GB694520A - Improvements in or relating to electrical high frequency diathermy apparatus - Google Patents

Abstract

694,520. Power measurements. ULTRASCHALL AKT.-GES. Jan. 11, 1951, No. 861/51. Class 37. [Also in Groups VI and XL (c)] A diathermy apparatus comprises a singlestage high-frequency valve generator A coupled to a mechanically resonant load device E by at least two resonant circuits B, C between the generator valve and load tuned to the natural frequency of the load device and the degree of coupling between the generator and load being such that, with slight or no damping of the load device, a double-humped band-pass filter characteristic is obtained with a deep depression in the middle occurring at the resonant frequency of the load device. The load deviceE may be of piezo-electric or magneto-strictive type generating ultrasonic pressure waves for diathermy treatment of a patient and associated with a power indicating meter D. It is shown that, as the load on the device E increases, the voltage across it increases and the central depression of the double-humped characteristic rises so that a coupling characteristic peaked at the resonant frequency of the load E is produced, and a meter D shunted across the device E may therefore directly measure the ultrasonic power radiated. The Q of the circuit C may be chosen so that its resonance curve agrees with that of the device E. The coupling between the circuits B, C may be varied by a movable earthed screen F or varying the number of turns in the coils. In a modification, the meter D is connected across a separate high Q tuned circuit loosely coupled to the tuned circuit B. In a further modification (Fig. 4), the circuit B is variably coupled by capacitors K, P to a flexible cable terminating in a head or handle comprising the resonant device E and a series resonant circuit H across the capacitance of which the device E is connected. The series inductance may be tuned by a movable ferro-magnetic core or a shunt trimming capacitor. The coupling capacitance P may be that of the cable. The valve generator A preferably operates under class B or C bias. For measuring the power flowing along the cable, two diodes Rl, R2 (Fig. 6) operating on a square law part of their characteristics, may be connected to be responsive to the sum and difference of a proportion of the voltage across the cable derived from potential dividing shunt capacitors M1, M2 and a voltage derived from current transformers K1, K2 linked in opposite ways with the cable, the difference of the rectified voltages between their cathodes indicated on a meter N then measuring the radiated power, including the power factor. An additional voltage derived from that across the device E may be inserted by a potentiometer O into the meter circuit to compensate for power losses in the operating head, or compensation may be effected by making the currents applied to the current transformers adjustably different as by introducing a variable capacitive shunt load at the output of the first transformer.