The present invention relates to communication devices used to connect computers, for example personal computers, in networks. This device performs the function of an interface between the computer and the communication channel utilised for the network. This channel can be a physical channel, for example a coaxial cable or an optical fibre, or space in the case of the so-called wire less or cordless type of network in which the communication takes place by means of radio waves.

The device according to the present invention has been developed particularly, although not exclusively, for use in cordless local area networks, and in the specific embodiment to operate according to the ETS 300-175 standard.

Communication devices of the said type are known in the art, however they have one or more disadvantages such as, for example, sensitivity to disturbances, high cost, circuit complexity and others.

The object of the present invention is that of providing a simpler and less expensive communication device than the prior art devices whilst providing optimum

performance .

According to the present invention this object is achieved by a communication device having the characteristics indicated in the claims which follow the present description.

Further advantages and characteristics of the present invention will become apparent from the following detailed description, in which reference is made to the attached drawings, provided purely by way of non- limitative example, in which:

Figure 1 schematically illustrates a communication device for computer networks;

Figure 2 is a block schematic diagram of an embodiment of the device according to the present invention;

Figure 3 is a block schematic diagram of an alternative embodiment of the device according to the prepent invention; and

Figure 4 is a block schematic diagram of a portion of the device shown in Figures 2 and 3.

In Figure 1 is shown a generic communication device of the type described hereinabove. The device 1 communicates, via a digital communication channel of a type which is widely known, with a computer not illustrated. This channel is bi-directional such that it

allows the transmission of digital input data, indicated DDI from the computer to the device 1 and the transmission of output digital data, indicated DDO from the device 1 to the computer.

The device communicates with other identical or compatible devices, still in a bi-directional manner, by means of radio waves using an antenna A.

In a possible embodiment it may be found convenient to use two antenna so as to be able to exploit the technology known as "space diversity".

In Figure 2 is shown the high level architecture of an embodiment of the device 1 according to the present invention.

The input digital data DDI constitutes the input of a modulator, indicated MOD, which generates a modulated signal at its output. This modulated signal enters a mixer MIX1 which raises its frequency by utilising a fixed frequency signal produced by a local oscillator LOC. The high frequency modulated signal at the output from the mixer MIX1 then passes through a ceramic filter FI after which it is amplified by the power amplifier AMP.

The amplified signal is sent to a commutation unit,

indicated C, which sends it to an antenna A through which the signal is thus transmitted via radio.

What has been just described is the transmitter section of the device 1, the receiver section is similar, having a complementary structure. The radio signals received by the antenna A are sent via the commutation unit C to a receiver preamplifier indicated PRE. The received signals, thus preamplified,- pass through a ceramic filter F2 and are applied to the input of a mixer MIX2. In the mixer MIX2 the signals are converted to a lower frequency by utilising the same local oscillator LOC as was used in the transmitter section.

The output signals from the mixer MIX2 are intermediate frequency signals; these are sent to a unit for conversion to low frequency, indicated IF. The low frequency signals at the output from the unit IF are sent to a demodulator indicated DEMOD which decodes them and emits at its output the digital data DDO which can be sent to the computer connected to the device 1.

This architecture can be simplified, thereby reducing its cost and complexity, by making some of the elements of the communication device 1 bi-directional. A possible architecture simplified in this way is illustrated in Figure 3.

A first element which can be made bi-directional is the amplifier AMP. In this way it is possible to integrate into a single bi-directional circuit the transmitter power amplifier AMP and the receiver input preamplifier PRE. The resulting circuit is a bi-directional amplifier indicated AMP/PRE. The bi-directional amplifier AMP/PRE is typically provided with filters, not illustrated in the drawings.

Consequent on the integration of the amplifier AMP/PRE it is possible also to integrate the transmitter ceramic filters FI and receiver filters F2 into a single bi¬ directional filter F.

Moreover it is possible also to make the two frequency conversion mixers, namely the transmitter mixer MIX1 and the receiver mixer MIX2 bi-directional by integrating them into a single bi-directional mixer MIX1/2. The bi¬ directional mixer MIX1/2 still uses the local oscillator LOCY for frequency conversion.

The arrangement of the elements varies slightly, as illustrated in Figure 3, since the amplifier AMP/PRE is situated between the mixer MIX1/2, and the modulator MOD and the demodulator DEMOD.

Naturally, since the amplifier AMP/PRE is bi-directional it is necessary to introduce a connection circuit COMM

for connecting it to the modulator MOD and. demodulator DEMOD.

The bidirectionality of the device 1 can be accentuated greatly by also partly integrating the modulator MOD and the demodulator DEMOD for the purpose of utilising, for example, a single filter instead of two, as will be described below.

However, in the typical case in which the major part of the electronic circuit of the device 1 is made in integrated form it is necessary to evaluate carefully, in dependence on the specific embodiment, whether or not it is appropriate to make given components bi-directional.

In fact the additional cost of a mixer or an amplifier in integrated form can be negligible.

A particularly important component of the device 1 is the modulator MOD which will now be described in detail making reference to Figure 4.

The modulator MOD comprises a main oscillator indicated OSC which is the main (or master) fixed frequency oscillator. In the specific embodiment this frequency lies between 55 and 100 MHz although this can be varied in dependence on the particular design of the modulator MOD. The main oscillator OSC is a high quality

oscillator, for example a quartz oscillator, since it must be very stable. Typically it is a digital oscillator and therefore the output signal from the master oscillator OSC is a square wave.

The output frequency from the main oscillator OSC constitutes the input to two divider circuits DIV3 and DIV4; DIV3 is a modulo 3 divider and DIV4 is a modulo 4 divider. The dividers DIV3 and DIV4 are also digital circuits.

