just less than when this intensity fluctuation begins to occur. Distortion of the optical signal due to the LED response not being exactly linear may be decreased by further reducing the analogue gain.

2. The received optical power must not be high enough to overload the receiver circuitry. To set an acceptable level, first connect up the optical route, set the transmitter to analogue and turn the output power to maximum by turning the control knob 21 fully clockwise. Then switch the receiver to digital using the switch 2, and set the threshold sensitivity comparator 31 at its lowest sensitivity by turning the sensitivity control knob 33 fully anti clockwise.

If the digital indicator light does not light, leave the transmitted power at its maximum position and switch the receiver to analogue. The apparatus is now set up for transmission of analogue signals, and the receiver gain may be adjusted to the required level using the control knob 32.

If the digital indicator light does light then the transmitter output power control knob 21 should be turned anti-clockwise until the indicator LED 34 just goes OFF, and the receiver then switched back to analogue operation. The apparatus is now set up for the transmission of analogue signals, and the receiver analogue gain main be adjusted to the required level.

Throughout the above setting-up procedure, the buzzer may be switched on and used instead of the digital indicator light 34.

Sources other than the transmitter can be used to provide the radiation signal to the receiver.

Thus, a metal diaphragmed earphone 40 with the cover removed, may be connected by its lead 40a to the earphone socket of the radio and a torch 41 shone onto the earphone diaphragm 42 so that light is reflected into an optical fibre cable 43 coupled to the receiving diode socket 29a ofthe receiver. To set up the arrangement, first turn the radio volume control up to high level, turn the loudspeaker on by switching the switch 39 of the receiver to the ON position. Then switch the receiver to analogue and turn its analogue gain up to maximum by turning the control knob 32 fully clockwise and connect a short optical cable to the receiver. If the torch, earphone and cable end are positioned as shown in Figure 5 sound will be heard coming from the loudspeaker.Do not place the torch 41 or the end optical fibre of the cable too close to the earphone, or the receiver may be overloaded by the signal. Adjust the positions of the torth and cable until a strong sound is produced by the loudspeaker. The sound produced is a high quality reproduction of the original audio signal.

The effect is caused by the metal diaphragm of the earphone modulating the reflected light, and so producing a light beam which varies in intensity with the original audio signal.

The cable end may alternatively be pointed in the direction of an electric light. Care must be taken not to point the cable end directly at a very bright light source, because this could cause receiver overload. A 100 Hz mains "hum" will be heard at the loudspeaker. This effect shows that the light from a light source that is run from the mains supply is in fact varying in intensity at 100 Hz, a frequency too fast for the eye to detect.

The apparatus may also be used to demonstrate optical feedback. Thus, with the receiver set up as described above, position the free end of the optical cable close to the analogue indicator LED 35 of the receiver. A noise will be heard at the loudspeaker, and the frequency and intensity of the sound may be changed by adjusting the position of the cable end or altering the receiver analogue gain by turning the control knob 32.

This effect is produced by optical feedback, because the analoque indicator LED 35 is driven by a voltage signal that is produced by the receiver amplifier 29. Directing the light of the indicator LED 35 into the input optical fibre completes the loop. The effect is similar to the "whistling" that can occur in a microphone/ amplifier/loudspeaker system, where the feedback is from the loudspeaker to the microphone.

A similar positive feedback may be demonstrated by connecting the 'high Z' input of the transmitter to the 'high Z' output of the receiver, with both units on analogue and the loudspeaker swiched on, and lining up the transmitting and receiving diodes.

A variety of demonstrations, applications or experiments involving the reception of analogue light signals may be devised by the user.

Possibilities include: 1. using a rotating disc with pre-coded markings and a light source to provide a modulated light signal to the receiver; 2. measuring the frequency of a rotating disc or a vibrating object using light reflection or transmission; 3. using the fibre as an "optical wand" to detect a pre-prepared "bar-code".

It is also possible to "listen" to a number of different light sources by using the receiver/loudspeaker arrangement. Detecting the 100 Hz hum of mains lighting has already been mentioned. Another example is the "hiss" that a torch light produces. However care must be taken not to overload the receiver by pointing the torch directly at the receiving diode, and not to let the intensity of the mains lighting drown out the effect with its 100 Hz hum. If the torth is knocked, a high pitched sound is heard in the loudspeaker caused by the torch bulb's filament vibrating at high frequency. It is also possible to "listen" to light intensity variations caused by nature, such as the high frequency produced by reflecting sunlight off a fly's wings when the fly is in motion.

