STATE OF THE ART Phase Locked Loops (PLL) are used, e. g. for obtaining an output clock frequency which follows an input clock frequency.

A phase locked loop can for example comprise a voltage-controlled oscillator (VCO), which generates a frequency depending on an input voltage. However, such an oscillator is not in practice ideal since the output frequency can be affecte by, for example, the supply voltage, temperature, aging of components, humidity and atmospheric pressure. Therefore, the oscillator should be comprise in some type of control circuit. A number of different control circuits are possible.

In the US patent US 4,787,097, a phase locked loop having a feed back control circuit is shown.

In the US patent US 5,463,351, a phase locked loop having two parallel feed back control circuits, where either the first or the second of the control circuits is selected is shown.

In the US patent US 4,829,258, a phase locked loop having two feed back control circuits connecte in series is shown.

SUMMARY The present invention is based on the understanding that there are two contradictory problems to solve in the design of a control circuit for a voltage controlled oscillator, which is to generate an output frequency which is to follow an inputfrequency.

The first problem is that normally it is not desired that the output frequency follows the input frequency exactly, jitter in the input frequency signal should be attenuated. This requires a relatively slow control circuit having a relatively low limitfrequency.

The other problem is that a voltage controlled oscillator is affecte by for example supply voltage, temperature, aging of components, humidity and atmospheric

pressure. This requires a relatively fast control circuit having a relatively high limit frequency.

None of the documents described in the state of the art has been able to identify these two contradictory problems and has not been able to solve them.

The object of the present invention is to solve these two problems using a circuit a bit like a cascade circuit. A fast inner control circuit, having an inner controller and a stable oscillator is used to remove the noise in the voltage-controlled oscillator.

An outer slow control circuit having an outer controller is used in order for the output frequency to follow the input frequency, without following possible jitter in the input frequency.

Simplified, one could say that the inner control circuit is used to provide a stable "basic frequency"in the output frequency. The deviations in the input frequency from the basic frequency is then added to this basic frequency, resulting in that the output frequency follows the input frequency in a"soft"way.

The avantage of the invention is that noise in the voltage-controlled oscillator is removed at the same time as the input frequency is followed and that jitter in the input frequency is attenuated. Other avantages are good frequency and phase stability and that a large working range is obtained and that a low limit frequency (cut off frequency, modulation band width) can be used for attenuating the jitter in the input frequency.

In one embodiment phase meters are used for calculating phase differences, despite that the voltage controlled oscillator primarily controls the frequency. The problem with this is that it becomes necessary to consider both phase and frequency simultaneously. The embodiment relies on the understanding that a phase difference can be seen as an integration of frequency difference and thus either a differentiation or an integration must be provided at suitable locations.

According to the embodiment an additional integration is provided in the outer control circuit making the outer control circuit generate an outer output signal consisting of a phase ramp. The avantage of this embodiment is that a simple useful circuit without unnecessary elements is obtained.

An alternative to this embodiment relies on the understanding that the frequency integrations in the inner control circuit will provide phase ramps, but that a phase meter only has a limited range, such ars-1800 to 180°, which results in a phase jump of +360° and-360°, respectively) at the interval boundaries. This problem is

solved by making a corresponding compensation of-360° and +360°, respectively, on the output signal from the outer controller.

In one embodiment the location of an addition and a subtraction according to A+ (B-C) = (A+B)-C is interchanged in the implementation. The avantage of this embodiment is that a simple circuit without any unnecessary elements is obtained.

One embodiment relies on the understanding that there may be problems if the input frequency signal disappears. This is solved according to the embodiments by that, simply put, the outer controller is"frozen"so that the outer output signal provides the same phase ramp as before the signal disappeared. The avantage of this is that a relatively good output frequency is obtained even in the case of an error in the input frequency signal.

The invention will now be described in more detail by means of preferred embodiments and with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows a voltage-controlled oscillator in a feedback control circuit.

Figure 2 schematically shows a cascade connecte control circuit according to the state of the art.

Figure 3 schematically shows a voltage controlled controller according to the invention in a circuit a bit like a cascade circuit.

Figure 4 shows a more detailed implementation of the control circuit in figure 3.

DESCRIPTION OF PREFERRED EMBODIMENTS A voltage-controlled oscillator (VCO) is an oscillator having an output frequency which can be controlled by a reference voltage. A voltage controlled oscillator is however rarely ideal and in order to be sure of the obtained output frequency it is suitable to let the oscillator be part of some type of feedback circuit. What this could look like is briefly shown in figure 1.

A controller 1 receives an input frequency fin which is to be followed, and an output frequency fut from a voltage controlled oscillator 2. In response to the difference

between the input frequency fin and output frequency fUt the controller 1 generates an output signal which controls the oscillator 2 towards the input frequency fin.

The interferences which may affect the oscillator 2 can for example be variations in temperature and supply voltage. It is desirable that these interferences are eliminated as quickly as possible. The control circuit 1 should therefore be fast.

