![]() ![]() For larger values of kQ, a dip occurs in the middle of the passband. Both LC circuits must be adjusted on the same frequency. For kΩ = 1.2 a virtually flat transmission characteristic is obtained. The quality factor Q per circuit and the coupling factor k jointly determine the bandwidth and flatness of the filter characteristic. The formerly commercially available pot cores with dimensions of 18 mm Ø and 11 mm height and the code 2000N28 of the Siemens manufacturer were suitable for self-induction at low frequencies. The ų value, converted to the value for the inductance per winding in the square, was about 4000 nH /w 2 at 1500 Hz. This value applied to the windings within the hermetically sealed pot core without an air gap. To be able to finetune the bandpass filter with the required centre frequency and bandwidth, the choice was made for pot cores with an air gap where the ų value for the inductance could be changed with the control pins to be fitted. The magnetic permeability of an object is a number that indicates how the object behaves within a magnetic field. In other words, the permeability corresponds to the degree to which an object can be magnetized. The absolute permeability ų of a magnetized body is: the inductance achieved by the strength of a magnetic field in which it is located: ų = B / H with B = total flux per surface unit (field density) or induction H = the magnetic field in Ampere/m 2. In order to keep the dimensions of the components for the filter limited, the structure was chosen with tuneable coils (self-inductance) in Ferroxcube (Siferrite) pot cores. The capacitors (capacities) to be used were also of a limited size. When choosing the pot cores, the high magnetic permeability of the Ferroxcube (Siferrite) material played an important role. ![]() In the design of the bandpass filter, it was assumed that the manually entered Morse information could not be generated faster than with a speed of 75 baud, which corresponds to 75 bits (marks) per second, or a frequency of 75 Hz. In order to be sure of the least possible distortion for a small shift of the carrier wave for this Morse signal of 1500 Hz, for example, the bandwidth for the band filter is set at 200 Hz, the bandwidth is defined as follows: the bandwidth is that width at which the bandwidth the amplitude of the lowest and highest frequency -3 dB (about 0.7 of the Vmax) is attenuated relative to that of the central transmission frequency. ![]() the inductive elements of the circuits must be tunable. ![]() A bandpass filter in its simplest embodiment is composed of two coupled single oscillator circuits. In the application of inductive and capacitive elements for the filter, the coupling is carried out with a capacitor. When processing into digital signals, the shape and thus the frequency content (loss of higher harmonics) is affected when using a too narrow filter. This means that the widest possible frequency passage for the modulated frequency is required. In order to ensure the suppression of the secondary frequencies, at least one more extensive filter, a so-called bandpass filter, was necessary. In order to be able to separate low-frequency signals (<10 kHz) from their environment spectrum of frequencies, a filter is required. A frequency filter built up with a single coil and capacitor is usually insufficient to transmit a (digital) signal in an undisturbed manner. With the sending of Morse signals, in which a carrier frequency is set to the ON and OFF (so-called A1 modulation) in the rhythm of an activated Morse-key, and the later digitisation of other information, the processing of signals as undistorted as possible after being received was needed in the ’70s. Bandpass filter design for processing Morse signals (1970) ![]()
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