FREQUENCY CHARACTERISTICS OF LINE

When the frequency of the input signals increases, the line attenuation due to both the ohmic resistance R′ and the conductance C′ progressively increases because of the so-called “skin effect”.

Starting from a given frequency onward, the line attenuation quickly increases. The “cut-off frequency” of the line is defined as the frequency value at which the attenuation reaches the value of 3 dB compared to the low-frequency attenuation value.
The purpse of this test is to measure the cut-off frequency for the coaxial line provided on the DL2597. This measurement is performed in conditions of line closed on a load equal to the characteristic impedance.
Required components, instruments and accessories

1. Cable with intermediate sockets
2. Variable line termination resistors
3. Connecting cables and chords
4. Function generator
5. Oscilloscope

Procedure

F10-4 shows the circuit used to perform the test. Substantially, the line is closed on the load Rt and the generator is connected to the input of the line. The internal impedance of the generator is matched to the line impedance through the resistor Ri = Rt. As already said in the previous test, this is true when a generator  is used whose impedance is neglectable when compared to 68 ohms. Otherwise, for Ri a value has to be selected equal to the required resistance (68 ohms) and the output impedance of the generator.
The test consists in scanning a wide frequency range, for instance from 100 kHz to 4 MHz, with suitable amplitude steps, measuring the attenuation in the different measurement points selected.
It is suggested to use for the input signal a peak-to-peak amplitude of 2 V, which is kept constant in the whole measuring range. The results can be gathered in tabular form.
From the table data, which are in the following translated in graphical form, the cut-off frequency of the line can be inferred. As stated above, this is the frequency at which the attenuation is reduced f 3 dB compared to the value measured at low frequency (10 kHz).
We recall that a voltage attenuation of 3 dB corresponds to a signal attenuated to one half compared to the input signal.

F10-4: Measuring the frequency characteristic of the line.