Another effective demonstration of 'listening to light' is to use the variable frequency square wave generator in the transmitter as the source.

Changing the frequency alters the pitch of the sound heard at the receiver. The higher frequencies cannot be detected by the eye, although they are readily detectable by the ear.

Turning the transmitter onto 'pseudo-random signal' produces a sound with which players of computer games will be familiar.

Demonstrations of the presence of infra-red radiation may be convincingly carried out. For example, a diffraction pattern produced by the transmitter's infra-red source together with a diffraction grating may be detected using an audio transmission procedure similar to those described above with the square wave as the transmitted signal.

If the high impedance output socket 24 of the receiver is used, then the LOUDSPEAKER/LOW Z switch 39 may be turned to the OFF position to reduce power consumption as well as disconnect the loudspeaker.

Digital Signal Transmission When the apparatus is used in the digital mode, the optical output of the transmitter is either ON or OFF, depending on the state of the input voltage signal.

As mentioned above, the apparatus may be used to send morse code. To set up the apparatus for morse transmission, first set the transmitter to digital operation, turn the rotary switch 1 5 to the TTL/CONTACT/MORSE position and set the output power at maximum by turning the control knob 21 fully clockwise. At the receiver, first set the analogue/digital switch 2 to digital, then turn the digital threshold sensitivity control knob 33 to a midway position, and turn the buzzer switch 36 to the ON position. Connect an optical cable between the transmitter and receiver.

The apparatus is now ready to transmit morse code and depressing the morse key 1 8 will cause the transmitter's output LED's 8 and 9 to emit radiation (the output indicator 22 will go on), and the receiver's buzzer and digital indicator LED 34 will also be turned on. (The buzzer may be switched off if desired, leaving the digital indicator LED 34 to show when the receiver detects a light signal). A morse message may thus be communicated through the system. A proper morse key may be used by connecting it between the TTL/CONTACT and GROUND terminals 11 and 20'b.

A morse signal may be communicated from the transmitter to the receiver over "free-space" rather than through an optical cable by removing the interconnecting optical fibre, and positioning the units so that the transmitting LED faces the receiving diode. The transmission distance is limited to about a half of a metre even with the output power on maximum and with the digital threshold sensitivity as high as the ambient light allows (the receiver should be pointed away from any bright external light source for the best results). This distance is shorter than the transmission distance achievable in free space with analogue signals, due to the fact that the receiver's digital sensitivity is not as high as its analogue sensitivity.

The digital receiver circuitry may also be activated by room light. Pointing the receiving diode or an optical fibre connected to the receiving diode socket 29a toward a light source of adequate intensity will cause the light signal to be detected by the receiver. In order to detect a light signal in the presence of background (ambient) light, it is necessary to reduce the digital threshold sensitivity, by turning the control knob 33 anti-clockwise, to a level at which the background light does not activate the receiver.

When the threshold sensitivity is set at maximum, the receiver is extremely sensitive, and will trigger off an optical level of just 50n W (nano Watt).

As described above, the transmitter is provided with an internal pseudo-random digital frequency generator 1 6a which may be viewed as an "automatic morse code generator" and also a square ware generator 1 6,b.

In order to use the internal digital frequency generator, first turn the transmitter's rotary switch to "Pseudo-random signal" or "square wave" and turn the Signal Generator Frequency to a minimum by turning the knob 17 fully anticlockwise. The frequency of the internal signal source may be increased by turning the "Generator Frequency" control knob in a clockwise direction.

General digital signals may be transmitted using the apparatus. The digital bandwidth of the system is D.C. to 20 k Bit/S. As described above, a number of digital interfaces are provided: TTL, CMOS, RS232 Signal Voltage, Contact Switch and Morse.

At the transmitter only one of the TTL, CMOS or RS232 interfaces can be used at any one time.

Selection is carried out by means of the multiposition rotary switch 1 5 together with the corresponding input socket. In the receiver, any or all of the TTL, CMOS and RS232 interfaces may be connected for use simultaneously, by making connections to the corresponding output terminals.

A "contact" interface may be used at the transmitter. This is selected by turning the rotary switch to TTL/CONTACT/MORSE, and connecting wires to the TTL/CONTACT and ground terminals.

