in the description that follows , the same reference signs are used for similar and similarly acting parts . fig1 shows the essential components of an electrosurgical instrument . these comprise an operating unit 5 for activating and selecting a cutting and / or coagulation mode , an electrosurgical instrument 20 for applying an hf treatment current i actual , an hf generator 10 for generating the hf treatment current i actual , a measuring device 50 for generating a current signal i ( t ) and a voltage signal u ( t ) from the hf treatment current generated i actual and a control device 30 for controlling the hf generator 10 . seen as a whole , at a time point t , the hf generator 10 provides a treatment current i actual at an actual voltage u actual , with which the electrosurgical instrument 20 is operated . from this value , the measuring device 50 determines the characteristic current signals i ( t ) and voltage signals u ( t ) for this apparent power s . the control device 30 processes the current signal i ( t ) and the voltage signal u ( t ) as well as the operator signals in , which are input by the user of the electrosurgical instrument 20 with the operating unit 5 . based on these signals , the control device 30 determines corresponding control signals d ( t ), by means of which the hf generator 10 is adjusted . these control signals d ( t ) comprise a voltage control signal u target . in order to enable appropriate control of the hf generator 10 , the control device 30 comprises a processor which is configured to carry out various operations for processing the current signal i ( t ) and the voltage signal u ( t ), and a corresponding memory device which enables results and / or settings and / or other data to be stored in the short term or lastingly . the control device 30 is therefore configured , inter alia , to implement the regulator 31 described below and a corresponding regulating loop . fig2 shows , in schematic form , a regulating loop which is used for control of the hf generator 10 according to the invention . the hf generator 10 operates a monopolar electrosurgical instrument 20 , which is connected thereto via a first line . the electrosurgical instrument 20 comprises a first electrode 21 for applying an hf treatment current i actual . the second electrode 22 , which is also connected via a line to the hf generator 10 lies directly against the tissue 1 to be treated . this is a neutral electrode which creates a large area contact with the tissue 1 to be treated or the body of the patient . during operation of the regulating system , a target value of the real current i target is pre - set by the operating unit 5 . a regulator 31 determines a voltage control signal u target from this target value of the real current i target and from a real current i real by means of a comparator or an error amplifier 34 . the hf generator 10 applies a corresponding voltage to the electrodes 21 , 22 . this produces a treatment current i actual . the measuring device 50 detects the current signal i ( t ) and the voltage signal u ( t ) and determines a real current i real by means of a real current calculation unit 33 . the real current i real is compared , as previously mentioned , in said error amplifier 34 , with the set target value of the treatment current i target and is converted into a corresponding voltage control signal u target . this voltage control signal u target is fed to the hf generator 10 as previously described . a regulating loop therefore comes into being which continuously determines the real current i real from the hf treatment current i actual and adjusts the voltage control signal u target such that the value of the difference between the real current i real and the target value of the real current i target is as small as possible . a substantial part of the present disclosure lies in determining , in a precise and error - tolerant manner , the real current i real or , more generally expressed , the ratio between the real power p and the reactive power q of the above - described system . for this purpose , four different methods are described below . calculation of the real current i real initially involves the formation of a power factor cos φ , which puts the real current i real and the apparent current i apparent in relation to each another . in the exemplary embodiment above , the apparent current i apparent corresponds to the treatment current i actual . therefore : according to the power triangle , the following relation exists between real power p , reactive power q and apparent power s : s =√{ square root over ( p 2 + q 2 )} ( formula 2 ) in a first exemplary embodiment , in order to calculate the real power p and the reactive power q , the hilbert transform is used ( see fig3 ). the known hilbert transform causes a frequency - independent and amplitude - neutral phase shift of time signals by 90 °. however , as far as the further processing of the signal is concerned , it is important that the digital hilbert operator , which is implemented as an fir filter , has a throughput time that is typical of fir structures . it is therefore important that chronologically associated