CCIF Intermodulation Distortion:

In the second method of carrying out an CCIF Intermodulation Distortion test, the two test signals used have equal amplitude, and relatively high but slightly different frequencies.

When non-linear distortion is present, a difference frequency component appears in the output and is used as a measure of the amount of distortion present.

Figure 16.8 illustrates the frequency relations, where f_a and f_b represent the two test frequencies of amplitude E_a and E_b, and E_d is the amplitude of the resulting difference frequency component f_d.

(Second order combination frequencies E_d2 and E’_d2 are also shown in

Fig. 16.8, and are a measure of the cubic type of distortion. These components

can however be detected only by the use of a highly selective wave analyzer, and are generally not made use of in this form of the intermodulation test.)

The intermodulation distortion is expressed by the relation

This form of intermodulation test has been recommended by the Interna­tional Telephonic Consultative Committee, and is referred to by the initials CCIF.

The basic block diagram setup for measuring intermodulation distortion tests by the CCIF method is shown in Fig. 16.9.

The difference frequency component E_d is separated by a low pass filter N and indicated separately by the voltmeter V₂. The amplitude of test signals E_a and E_b can be determined with the help of V₁. If V₁ is the peak reading instrument, then the peak voltage observed at terminals a — b will be very close to the peak of E_a + E_b.

Alternatively, E_a and E_b can be determined individually if test voltages of frequencies f_a and f_b are applied one at a time to the input terminals of the amplifier under test. (Both these procedures involve approximation as a result of failure to allow for other frequency components that are simultaneously present at terminals a — b, as a result of the distortion of the amplifier. For exact determination of E_a and E_b, a highly selective wave analyzer is used. However, the approximation procedure is adequate for most practice applications.)

For intermodulation test by the CCIF method, the test frequencies f_a and f_b are generally centered towards the high frequency end of the response range, and are sometimes placed purposely in the region where the high response begins to fall off appreciably. The difference frequency f_d = f_b — f_a, between the two test frequencies, is generally a moderate value from 50 — 200 Hz. This difference frequency should not be so small as to be below the normal low frequency response range of the amplifier, as the amplifier circuits tends to suppress the difference frequency output.

The intermodulation distortion observed in the CCIF method are indicative of the non-linear distortion present in the system in the frequency range f_a to f_b.

In particular, the CCIF method is able to measure the result of the non-linear distortion present at the upper frequency limit of the amplifier, where the high response starts to fall off.

The observed value of the intermodulation distortion is substantially inde­pendent of the exact values of f_a and f_b, and of the difference frequency f_b — f_a = f_d) of the two tests, provided this difference frequency is a small fraction of the test frequencies.