The transmitting LED 8 or 9 is turned on by making contact between these two wires, thus shorting the sockets.

The morse key 18 may be pressed when any other interface is being used, because the morse key overrides ail other digital signals. This feature is useful for checking the integrity of the link between the transmitter and the receiver.

During use of the apparatus to transmit digital signals the digital indicator LED 34 and also the buzzer, if switched on, of the receiver goes ON if the signal is a "space" (corresponding to light at the input), and goes OFF if the signal is a "mark" (corresponding to no light at the input). In order to obtain the optimum signal characteristics at the receiver, the following procedure, which sets up the best values of output power and threshold sensitivity, should be carried out before transmitting digital data.

First connect up the optical route, set the receiver and transmitter to digital operation using switches 1 and 2 and turn the transmitting LED to a continuous "ON" state by, for example, turning the rotary switch 1 5 of the transmitter to CMOS, and leaving the CMOS terminal unconnected.

Next, turn the transmitter output power to maximum by turning the control knob 21 fully clockwise. Then, arrange the receiver digital threshold sensitivity to be at minimum sensitivity by turning control knob 33 full anticlockwise.

If after the above procedure the receiver's digital indicator LED 34 is off, increase the digital threshold sensitivity at the receiver by turning the control knob 33 clockwise until the indicator LED 34 just goes on. Note the position of the threshold control. Now turn the threshold control knob 33 clockwise to the point mid-way between the noted position and the fully clockwise (i.e.

maximum sensitivity) position. The indicator light should now be ON. The system is now set up for the transmission of digital data.

Instead of carrying out the above step the threshold sensitivity may be set at maximum (fully clockwise). However, this results in up to typically 7.5usec. puise width distortion (equivalent to 15% at 20kBit/s data rate, with the percentage distortion reducing in proportion to the data rate).

Setting the threshold at maximum sensitivity enables a wide range of input power levels to be used if the 7,5used. distortion is not critical to the application.

If the indicator light remains off when the sensitivity is increased to its maximum position, and the above steps have been carried out correctly, then the optical signal at the receiver is too weak for a digital link to be established. If this is the case, check that all the optical connections are good, and that the fibre ends are clean.

If, alternatively, the receiver's digital indicator LED 34 is ON when the above procedure is carried out, reduce the transmitter's output power by turning the control knob 21 anti-clockwise until the indicator LED 34 just goes OFF. Then, increase the digital threshold sensitivity at the receiver by turning the control knob 33 clockwise until the indicator LED 34 just goes on. Note the position of the threshold control. Now turn the threshold control knob 33 clockwise to the point mid-way between the noted position and the fully clockwise (i.e. maximum sensitivity) position. The indicator light should now be ON. The system is now set up for the transmission of digital data.

Throughout the above setting-up procedure, the buzzer may be switched on and used as the indicator instead of the digital indicator LED 34.

A large number of applications, demonstrations, or experiments involving the reception of digital-type signals may be devised by the user. Some examples are: detection of very small signal levels by the receiver; construction of an alarm system based upon the presence of a light signal; using the transmitting and receiving apparatus in an optical pulse counting system for applications such as quality assurance, and scientific and engineering experiments.

Optical-Fibre Attenuation Measurements Standard Measurement 1 The apparatus, in conjunction with a digital voltmeter set to read a.c. voltage, can be used to measure the attenuation of a fibre-optics route.

The standard measuring procedure is given in the rest of this Section, and it is advisable for the user to familiarise himself with this first method before attempting the others.

Steps a and b must be carried out initially to ensure that there is no optical overload at the receiver.

a. Set the receiver to digital operation, and reduce the threshold sensitivity to a minimum (control fully anti-clockwise).

b. Set the transmitter to digital operation by means of the switch 1, and set the emitting diode to a continuous ON by switching the switch 15 to CMOS and leaving the CMOS terminal unconnected. Set the output power onto maximum by turning the knob 21 fully clockwise. Connect a short length (about a metre) of optical cable between the transmitter and receiver. This short length of cable must have flat, clean ends in order to achieve a good measurement accuracy. Reduce the transmitted output power level by turning the knob 21 anticlockwise until the receiver's digital indicator light 34 just goes OFF. Do not alter the output power level throughout the remainder of the measurement.

c. Turn the transmitter onto "Square Wave" by means of the selector switch 1 5 and set the Signal Generator Frequency control knob 1 7 to maximum by turning it fully clockwise.

