GB2237708A - Radio transmission of digital data - Google Patents

Abstract

Digital serial data streams are transmitted by shifting the frequency of a radio frequency carrier at each change in the data stream and the carrier is radiated either directly or after amplification. The rate of change of frequency and its direction comprises the transmitted intelligence. The sidebands in the transmitted radiation are reduced by the substantial absence of inductive elements in the associated circuits. Preferably the digital information is fed to a semiconductor switch which connects a capacitor or resistor into the frequency determining network of an oscillator 4 which comprises the carrier either directly or after amplification or frequency multiplication or division; the capacitor or resistor may be switched by means of a series connected transistor 3. For increase in the radiated carrier power the output from the oscillator may be fed to a linear amplifier 14 connected to an aerial 13. <IMAGE>

Description

RADIO TRANSMISSION OF DATA This invention relates the transmission of digital data by radio and methods for performing such transmissions.

The current practice is to use a radio link as if it were a telephone line. Digital data is converted into audio frequency tones which are then used to modulate the radio frequency (RF) carrier in a similar manner to voice signals.

The present invention relates to a method of modulation in which the digital data is used directly to shift the frequency of the RF carrier. The method requires a smaller bandwidth than the conventional tone modulation system and allows more separate channels to be carried withn an allocated frequency band.

According to the present invention there is provided a method of transmitting serial digital information wherein the frequency of a radio frequency carrier is changed at each change in the data stream and the carrier is radiated either directly or after amplification.

In this method of transmitting serial digital information the rate of change of frequency and its direction comprises the transmitted intelligence. Preferably the sidebands in the transmitted radiation are reduced by the substantial absence of inductive elements in the associated circuits.

There is also provided apparatus for performing the method.

By direct change of the frequency of the radiated RF signal the modulation circuitry is less complicated and expensive than those using tone modulated RF carriers. Furthermore as the carrier is radiated continuously a better signal to noise ratio is obtained than when using a carrier switched by a digital signal.

The basis of the system is that the entire circuit has the minimum storage of energy so that rapid frequency control can be obtained using semi-conductor devices. In practice the transition between the digits '0' and '1' can take place with a considerable reduction in the number of sidebands compared with prior modulation systems. The circuit design avoids the use of inductors and the use of capacitative elements is minimised where they affect the generation of impulsive changes. The result is that the data rate practicable with a simple digital system is commensutrate with the bandwidth of the signal and not, as is usual in NBFM and audio type modulation, a bandwidth several times the frequency deviation togther with an allowance for sidebands.

An arrangement is described for a data transceiver which may be used to transmit and receive data within a building and its surroundings. Using standard semi-conductor integrated circuit elements, e.g. 5 volt chips, the necessary driving energy can be obtained from the standard V24 or RS232 outlets on computers and similar apparatus; the need to lay cables throughout the building is avoided.

The data may comprise streams of digits at a constant baud rate or at a variable rate.

For longer distance transmission a higher power output and directional aerial arrays are needed which may require the use of an amplifier. Wherever possible power losses are avoided by the use of well matched low loss coaxial cables.

A directional aerial will ensure that the majority of the RF signal is radiated in the desired direction.

If higher powers are needed than are available from the RF oscillator then amplification may take place at the aerial using a wide band linear amplifier. Such amplifiers are known and are substantially untuned so that they will amplify a range of radio frequency signals. The power for any linear amplifier may be supplied as direct current through the coaxial feeder cable in known manner or by means of a separate pair of wires. Solid state linear amplifiers using semi-conductor devices are known in the art. Any linear amplifier used in the system may accept signals from more than one source and, provided the two frequencies differ, will amplify each signal independently.

Locating any signal power amplifier away from the main unit close to the aerial ensures the minimum of interference with the digital operating equipment.

Similarly, if the input signals to the aerial are weak a preamplifier may be included in the path to the receiver.

The preamplifier is preferably mounted with the aerial and fed with direct current through the coaxial cable leading to the input mixer. To prevent damage and interference the aerial may be switched from the RF feed coaxial cable to the reciever input cable by means of a relay or other known device according to whether signals are to be transmitted or received. The energisation of the relay may take place in response to a manually initiated signal or automatically by means of an RF signal sensor.

Although the invention may be used with any digital code and form of data at high speed it is particularly useful in 'packet' operation where a number of transmit/receive stations all use the same send/recieve frequencies.

Because the signal bursts are intermittent a number of stations can all be accomodated within a 12 or 25 kfiz channel. The channel width will depend upon the deviation between the signal frequencies.

Use of drift sensing and frequency stablising systems enable the reciever to remain in tune with the transmitter although both have departed from their intial value. Only the rapid changes of frequency provide intelligence and its instantaneous value is relatively unimportant. This means that carrier may drift over quite a wide range and, by noting the interference pattern, a pair can be made to move in such a manner that stations endeavour to avoid each other within the operating bandwidth.

In order that the inventicn may be clearly understood it will now be described with reference to the accompanying drawing in which the single figure shows a schematic diagram of a data transmission system according to the invention.

The data transmission and reception system consists of input means in the form of an opto-isolator 1 having a light emitting diode (LED) 2 input and a phototransistor 3 output. The diode 2 receives data signals in digital form which generate light or infra-red radiation pulses which are received by and switch the phototransitor 3. Radio frequency oscillations are provided by an oscillator 4.