The output signals from the dividers DIV3 and DIV4, that is to say signals having frequencies equal respectively to a third and a quarter of the frequency of the main oscillator OSC, are selectively connected to the input of a further modulo 8 divider DIV8 by a switch SW. This switch SW is also a digital circuit and is controlled by the digital input signals DDI which in the specific embodiment have a frequency of 576 KHz (or 1152 Kbit per second) .

This configuration makes it possible to effect a modulation of the FSK (frequency shift keying) type. At the output of the divider DIV8 there therefore appears a modulated signal of the pure FSK type, centred on the mean of the frequency of the main oscillator OSCdivided by 24 or by 32, that is to say divided by 8x3 or by 8x4. Therefore this PURE FSK signal is centred on a frequency

which in the specific embodiment lies between 2 and 6 MHz.

The choice of the frequency generated by the main oscillator OSC is determined by the frequency desired in the specific case, lying between 2 and 6 MHz, for the modulated FSK signal. The modulo 8 divider DIV8 has the function of cleaning the signal at the output of the switch SW of harmonics at unwanted frequencies produced by the lack of synchronization between the frequency generated by the main oscillator OSC, and therefore the output frequency from the dividers DIV3 and DIV4 , and the flow of data DDI which controls the switch SW.

If the flow of data which controls the switch SW is synchronised, by means of a synchronisation circuit SINC, with the frequency generated by the main oscillator OSC the output signal from the switch SW is very much cleaner of unwanted harmonics. In this case it is therefore possible to avoid the use of the modulo 8 divider DIV8 and consequently the frequency generated by the main oscillator OSC can be reduced by a corresponding factor.

The output signal from the switch SW or possibly from the divider DIV8 is applied to the input of a mixer MIX in which the frequency is raised to 57 MHz, in the specific embodiment by means of a fixed frequency signal FO. The frequency FO can be produced by a dedicated oscillator or

derived in a known way from other frequencies generated in the device 1.

The mixer MIX is also a digital circuit, and can therefore be formed as a simple logic port or gate in a simple and inexpensive manner. At the output of the mixer MIX appears a square wave digital signal at a frequency of 57 MHz with a very precise FSK deviation. This signal however is spectrally very dirty and it is therefore necessary to filter it .

The output signal from the mixer MIX then passes through a filter of surface-acoustic-wave type indicated SAW, which is a passive filter used in the radio field.

The output signal from the SAW filter is the signal generated by the modulator MOD.

The advantage of this system lies in the ease with which the' power spectrum is thoroughly filtered. In other systems according to the prior art the filtering is extremely critical and in general does not give good results . With the system according to the invention on the other hand the filtering is very simple, gives optimum results, and can be achieved by means of an SAW filter. The SAW filter is the same filter as is utilised to filter the intermediate frequency during the reception phase. This, too, is a very important point; that is to

say the same filter is utilised in reception. With this system there is the advantage of saving filters, which are the most expensive elements in the system, in that the same filters can be used both for reception and transmission. The rest of the modulator moreover is extremely simple since it involves only a completely digital circuit. The master oscillator is also a square wave digital oscillator.

A further significant advantage of the system is given by the modulation of power developed in a manner implicit by the filter SAW. Networks of this type operate in TDD (time division duplex) or else in TDMA (time division multiple access) . That is to say the radio does not transmit for 100% of the time but transmits only for limited time periods. This means that the transmitter is turned on and off.

The problem is that if the transmitter is turned on and off,- sharply, that is to say in practice with ^: a square wave power variation in time, there is a spurious harmonic emission at the moment it is turned on or off.

The spurious harmonic emission is damaging in that it could disturb other users of the system. It is therefore necessary to modulate the power emitted by the transmitter in such a way that the variation is not a square wave variation but that th ^* transmitter is turned

on and off in a gradual or smoothed manner.

However this is very difficult since the transmitter is a non-linear element, and it is therefore rather complicated to reconcile this characteristic with other characteristics of the transmitter. Principally it is practically impossible to construct a transmitter the transmission power of which can be modulated, and which at the same time can jump quickly from one channel to the other. In particular these two ■characteristics are both very useful and the prior art has not succeeded in combining them in the same transmitter.

In fact prior art transmitters cannot change quickly from one channel to the other to the extent that, in certain cases, the solution has been adopted of having two synthesizers which serve the purpose since, whilst one is transmitting in one time slot the other is changing channel to arrive at the right channel to be able to transmit in the next time slot .

In the case of the present invention on the other hand the transmitter is non-linear and not modulated and therefore can change quickly from one channel to the other whilst still being inexpensive and simple. This is a significant advantage.

This is possible because the SAW filter adopted makes it

possible to achieve a precise control of the spectral form, it is therefore possible to design the SAW filter in such a way that the harmonics generated by its time response to a pulse is below the limits envisaged by any standard. This obviates the necessity for power control since the shaping, that is to say the forming in time, is done automatically by the filter SAW, namely by a passive element. This is of very low cost and is very advantageous . The SAW technology filter has not been chosen only for this, but also for other reasons in that it is the best technology in cases such as this, the most economical and precise, and the least bulky.

The SAW filter is therefore designed not only to have the optimum output spectrum but also to shape the power variation during transmission, that is to say for eliminating unwanted transmission harmonics.

In this system there is the advantage that the number of components for the 2 GHz part is minimised thereby reducing costs and size. In systems in which space is not a problem, it is possible to make the 2 GHz filters in the form of passive filters, made simply by tracks on a printed circuit, at a cost which is therefore absolutely insignificant. Alternatively ceramic filters are used.

Naturally, the principle of the invention remaining the same, the details of construction and the embodiments can

be widely varied with respect to what has been described and illustrated, without by this departing from the ambit of the present invention.