Background of the invention In a radio ASIC there is a need for a stable frequency to move information up and down in frequency. This stable frequency is normally generated by locking an unstable VCO (Voltage Controlled Oscillator) to a very stable reference fre- quency, e. g. 13MHz, using a PLL (Phase Locked Loop)-circuit. In figure 1 a PLL- circuit is disclosed containing a phase detector 150, a filter&lifier 160, a VCO and a divider with e. g. a dividing factor of 72. The PLL strives for maintaining the same signal frequencies at the inputs I, II of the phase detector. If for example the signal at input I of the phase detector has a reference frequency of 13 MHz (13MHz-clock in the telephone) the other input II of the phase detector 150 strives for having the same frequency. This means that the VCO must run at a frequency of 72 multiplied by 13MHZ equals 936MHz, since the divider divides the signal from the VCO by a factor 72. Thus, the output signal III of the VCO is a very sta- ble 936 MHz-signal, which could be used for the GSM-band.

As mobile terminals constantly decrease in size, more and more compo- nents have to be integrated on the same ASIC (Application Specific Integrated Circuit). The integration of the VCO on the ASIC will cover a major part of the

ASIC area, and since the area cost is the main part of the total ASIC cost it is im- portant to minimise it. It should be realised that up to now the inductor in the reso- nator circuit of the VCO has always been implemented outside the ASIC-circuit since it was not possible to make sufficiently good inductors on the chip. Thus, the invention deals with a new inventive area, i. e. how to implement the whole VCO including the inductor of the resonator inside the ASIC-circuit. A problem when integrating a VCO on an ASIC is that there are no external components, which could be used to trim the frequency of the VCO. Therefore, the VCO tuning range has to be large enough to cover all component spread and temperature variations.

When increasing the tuning range of the VCO, the phase noise of the VCO in- creases, which implies bad noise performance of the VCO. Another problem is that if a large band is to be covered, it may be necessary to use two separate VCO: s, which of course will consume a lot of space on the radio ASIC. The object of the present invention is to solve these problems.

Summary of the invention The above object is achieved by means of an integrated VCO, preferably in a radio ASIC, wherein a switch is arranged for switching the frequency of the VCO between arbitrary frequency bands to be able to perform frequency band switching and/or frequency tuning.

Instead of using one VCO for the DCS-band and another one for the PCS- band as in prior art, the present invention uses only one VCO for both bands sav- ing lots of valuable space on the ASIC.

Another benefit as can be seen in claims 2 to 5 is that the band switch for the VCO is software controlled implying that no manual trimming is required. The trimming is carried out by the mobile phone itself.

The switch of the invention points out the importance of having a slow di- ode, i. e. a diode with a large transit time. This is achieved with a collector-base junction in a bipolar process as is claimed in claim 8.

A further advantageous embodiment of the switch for the VCO is disclosed in claim 10 and 11, wherein the switch is on when the diodes are forward biased and off when the diodes are reverse biased acting as varicaps.

In a preferred embodiment of claim 13 several switches are arranged for the VCO to be able to control and switch between an arbitrary number of frequency bands of the switch.

In an embodiment as is claimed in claim 3, it is proposed that the switch for the VCO could be controlled by means of a logic circuit via a wireless communi- cation link, e. g. Bluetooth.

In another embodiment of claim 14 the switch is integrated with the VCO.

Other characteristics of the invention are set out in the other dependent claims.

Brief description of the drawings The present invention will now be described in more detail with reference to preferred embodiments of the present invention, given only by way of exam- ples, and illustrated in the accompanying drawings in which: Fig. 1 illustrates the VCO-function in a PLL; Fig. 2 discloses a schematic view of the switching function of the present invention; Fig. 3 illustrates an implementation of the VCO in the present invention; and

Fig 4 is a preferred implementation of the switch in the present invention.

Detailed description of embodiments of the invention The embodiments that now will be described with reference to the figures relate to a circuit for switching the centre frequency of the VCO to be able to per- form band switching or frequency tuning in an integrated radio circuit, e. g. a radio ASIC. It should of course be realised that the switching technique of the invention could be used in any VCO: s in other electrical circuits not related to radio. It should also be emphasised that this invention is related to pending applications ti- tled"A Dual band VCO"and"A Stacked VCO Resonator", applicant: Telefonak- tiebolaget LM Ericsson, inventor: Magnus Nilsson. These applications,"A Dual band VCO"and"A Stacked VCO Resonator", respectively are herewith to be in- corporated with this application by reference.

Fig. 2 discloses in general terms the switching function in the resonator 100 of the integrated VCO. The coils 10,20 are coupled in parallel with the capacitor 30, the varicaps 40,50 and the switching means 60,70,80,90. When the switch- ing means 60,70 is closed the capacitors 80,90 are connected to the resonator im- plying a higher capacitance in the resonator, which in turn results in a lower fre- quency. On the contrary when the switching means is open, the capacitors 80,90 are disconnected from the resonator implying a lower capacitance in the resonator, which results in a higher frequency. Thus, the switching means in the present in- vention is used for moving the frequency from one arbitrary frequency band to an- other arbitrary frequency band.

In a preferred embodiment the switching means in the VCO moves the fre- quency between the DCS-band (1800-1900 MHz) and the PCS-band (1900-2000 MHz). The switching means is also used for trimming the centre frequency of the VCO. This means that if the VCO frequency is slightly deviating from the centre

frequency, the switching means 60,70,80,90 trims the VCO frequency back to the centre frequency.