The oscillator 4 is tuned to oscillate at a first frequency fl substantially determined by the values of the resistor 5, capacitor 6 and varactor (voltage controlled capacitor) 7. A modulator capacitor 8 may be connected in parallel with the capacitor 6 and varactor 7 by means of the phototransistor 3 in the opto-isolator 1. Energisation of the LED 2 switches on the transistor 3 causing the oscillator frequency to change to a second, lower, frequency f2.

The output of the oscillator 4 normally acts as the carrier wave for data transmissions, and after optional amplification by an amplifier 9, are fed by a low loss coaxial cable 11 and relay contacts 12 to an aerial array 13. The array 13 may include at its input a linear amplifier 14 for increasing the power transmitted. The relay contacts 12, shown as a single pole changeover, are operated by a relay 15.

The relay 15 is only energised when signals are being received. The aerial 13, when the relay 15 is energised, is connected through the contacts 12 to a pre-amplifier 16.

The output of the pre-amplifier 16 is carried by a low loss coaxial cable 17 to a mixer circuit 18. The output from the mixer 18, after passage through an optional amplifier 19, is fed through a short time constant network 21 to output means in the form of an opto-isolator 22 and through a long time constant network 23 to an amplifier 24. The opto-isolator 22 has an input LED 25 input and a phototransistor output 26.

A crystal oscillator 27 provides a reference frequency signal which is fed to the mixer 18 through a frequency multiplier 28. The output of the oscillator 27 is also fed to a phase locked loop (PLL) 29 where it is compared with the carrier signal from the oscillator 4 after passgae through a frequency divider 31. A control signal from the PLL 29 is applied to the varactor 7 in the frequency determining network of the oscillator 4. This control signal stabilizes the frequency fl by reference to the crystal oscillator 27.

When the relay 15 is energised and the system is in its 'receive' mode the recived signals from the aerial 13 are demodulated by a 'direct conversion' system by mixing the received signals with a portion of the transmitter oscillator either directly or, as shown, after multiplication by the multiplier 28. The difference beat note is fed to a short time constant derivative network 21 whose output rises rapidly at each digital excursion of the signal from the amplifier 19 and energises the LED 26 in the output opto-isolator 22. The isolator 22 provides a switching signal on a photo-transistor 25.

The difference beat note is also fed to a long time constant derivative network 24 whose output changes slowly and follows the average value of the signal. The time constant of the network 24 may be of the order of 0.25 seconds. The output from the network 24 is fed to amplifier 24 whose output is connected to PLL 29. The output form PLL 29 applied to the varactor 7 enables the oscillator 4 to track the received carrier.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims (17)

1. A method of transmitting serial digital information wherein the frequency of a radio frequency carrier is changed at each change in the data stream and the carrier is radiated either directly or after amplification.

2. A method of transmitting serial digital information as claimed in claim 1 wherein the rate of change of frequency and its direction comprises the transmitted intelligence.

3. The method as claimed in either claim 1 or claim 2 wherein the sidebands in the transmitted radiation are reduced by the substantial absence of inductive elements in the associated circuits.

4. The method as claimed in any of the preceding claims wherein the digital information is fed to a semiconductor switch which connects a capacitor or resistor into the frequency determining network of an oscillator which comprises the carrier either directly or after amplification or frequency multiplication or division.

5. The method as claimed in claim 4 in which the capacitor or resistor is switched by means of a series connected transistor.

6. The method as claimed in any of the preceding claims wherein the output from said oscillator is fed to a linear amplifier connected to an aerial.

7. The method as claimed in claim 6 wherein the outputs of a plurality of oscillators are fed to a single linear amplifier connected to an aerial.

8. The method as claimed in any of the preceding claims wherein the carrier frequency is changed by 25 kHz or less.

9. The method as claimed in claim 8 wherein the carrier frequency is changed by 25 kHz or less.

10. The method as claimed in any of the preceding claims wherein the input data signal comprises a serial data chain.

11. The method as claimed in any of the preceding claims 1 to 10 wherein the input data signal comprises a series of digits at a variable repetition rate.

12. Methods of transmitting digital information as claimed in claim 1 and as herein described.

13. Apparatus for transmitting digital information comprising a radio frequency oscillator adapted to shift frequency in response to a digital signal applied to an input means, means adapted to direct a portion of the oscillator output to an aerial, a mixer circuit adapted to accept and mix signals from the oscillator and received by the aerial, and output means adapted to provide a digital signal output corresponding to any received signal and switching means to connect the aerial to the oscillator output or the mixer.

14. Apparatus as claimed in claim 13 in which the a crystal oscillator provides a reference frequency and control means maintain the oscillator frequency in a fixed relationship to the reference frequency.

15. Apparatus as claimed in claims 13 or 14 in which a phase locked loop is adapted to maintain the oscillator frequency in a fixed relationship to the received carrier frequency.

16. Apparatus as claimed in any of the preceding claims in which a the digital signal is applied to a semiconductor switch adapted to connect a capacitor or resistor into the frequency determining network of the oscillator.

17. Apparatus as claimed in claim 13 and as herein described with reference to the accompanying drawing.