On the other hand it is not desirable from a stability point of view that the output frequency fut follows the input frequency fin too quickly. Possible jitter in the signal with the input frequency fin should be attenuated. The controller 1 should therefore be slow.

Conclusion: the controller 1 should both be fast and slow. Impossible? The present invention is based on that these contradictory problems are realized and by solving them with a circuit a bit like a cascade circuit, which in a simplifie version is shown in figure 3.

However, first a cascade type control circuit previously known from general control applications is considered in figure 2. The system to be controlled comprises two subsystems 11,12 connecte in series which are controlled by two feedback control circuits 13,14 located inside each other.

In an inner fast control circuit 13 a first controller 15 control the first subsystem 11 in response to a first input parameter SPi, which results in a first output parameter PVi ! which is fed back to the first controller 15. The first output parameter PVi in turn acts on the second subsystem 12, which in its turn provides a second output parameter PVy, which is fed back in an outer slow control circuit 14 to the second controller 16. The second controller 16 in turn controls the first controller 15 using a second input parameter SPy.

Thus, the object is to control the second output parameter PVy by using the second input parameter SPy in order to control the first output parameter PVi. A common example is to control the temperature of a fluid by means of controlling a cooling water flow. Interested readers can read more in for example Glad, Ljung: "Reglerteknik. Grundlaggande teori", chapter 6.2 or Hamos:"Reglerteori", chapter 15.5.

It is however not known to use a cascade circuit in order to control a voltage- controlled oscillator. To control frequencies and phase poses special problems.

Also, it is not possible to divide the system into subsystems, which is one of the main ideas for using a cascade circuit. Instead, a control circuit which is briefly shown in figure 3 is used according to the invention.

Very simplifie the circuit operates according to the following: A stable non-modulatable oscillator 27 is used, the frequency of which is divided to a reference frequency fref which is close to an input frequency fin which an output fut is to follow. This reference frequency fref is fed, via a sum circuit 28, into an inner controller 25 in an inner fast control circuit 23. The function of the sum circuit 28 will be explained below.

The inner controller 25 then provides an inner output signal PDi, which controls a voltage controlled oscillator 21, which in turn provides an output frequency futX which is fed back to the inner controller 25.

Thus, in order to eliminate noise N in the voltage controlled oscillator 21 a fast inner control circuit 23 and a constant frequency fref from a stable oscillator 27 is used.

The input frequency fin is fed to an outer controller 26 in an outer slow control circuit 24. The outer controller 26 then provides an outer output signal PDy which is summed 28 with the reference frequency fief, whereupon the sum is fed to the inner controller 26. The outer controller 26 thus controls the output frequency fut via the inner controller 25. Finally the output frequency fut is fed back to the outer controller 26.

Thus, in order for the output frequency fut to follow the input frequency fin, without following possible jitter in the input frequency fin, a slow outer control circuit 24 is used.

One could say that the inner control circuit 23 is used to provide a stable"basic frequency"in the output frequency futs having the interferences in the voltage controlled oscillator 21 eliminated. The deviations of the input frequency fin from the basic frequency are then added to the basic frequency, which results in that the output frequency fUt follows the input frequency fin in a soft way.

In one area of automatic engineering it is common to look at it from the controller point of view and then draw the signals directly to the control circuit, as in figures 2 and 3, the signal to be controlled is called"process value", the signal to be

controlled after is called"setpoint"or"reference value"and the output signal from the controller is called"process demand"."Control errors"are often defined as setpoint minus process value.

Accordingly, in the outer control circuit 24 the input frequency fin is the setpoint, the output frequency fut is the process value and the output signal PDy from the outer controller is the process demand. However, in the inner control circuit 23 the outer output signal PDy from the outer control circuit added with the reference frequency fref is the"setpoint", the output frequency fut is the process value and the inner output signal PDi from the inner controller is the process demand.

In practice some complications can arise in the implementation. Therefore, a practical embodiment is explained below. It is however to be understood that also other embodiments are possible as long as the function in figure 3 is obtained.

The embodiment is shown in figure 4, using the same reference numerals as in figure 3. It is however to be noted that the subtractions in figure 4 are shown separately from the controllers in contrast to figure 3. Software implementation is shown with a dashed line in the upper part of the figure, whereas hardware implementation is shown with continuos lines in the lower part of the figure.

It is most suitable to implement the controllers in software in order to achieve desired limit frequencies. It is however possible to implement the controllers in hardware instead.

It was previously stated that the frequency was to be controlled. In practice it is however probably easiest to control the phase. Thus, a phase meter 3231, is used to detect phase differences. For reasons of implementation it can then also be suitable to interchange the location of the addition and subtraction before the inner controller 25. As is known A+ (B-C) = (A+B)-C.

This results in that the setpoint in the inner control circuit 23 can not be obtained directly (if anyone would like this), but it is only implicit in the circuit.

