Recording the Passband Curves of HF Circuits, Radio and Television Receivers

Abstract

In recording the passband curves of HF four-poles, what is required is to show the dependence of the output voltage on the frequency of the input voltage. This means that the horizontal deflection must be proportional to the frequency and the vertical deflection must be proportional to the output voltage. The input voltage frequency must therefore be varied rhythmically to produce the horizontal deflection of the spot. In the process described in Ch. 26, for recording “low frequency characteristic curves”, the horizontal deflection proceeded by means of the variable frequency input voltage obtained from a frequency-dependent network. In recording HF characteristic curves it is often the case that the frequency change of the input — the “frequency modulation” — is derived from the X-deflection voltage. It is obvious that this frequency change — “wobbulating” — can be achieved mechanically by periodically detuning an oscillator. In early publications in which the display of resonance curves on an oscilloscope tube was described, the frequency change was carried out by a motor-operated variable capacitor [1]. The slider of a potentiometer was driven by the same spindle, the potentiometer resistance track being connected to a direct voltage source. Between the slider and one end of the resistance track a voltage which increased linearly with the frequency change was obtained. It was used to deflect the spot horizontally, the horizontal position of the spot thus corresponding at each instant to a particular frequency. If the vertical deflection of the spot is carried out simultaneously by the output voltage of the HF four-pole, then the spot path corresponds to its passband curve. As the output voltage is used directly to deflect the spot vertically, the frequency characteristic is seen as the boundary curve of a luminous area, as will be seen later in the oscillograms of Figs. 18-5 and 18-8.