Patent GB-1273792-A (GB)
Title: A circuit arrangement for measuring non-linearity of an information-transmission system
Inventor: LANDWEHR FRANZ-JOSEPH
IPC: H04B3/46
Filed: 1969-07-29
Abstract: 1,273,792. Measuring non linear distortion. WANDEL U GOLTERMANN and F. J. LANDWEHR. July 29, 1969 [July 30, 1968; July 11, 1969], No.37997/69. Heading G1U. To measure the overall non linear distortion due to an information transmitting circuit or system X of a predetermined frequency band, the output of a summing circuit 8 is connected to the input of the system under test, the summing circuit having several inputs, two of which are connected via respective channels 4, 5 to a source 1 of a spectrum of random signals. At least one channel contains a frequency changer e. g. 4 to one input of which the signal source is connected while to the other is connected a local oscillator e. g. 6. The output of the system under test is connected to a frequency selective amplitude measuring device 14 able to select at least a sum or difference intermodulation frequency resulting from the non linear distortion due to the system for any two-coherent components of the signals transmitted via the two different channels to the inputs of the summing circuit. In one embodiment, Fig. 1, a random noise source 1 is connected to a band pass filter 2, the passband of which is outside that of the system X. The filter output is connected to two frequency changers 4, 5 where it is mixed respectively with fixed frequency signals from local oscillators 6, 7. The outputs from the frequency changers are applied via the summing circuit 8 and filter 11, determining the band to be tested, to the system X. The local oscillators operate at frequencies close together outside the test frequency band, and the frequency changers operate to provide two bands of noise slightly displaced from one another, in the passband of the system under test. Any non linearities in this enable coherent signals in the two displaced bands to mix together to produce signals at the difference frequency of the two local oscillators. This will occur right through the test frequency bands to produce an overall output signal from the system at the difference frequency whose amplitude is related to its degree of non linearity. This output is passed via a filter 16 to a meter 15. Third order non linearity may be assessed by providing a further local oscillator 10 connected to the summator 8. Third order non linearity will enable this signal to mix with the difference signal produced by second order effects, as described above, to produce a signal at another frequency representing the extent of this non linearity. This is passed via a suitable filter 17, to be measured on the output meter. In alternative arrangements, the local oscillator frequencies G1, G2 may be widely separated, to equal extents at opposite ends of the noise band from the filter 2. The difference frequency signal representing non linearity will then be at the frequency difference between either local oscillator and the corresponding end of the band. In another system, Fig. 4 (not shown), only one local oscillator and frequency changer are used, the noise band being passed directly to the second input of the summing circuit. In another system, Fig. 6 (not shown), the arrangement of Fig. 1 is used to test part of a radio circuit system while actually in operation, the summing circuit (53) being connected to receive also the input signals from other parts of the equipment, and the output also being connected to further parts of the equipment. In yet another radio system, Fig. 8 (not shown), a similar arrangement to that of Fig. 4 (not shown), is used, but the normal inputs to the circuit are used in place of the random noise test signal.