The reason for that it is suitable to interchange the place of the addition and subtraction is that the outer controller 26 provides a digital signal, whereas the stable oscillator 27 provides a frequency signal. In order to add these two signals some kind of converter would be required.

The phase meter however receives two frequency signals, mesures (i. e. counts) the phase difference between them and generates a digital signal which represents the phase difference. The phase meter can therefore in a way be seen as a converter. To interchange the place of the subtraction and addition it is therefore an intelligent way of obtaining a simple circuit solution.

Thus, in the inner control circuit 23 the phase of the output frequency fUt is subtracted from the phase of the reference frequency fret in an inner phase meter 31. The inner phase difference freffut, which is in a digital form, is added in software 33 to the outer output signal PDy from the, in this case, software implemented, outer controller 26. The sum PDy+ (fre,-fut) corresponds to an inner control error (PDy+fre,)-fut which is fed to the, in this case software implemented, inner controller 25.

The inner output signal PDi from the inner controlier is fed, via a not shown D/A converter, as a voltage to the voltage controlled oscillator 21, which as stated earlier provides an output frequency fut.

In the outer control circuit 24 the phase of the output frequency fut ils subtracted from the phase of the input frequency fin in an outer phase meter 32. The difference in phase, i. e. the outer control error fin-futl which in this case is in a digital form, is fed to the outer controller 26 which provides an outer output signal PDy, which, as has already been stated, is added in software 33 to the inner phase difference fret-fut from the inner phase meter 31.

The complete control circuit thus controls phase, but the voltage controlled oscillator 21 strictly speaking does not control the phase, but the frequency. Since phase difference can be seen as an integration of frequency difference, the oscillators 21,27 can be seen as having built-in integration functions. This should be observe in the remainder of the circuit.

The inner phase meter 31 counts the difference between the phase of the reference frequency fref and the phase of the output frequency fut Since the output frequency fUt is also controlled by the input frequency fin, there will in practice always be an inner phase difference freffut which is not equal to zero from the inner phase meter 31, i. e. a phase ramp is obtained.

The inner phase difference freffut shall, as has been stated, be added to the outer output signal PDy. This results in the inner control error PDy+fref-fut, which is fed to the inner controller 25. If the inner controller 25 were to control using a phase

ramp as input signal a complicated controller would be required. Furthermore, the contribution from the outer output signal PDy would loose in significance the larger the phase ramp became, which in the long run would result in that the output frequency fUt would stop following the input frequency fin In order to obtain the desired function in the control circuit, the inner control error PDy+f,, f-fut should therefore preferably not be in the shape of a phase ramp. The simples way of obtaining this is to let the outer controller 26 provide a phase ramp which corresponds to the phase ramp from the inner phase meter 31, but with a different sign. This can be obtained by letting the outer controller 26 comprise an additional integration.

Theoretically one could instead differentiate the frequency signals from the oscillators 21,27 but this would probably be more complicated to implement in practice.

A conventional controller can, as a basic rule, have three different control parameters, which can be summed as follows: A large proportional parameter kp will result in an increased speed in the controller, but also most often to a reduced stability. The use of an integral parameter k ; will eliminate remaining errors in the output signal, but reduces stability the larger the parameter becomes. The use of a derivative parameter kd can increase stability, but differentiation of noisy measured signals can be hazardous. A careful balance of said parameters is thus required depending on the requirements to the controlling.

The inner controller 25 can suitably have a control function (filter function) which in Laplaceform is written as follows: kpi #(1+###), i. e. a proportional parameter and an integral parameter. The addition of a derivative parameter is possible, but not necessary.

The outer control circuit can on the other hand suitably have a function which in Laplaceform is written as follows: kpy #(1+###)##, i. e. a proportional parameter and an integral parameter, and an additional integration in order to provide a ramp. The addition of a derivative parameter is possible, but not necessary.

It can be added that in practice there are three different types of controller implementations which usually are used-ideal, serial (also called lead-lag) and parallel. In these different types the proportional parameter, the integral parameter

and the derivative parameter are designed and combine in somewhat different mannes. They are however so alike and function in such similar manners that in this context they must be regarde as equivalent.

The fact that the inner phase meter 31 generates a phase ramp also results in another problem. A phase meter normally operates in the range -180° to +180°. Every time the phase meter crosses the range boundary it thus jumps +360° or -360°. The outer controller 26 however provides a continuous output signal PDy.

This is simples compensated by adding-360° and +360°, respectively to the output signal PDy from the outrer controller, so that after the addition 33 there is no jump in the inner control error PDy+fref-fut which is fed to the inner controller 25.

If the signal with the input frequency fin shouid disappear another problem occurs. The voltage-controlled oscillator 21 can in this case in an easy manner be set to continue generating the same frequency as before the disappearance of the signal having the input frequency fin This is obtained by feeding a constant value zero to the outer controller 26 instead of the outer control error from the outer phase meter 32.

Hereby the outer controller 26 believes that there is no control error and will therefore continue to generate the same phase ramp as before.