d. Switch the switch 2 of the receiver to analogue, and turn the receiver's analogue gain down to the MINIMUM by turning the analogue gain control knob 32 fully anticlockwise and switch the loudspeaker switch 39 to the OFF position. Connect a digital voltmeter, which can measure a.c.

values to a tenth of a milli-Volt, to the 'high Z' socket 24 of the receiver. A 3 1/2 digit Digital Voltmeter (DVM) with a 200mV a.c.

scale is ideal for this purpose.

e. Take the voltmeter reading (=VREF), with the short cable length connected between the transmitter and receiver.

f. Connect up the route to be measured between the transmitter and receiver, in place of the short length, and take the voltmeter reading (=to). (Do NOT alter any controls during Step f).

Check the electrical noise level by switching off the transmitter and noting the voltmeter reading (=VA). VA will be zero if the DVM specified in Step d is used.

g. The attenuation (or insertion loss) of the route is calculated using the following formula:

VREF attenuation=10 logaO dB, (1) VREF VREF which approximates to 101ogre dB V0 (2) when VA is small compared to V0 (VA is zero for Standard Measurement 1 if a 0.1 mV sensitivity DVM is used).

For measurement of a number of routes, only the final two steps f and g need be repeated. If the output power level of the transmitter is altered, however, steps a to e must be carried out to reset a suitable level. After measuring a number of routes, check that VREF is essentially the same as the original value.

The attenuation range using the above procedure is 25dB, and the accuracy is within ±1 dB. This accuracy figure is predominantly determined by the inconsistency of connector losses.

Standard Measurement 2 If the complete measurement procedure (including Steps d and e) of the Standard Measurement 1 is carried out with the DVM connected to the 'low Z' socket 23 of the receiver (and with the loudspeaker/low Z switch 39 on), the attenuation range is increased to 30dB.

However, VA, the voltage contribution from noise may not be zero and so its value must be taken into account in the attenuation formula (1), if a relatively high loss route is being measured. The accuracy is again within +1dB.

High Loss Methods 1 and 2 In order to increase the attenuation range capability of the apparatus steps f and g of the Standard Measurement 1 procedure are replaced by the following (which may be used to measure losses of greater than 1 5dB).

Connect up the route to be measured between the transmitter and receiver, in place of the short reference length. Then, increase the receiver analogue gain to maximum by turning the control knob 32 fully clockwise. Take the voltmeter reading (=V,).

Measure the electrical noise level by switching off the transmitter, and noting the voltmeter reading (=we).

The attenuation of the route is given by the formula:

VREF attenuation=10 log1 +kdB V/(V12~VB2) (3) where the approximate value of k=1 5 (k is derived from the ratio of the maximum to minimum gain of the receiver).

Remember to turn the receiver analogue gain back down to a minimum by turning the knob 32 fully anticlockwise if re-measurement of VREF is required.

The range of the instrument using this method is 40dB, while the accuracy is +2.5dB.

If the above method is carried out with the DVM connected to the 'low Z' socket 23 (and with the loudspeaker/low Z switch 39 ON), the range is increased to 50dB, and the measurement accuracy is +3dB.

In order to increase the dB accuracy of the High Lost Methods, the constant k may be accurately determined by equating the measurement results of a route with a loss between 1 5 and 25 dB, using the Standard and High Loss Methods. A precise value of k leads to accuracies of + 1.5 dB and +2dB for the High Loss Methods 1 and 2 respectively.

Measurement with Non AMP DNP-terminated Cable The attenuation of a cable route with end connectors other than AMP DNP may also be measured. Identical methods to those described above are used, but with short (one metre or less) interface cables connecting the apparatus transmitter and receiver to the cable to be measured. Steps a to e of the standard method 1 must be carried out with a short length of cable terminated with the same type of connectors as in the route to be measured, and this short reference cable is also connected to the transmitter and receiver via the two interface cables.

Although device/connector/cable mis-match is compensated for to some extent by the setting of the transmitted power level in Steps a and b above, a large mis-match will result in a reduced measurement range of the equipment (although the accuracy will not be affected).

Test Equipment Applications The transmitter and receiver form useful pieces of test equipment in a fibre-optics and general optics laboratory, production facility or at an installation site, and may be used for many other test applications besides the measurement of fibre attenuation as described above.

The transmitter may be used as a versatile optical source for testing out optical receivers.

