a high frequency welding device for welding plastic sheets , in which the high frequency energy is provided by fixed - frequency oscillator , preferably by using a quartz crystal amplifier , whose frequency does not change during the welding process . the output circuit is designed to have a resonance curve which , before the occurence of any change in the output and coupling circuit characteristics due to the welding process , intersects with the fixed frequency of the oscillator at a &# 34 ; operating point &# 34 ; which is on the slope of the resonance curve descending to lower frequencies . the circuit components are selected to behave in such a manner that the resonance curve undergoes a shift toward lower frequencies , with the fixed - frequency &# 34 ; operating point &# 34 ; rising across the peak of the resonance curve to a point on the slope of the curve descending to lower frequencies , as the welding process progresses .