values of current and voltage are always processed together . in the first exemplary embodiment , a reactive power mean value q m is determined by the use of the hilbert operator on the time - dependent current signal i ( t ) and subsequent multiplication by the delayed voltage signal u ( t ). the mean value of the reactive power q , i . e . the reactive power mean q m , can be obtained by averaging over at least one voltage waveform period or current waveform period . calculation of the real power mean p m is carried out by direct multiplication of the current signal i ( t ) by the voltage signal u ( t ), wherein , here also , averaging takes place over at least one period . by evaluating the relationships in the right - angle power triangle ( apparent power s is the hypotenuse , real power p is the adjacent side and q is the opposite side to the angle φ ), the power factor cos φ can be calculated by the following two methods ( see also formula 1 ): the control system 30 described can therefore calculate the real current i real using the hilbert transform and by means of the formulae 1 and 3 or 1 and 4 . a second method for calculating the real power p and the reactive power q can also be provided using the hilbert transform ( see fig4 ). in this case , the voltage signal u ( t ) is transformed and multiplied by the delayed current signal i ( t ). the reactive power mean value q m is found by averaging the values calculated in this way . the real power mean value p m is found by multiplication of the current signal i ( t ) by the voltage signal u ( t ) and subsequent averaging of the values . using the reactive power mean q m and the real power mean p m , the power factor cos φ can be determined with the above formula , so that a relation can be established between the real current i real and the apparent current i apparent . a third method for determining the real power p is as shown in fig5 . firstly , the current signal i ( t ) and the voltage signal u ( t ) are subjected to a discrete or fast fourier transform ( dft , fft ). in order to minimise the calculation effort required , an fft is preferably selected as illustrated in fig5 . as the result of the fft , a vector of complex number values is obtained , comprising real and imaginary components of the current signal i ( t ). following complex conjugate multiplication , the power is given , separated into real component ( p ) and reactive portion ( q ) as follows : u · i * =| u | e jφ u ·| i | e − jφ i =| u |·| i | e − j ( φ u − φ i ) = p + jq ( formula 5 ) by totaling the vector values of the real power p and by a separate totaling of the reactive power q , the real power mean p m and the reactive power mean q m are obtained . the power factor cos φ can be determined as described above by using the formulae 3 or 4 . according to the disclosure , there are two methods for determining the apparent current i apparent . the value of the apparent current i apparent can be found from the complex results of the fft by absolute value formation : alternatively , totaling of the real - value instantaneous values of the current over a period can be undertaken . in a fourth method , the real current p can be determined from the effective values of current i eff and voltage u eff . for sinusoidal current and voltage forms , the real power p is defined by means of the power factor cos φ as follows : if the real power p is replaced with the real power mean p m , then the power factor cos φ can be determined from the real power mean p m and the effective values of the current and voltage variations i ( t ), u ( t ). thus , for the power factor cos φ the following then applies : a calculation of the real power mean value p m can be carried out for n sample values as follows : the effective value u eff of the voltage is found , over n sample values , from the equation which defines it : in a similar way , the effective value of the current i eff can be found : making use of the power factor cos φ thus obtained , the real component of the current is found from : the last method is characterised , in particular , by the fact that relatively few operations have to be performed to determine the real current i real . however , using modern digital signal processing electronics , methods 1 to 3 can also be implemented .
an electrosurgical generator for the treatment of biological tissue . the electrosurgical generator comprises a generator part which supplies a high - frequency treatment current with an hf voltage set according to a voltage control signal . a measuring device detects the hf treatment current and the hf voltage and generates corresponding current and voltage signals . the current signal and the voltage signal are fed to a conversion device which forms a real current signal corresponding to the real component of the hf treatment current . a regulation device compares the real current signal with a pre - settable target value and generates the voltage control signal on the basis of the comparison . the real component of the treatment current is determined and the generator part is adjusted so that said real component approximates to a pre - determined target value .