Possible test configurations include: 1. utilising the internal pseudo-random and square wave generators to provide a realistic data format (the signal generator output may be monitored as the RS232/Sig.Gen.Monitor socket); 2. connecting an external signal generator to the TTL input to produce data trains up to a rate of 0.5 MBits/s; 3. using an analogue signal generator in conjunction with the transmitter in analogue mode, to produce optical signals that correspond to those formed by a dispersive medium, for testing digital receivers.

This analogue set-up may also be used to test analogue optical receivers.

In all of the above configurations, the output power control, which has a range of approximately 20dB, provides a useful feature for receiver sensitivity testing.

Equipment terminated with connectors other than AMP DNP may also be tested by using an appropriate interface cable between the apparatus and the equipment under test, or by utilising the infra-red l.e.d.

The receiver may be used in the digital or analogue mode to test out optical transmitters.

Also, it may be used in the analogue mode for giving an audible indication of the presence of infra-red radiation at locations such as the remote ends of optical links, cable breaks, bad joints and "lossy" optical coupling arrangements, if the transmitted signal contains audio frequency components. The receiver's digital circuity with buzzer output also provides a convenient method for detecting infra-red radiation. A short length of optical cable may be used as a probe for optical radiation detection purposes.

It is also possible to measure the optical absorption or reflection properties of various materials at the wavelengths of the emitting devices by detecting the level of radiation at the receiver (in analogue mode), with the transmitter set on 'square wave' acting as the source. The material to be tested is inserted between the transmitter and receiver (or between the ends of the cable attached to these) for transmission experiments, and at a suitable angle for reflection measurements. A DVM set to a.c. may be used to accurately measure the signal at either the 'high Z' or 'low Z' analogue outputs. For accurate measurements over free-space, it is important to minimise the contribution to the signal of ambient light from mains lighting. Also, when very small signal levels are being measured, the effect of electrical noise of the receiver may be taken into account by using the square root of the difference of the squares formula, which has been used above, that is, Signal Voltage= [(Total Voltage)2-(Noise Voltage)2]112.

Claims (30)

1. Signal transmitting apparatus comprising: means for transmitting an optical electromagnetic signal of analogue form; means for transmitting an optical electromagnetic signal of digital form; and means for switching between the digital and analogue signal transmitting means.

2. Signal receiving apparatus, comprising: means for receiving an optical electromagnetic signal of analogue form; means for receiving an optical electromagnetic signal of digital form; and means for switching between the digital and analogue receiving means.

3. Signal transmitting and receiving apparatus comprising: means for transmitting an optical electromagnetic signal of analogue form; means for transmitting an optical electromagnetic signal of digital form, means for switching between the digital and analogue signal transmitting means; means for receiving the analogue signal transmitted by the analogue signal transmitting means; means for receiving the digital signal transmitted by the digital signal transmitting means; and means for switching between the digital and analogue receiving means.

4. Apparatus according to Claim 1 or 3, wherein the electromagnetic radiation is visible light.

5. Apparatus according to Claim 1 or 3, wherein the electromagnetic radiation is infra-red radiation.

6. Apparatus according to any one of Claims 1, 3, 4 and 5, wherein the signal transmitting means are arranged to transmit electromagnetic radiation signals in response to a signal input thereto.

7. Apparatus according to Claim 6, wherein the input signal is an audio signal.

8. Apparatus according to Claim 6 or 7, wherein the analogue signal transmitting means comprises a flexible member arranged to vibrate in response to a signal input to the analogue transmitting means and an electromagnetic radiation source for directing a beam of optical electromagnetic radiation onto the member so that, in use, radiation reflected from the member is modulated by the vibration of the flexible member in direct proportion to the input signal.

9. Apparatus according to any one of Claims 4 to 8, further comprising one or more optical fibre cables connectable to receive and guide signals from the transmitting means.

1 0. Apparatus according to any one of Claims 1, 3 and 4 to 8, wherein the transmitting means are arranged to transmit optical electromagnetic signals over free space.

11. Apparatus according to any one of Claims 6 to 9, wherein the digital signal transmitting means has input means for accepting a digital input signal in the form of TTL, CMOS, RS232 voltage levels, MORSE or CONTACT inputs.

1 2. Apparatus according to any one of Claims 1,3 and 4 to 11, wherein the digital signal transmitting means comprises a digital frequency generator.

