Frequency rejection

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The frequency distortion is an (undesirable) modification of the reception frequency due to a change of the input signal strength.

conditions

Only heterodyne receivers that are fitted with amplifier tubes and equipped with a signal strength controller that influences the oscillator frequency are affected by the frequency rejection . It is particularly relevant in the reception frequency range from 2.3 MHz to 26.1 MHz. The frequency distortion arises relative to the reception frequency. It follows from this: the higher the reception frequency, the greater the frequency distortion in absolute terms and the more disruptive it is.

Mechanism of action

A signal strength control ensures that the output signal strength is constant even if the input signal strength fluctuates. It works as an amplifier with a variable gain. The gain is inversely proportional to the input signal strength.

In an amplifier tube, the amount of current flowing through its electrodes affects (undesirably) the capacitance between those electrodes.

In the case of an oscillator, the capacitance influences the oscillator frequency.

In the case of a heterodyne receiver, the oscillator frequency influences the reception frequency.

Because the signal strength regulator of the receiver affected by the frequency distortion affects the oscillator frequency, changes in the input signal strength cause changes in the receiving frequency.

Effects

If the reception frequency corresponds to the transmission frequency for a certain input signal strength, the reception frequency will move away from the transmission frequency if the input signal strength is increased or decreased. The greater this distance, the more the reception quality deteriorates. This deterioration in reception quality can be heard in two ways with amplitude modulation . On the one hand, the sound of the useful signal becomes brighter. On the other hand, the relationship between the useful signal and the interfering signal deteriorates. The interference signal can consist of noise, crackling thunderstorms, electrical device radio interference and competing radio signals.

In the case of a transmission path that causes strongly fluctuating input signal strengths in the receiver, reception can become unusable because of the frequency rejection. If these fluctuations are slow enough, the operator of the receiver can maintain the usability of the reception by constantly adjusting the reception frequency. The usefulness of the reception can therefore depend on the skill of the operator.

literature

  • BJ Deketh: Fundamentals of tube technology . In: Philips series of books on electron tubes . 2nd Edition. tape I . NV Philips' Gloeilampenfabrieken (Philips Industries), Eindhoven 1941.
  • BG Dammers, J. Haantjes, J. Otte, H. Van Suchtelen: Applications of the electron tube in radio receivers and amplifiers . In: Philips series of books on electron tubes . tape IV . NV Philips' Gloeilampenfabrieken (Philips Industries), Eindhoven 1949.
  • Ludwig Ratheiser: The great tube manual . Franzis-Verlag, Munich 1995, ISBN 978-3-7723-5064-1 .