In prior art one VCO has been used for the DCS-band and another VCO has been used for the PCS-band. Thus, in the present invention only one VCO is used for controlling both the DCS-band and the PCS-band. The switching means 60, 70,80,90 of the VCO integrated in a radio ASIC is externally software controlled from a logic circuit via a data bus. In this case both the radio ASIC and the logic circuit are arranged on the same PCB (printed circuit board) in a mobile telephone.

It should be realised that the ASIC and the logic circuit could be arranged in an arbitrary electrical machine such as for example a computer. The ASIC could also be controlled from a separate electrical device via a wire based or wireless com- munication link. An external device could for example use the communication protocol Bluetooth to control the switching means for performing band switching or fine tuning of the VCO in the ASIC.

As could be seen from above, if the centre frequency of the VCO could be switched, the tuning range of the same VCO could be reduced. This would im- prove the noise performance of the VCO and the VCO would be able to cover multiple frequency bands.

As could be seen from figure 2, the ideal situation is to switch in a capacitor 80,90 into the resonator using a switch 60,70, the impedance of which varies between zero and infinity. It is important that the ratio between on-and off-im- pedance is large in order not to reduce the tuning range of the VCO and not to re- duce the Q-value of the resonator. The Q-value is defined as quotient between the reactive power and the inserted power of the resonator.

Figure 3 discloses an integrated VCO 130 according to the present inven- tion. As can be seen in figure 3, the VCO comprises a differential stage 140 (ac- tive part) and a resonator 120. The resonator contains inductors 110, coupling ca-

pacitors 210,240, resistors 170 and varicaps 220,230. A varicap is defined as a voltage controlled capacitance.

Figure 4 discloses an implementation of the switch of the invention corre- sponding to the switching means 60,70,80,90 in figure 2. In a preferred em- bodiment of the invention, a slow diode 60,70 is used as a switch component as can be seen in figure 4. Using a slow diode means that it has a large transit time.

Such a diode is normally present in a bipolar process in the collector-base junc- tion.

In the on state, the diodes 60,70 are forward biased and the capacitance of the diodes is proportional to the current through the diodes and the transit time of the diodes. In order to get a large capacitance, the product of the current and the transit time has to be large. To reduce the current consumption of the switch 60, 70,80,90 it is important to have a large transit time of the diodes. The series re- sistance of the diodes has to be low in order not to reduce the Q-value of the reso- nator. A large diode area gives a low series resistance.

In the off state the diodes 60,70 of are reverse biased and the diodes act as varicaps. The capacitance of the varicaps is proportional to the area and this has to be small in order to make the capacitance of the varicaps much smaller than the coupling capacitance.

The two area requirements give a trade-off when selecting diode size.

The switch 180,60,70,80,90 in figure 4 discloses an implementation of the switching function for the VCO 130 in a radio ASIC, which has been success- fully tested in the laboratory. The switch 180 integrated in the radio ASIC com- prises a control part 190 (differential stage) and a switching part 200. The band switch is represented by the coupling capacitors 80,90 and the diodes 60,70, which are externally controlled by a logic circuit via Bsel and Bsel_bar, respec-

tively. Thus, the signals on these nodes (Bsel Bselbar) determine the switching of the centre frequency of the VCO in figure 3. A logic circuit (base-band ASIC, not shown) arranged outside the radio ASIC controls the switch 180,60,70,80,90 by applying a specific predetermined voltage on these nodes. Typically, the switch in the ASIC or integrated in the VCO is software controlled from said logic circuit via a data bus. The nodes Out and Out bar, respectively of the switch in figure 4 are connected to the nodes Out and Out bar of the VCO in figure 3. Depending on the control voltage, from the external circuit, over the nodes Bsel and Bselbar of the switch, a predetermined capacitance 80,90 will be switched in or out, via nodes Out and Out bar, in parallel with the resonator 120 of the VCO 130 chang- ing the frequency band of the VCO.

When analysing figure 4 in detail, it is seen that a bipolar band switching circuit is used. When the band switch is turned on, current runs from transistors Q2 and Q3, respectively through the diodes 60,70 and through transistor Q4.

Now, the band switch capacitance is dominated by the coupling capacitors 80,90 and the VCO frequency is lowered.

When the band switch is turned off, Q 1 is enabled which acts to pull N 1 close to Vcc. The resistors R1, R2 and R3 pull N2 and N3 to ground, which im- plies that the diodes 60,70 are reverse biased. Now, the capacitance of the band switch is dominated by the varicap capacitance (diodes 60,70) resulting in an in- creased VCO frequency. In this way a high performance integrated band switch is implemented in the VCO, which makes it possible to perform band switching and/or frequency tuning.

In a preferred implementation, the VCO is integrated in a radio ASIC ar- ranged in a mobile terminal. The switch 180,60,70,80,90 in the ASIC is electri- cally controlled by a base band ASIC through a bus between the two ASIC: s.

The frequency band switch 180,60,70,80,90 mentioned above is most useful in an integrated VCO. This switch is preferably dimensioned to switch the

frequency bands between DCS and PCS. It is of course possible to have several different switches for tuning and switching the frequencies between other bands than PCS or DCS.

In an integrated application, component spread is given by the IC-process and cannot be reduced. Therefore, these switches constitute a requirement for building an integrated VCO. The switches 180,60,70,80,90 are very small and the IC-process is unchanged, which implies that the excess costs are very small.

It would be appreciated by those of ordinary skill in the art that the present invention could be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing de- scription, and all changes which come within the meaning and range of equiva- lence thereof are intended to be embraced therein.