1 3. Signal transmitting apparatus comprising: an output transducer for receiving an electrical input signal of analogue or digital form and delivering a corresponding optical electromagnetic radiation signal; a first means for accepting an analogue input signal and delivering an analogue electrical output signal suitable as an input signal for the output transducer; a second input means for accepting a digital input signal and delivering a digital electrical output signal suitable as an input signal for the output transducer; and signal switching means connected to the first and second input means and to the output transducer and selectively operable to supply either the analogue or digital electrical output signal to the output transducer.

1 4. Signal transmitting apparatus according to Claim 13, including digital signal generating means for producing a digital input signal for the second input means.

15. Apparatus according to Claim 2 or 3, wherein the electromagnetic radiation is visible light.

16. Apparatus according to Claim 2 or 3, wherein the electromagnetic radiation is infra-red radiation.

1 7. Apparatus according to Claim 2, 3, 1 5 or 16, wherein the receiving means include means for converting a received digital or analogue signal into an audio signal.

1 8. Apparatus according to any one of Claims 2, 3 and 15 to 17, wherein the receiving means include means for converting a received analogue signal into a corresponding visible signal.

1 9. Apparatus according to any one of Claims 2, 3 and 1 5 to 18, wherein the receiving means include means for converting a received digital signal into TTL, CMOS, RS232 voltage levels, audio and visible outputs.

20. Apparatus according to any one of Claim 2, 3 and 1 5 to 19, wherein one or more optical fibres are connectable to the receiving means to supply a digital or analogue signal to the receiving means.

21. Apparatus according to any one of Claims 2, 3 and 1 5 to 19, wherein the receiving means is arranged to receive optical electromagnetic signals transmitted over free space.

22. Signal receiving apparatus comprising: an input transducer for receiving an optical electromagnetic radiation signal of analogue or digital form and delivering a corresponding electrical input signal; a first output means for accepting an analogue electrical input signal and delivering a corresponding output signal suitable for operating an analogue device; a second output means for accepting a digital electrical input signal and delivering a corresponding output signal suitable for operating a digital device; and signal switching means connected to both the first and second output means and to the input transducer and selectively operable to supply the electrical input signal to either the first or second output means.

23. Signal transmitting and receiving apparatus, comprising an output transducer for receiving an electrical input signal of analogue or digital form and delivering a corresponding optical electromagnetic radiation signal; a first means for accepting an analogue input signal and delivering an analogue electrical output signal suitable as an input signal for the output transducer; a second input means for accepting a digital input signal and delivering a digital electrical output signal suitable as an input signal for the output transducer; signal switching means connected to the first and second input means and to the output transducer and selectively operable to supply either the analogue or digital electrical output signal to the output transducer; an input transducer remote from the output transducer for receiving an analogue or digital electromagnetic radiation signal of analogue or digital form from the output transducer and delivering a corresponding electrical input signal; a first output means for accepting an analogue electrical input signal and delivering a corresponding output signal suitable for operating an analogue device; a second output means for accepting a digital electrical input signal and delivering a corresponding output signal suitable for operating a digital device; and signal switching means connected to both the first and second output means and to the input transducer and selectively operable to supply the electrical signal to either the first or the second output means.

24. Apparatus according to Claim 23, wherein an optical fibre connection is provided between the output and input transducers.

25. Apparatus according to Claim 23, wherein the output and input transducers are arranged respectively to transmit and receive optical electromagnetic signals over free space.

26. Signal transmitting apparatus comprising an electromagnetic radiation source for directing a beam of optical electromagnetic radiation onto a flexible member; means for causing the flexible member to vibrate in response to a signal input to the vibrating means; and one or more optical fibres for receiving radiation reflected from the flexible member so that, in use, an electromagnetic radiation signal modulated by the vibrations of the flexible member and directly proportional to the input signal is transmitted along the or each optical fibre.

27. Signal transmitting apparatus substantially as hereinbefore described with reference to, and as illustrated in Figures 1 and 3 or Figures 3 and 6 of the accompanying drawings.

28. Signal transmitting apparatus substantially as herein before described with reference to, and as illustrated in Figure 5 of the accompanying drawings.

29. Signal receiving apparatus substantially as hereinbefore described with reference to, and as illustrated in Figures 2 and 4 or Figures 4 and 7 of the accompanying drawings.

30. Any novel feature or combination of features described herein.