Method and apparatus for testing microwave repeaters for im distortion

Claim 2

Apparatus for measuring the non-linearity of a device subject to distortion, which comprises: means for generating a first signal of frequency f o ; means for generating a second signal of frequency (f o +δ), where δ is an offset frequency which is small compared to the bandwidth of the device under test; means for generating a third signal of swept frequency f; means for combining said first, second and third signals thereby to generate first and second test signals of frequency (f o +f) and (f o +δ+f) respectively, the frequencies f o , f and δ being selected such that the test signals scan the band of interest, said test signals being applied to the input of the device under test; and detector means, connected to the output of the device under test, for measuring the amplitude of any component in the output signal for the device under test of frequency (f o -δ+f) or (f o +2δ+f), said measuring means including means for cancelling any component of frequency 2δ caused by distortion within the detector means itself.

Claim 3

The apparatus according to claim 1 wherein said detector means comprises: a first non-linear device for generating a signal of frequency 2δ from said first and second test signals and the third-order IM distortion products of the device under test, said device also producing an undesired signal of frequency 2δ caused by second-order distortion of the test frequencies alone; a signal splitter connected to the output of said first non-linear device; filter means connected to a first output of said signal splitter for blocking all signal components present in the output of said first non-linear device other than those of frequency δ; a second non-linear device connected to the output of said filter means for generating a fourth signal of frequency 2δ; and means, connected to the output of said second non-linear device and to a second output of said signal splitter, for cancelling the unwanted signal component of frequency 2δ generated by said first non-linear device.

Claim 4

The apparatus according to claim 2 wherein said cancelling means comprises: a signal combining circuit connected to the second output of said signal splitter and to the output of said second non-linear device; a phase-adjusting circuit intermediate said signal splitter and said signal combining circuit; an adjustable attenuator intermediate said second non-linear device and said signal combining circuit; and a bandpass filter connected to the output of said signal combining circuit, said filter rejecting all signal components other than those of frequency 2δ.

Claim 5

The apparatus according to claim 3 further comprising: means, for visually displaying the output of said bandpass filter; and means, connected to said third, swept-frequency, signal generating means, for synchronizing said visual display means with said swept frequency.

Claim 6

The apparatus according to claim 3 further including: means for automatically maintaining constant the level of the first and second test signals applied to the input by said detector means even in the presence of gain variations in the device under test.

Claim 7

The apparatus according to claim 5 wherein said level maintaining means comprises: means for inserting a selectively variable level of attenuation in the output signal from said first, second and third signal generating means; and control means, connected to and driven by the output signal of frequency δ from said bandpass filter, for controlling the attenuation inserted by said inserting means so that the level of the first and second test signals applied to the input of the device under test is altered, in an offsetting manner, as the level of the signal of frequency δ from the bandpass filter rises and falls in accordance with corresponding changes in the gain of the device under test.

Claim 8

Apparatus for measuring the non-linearity of a device subject to distortion, which comprises: means for generating a first signal of frequency f o ; means for generating a second signal of frequency (f o +δ), where δ is an offset frequency which is small compared to the bandwidth of the device under test; means for generating a third signal of swept frequency f; means for combining said first, second, and third signals thereby to generate first and second test signals of frequency (f o +f) and (f o +δ+f), respectively, the frequency f o , f and δ being selected such that the test signals scan the band of interest, said test signals being applied to the input of the device under test; means for generating a fourth signal of swept frequency (f+α), where α is an offset frequency which is small compared to the bandwidth of the device under test; and detector means, connected to the output of the device under test and to the output of said fourth signal generating means, for measuring the amplitude of a selected intermodulation product, the amplitude of said selected intermodulation product being directly proportional to the non-linearity of the device under test, said detector means further including means for cancelling any signal component of the same frequency as the intermodulation product of fixed frequency generated by fourth-order non-linearity in the output from said fourth signal generating means.

Claim 9

Apparatus for measuring the non-linearity of a device subject to distortion, which comprises: means for generating a first signal of frequency f o ; means for generating a second signal of frequency (f o +δ), where δ is an offset frequency which is small compared to the bandwidth of the device under test; means for generating a third signal of swept frequency f; means for combining said first, second, and third signals thereby to generate first and second test signals of frequency (f o +f) and (f o +δ+f), respectively, the frequency f o , f and δ being selected such that the test signals scan the band of interest, said test signals being applied to the input of the device under test; means for generating a fourth signal of swept frequency (f+α), where α is an offset frequency which is small compared to the bandwidth of the device under test; and detector means, connected to the output of the device under test and to the output of said fourth signal generating means, for measuring the amplitude of a selected intermodulation product, the amplitude of said selected intermodulation product being directly proportional to the non-linearity of the device under test, said detector means further including means for cancelling any signal component of the same frequency as the intermodulation product of fixed frequency generated by fourth-order non-linearity in the output from said fourth signal generating means.

Claim 10

The apparatus according to claim 7 wherein said detector means includes a frequency converter driven by the output of said fourth signal generating means for converting the frequency of the intermodulation products in the output of the device under test to a fixed frequency (f o +2δ-α).

Claim 11

The apparatus according to claim 7 wherein said detector means includes a frequency converter driven by the output of said fourth signal generating means for converting the frequency of the intermodulation products in the output of the device under test, and said selected intermodulation product of fixed frequency has a frequency (f o -δ-α).

Claim 12

The apparatus according to claim 9 wherein said signal cancelling means comprises: a first signal splitter connected to the output of said frequency converter; a second signal splitter connected to a first output of said first signal splitter; a non-linear device connected to a first output of said second signal splitter; a multiplier connected to the output of said non-linear device and to a second output of said second signal splitter; a first bandpass filter intermediate said non-linear device and said multiplier, said filter rejecting all signals from the output of said frequency converter other than of the frequency δ; and means, connected to a second output of said first signal splitter and to the output of said multiplier, for cancelling the unwanted signal component of frequency (f o -δ-α) generated by distortion in said frequency converter.

Claim 13

The apparatus according to claim 10 wherein said cancelling means comprises: a signal combining circuit connected to the output of said multiplier and to the second output of said first signal splitter; a phase-adjusting circuit intermediate said first signal splitter and said combining circuit; an adjustable attenuator intermediate said multiplier and said combining circuit; and a bandpass filter connected to the output of said combining circuit, said filter rejecting all signals other than those of frequency (f o -δ-α).

Claim 14

The apparatus according to claim 11 further comprising: means for visually displaying the output of said bandpass filter; and means, connected to said third swept frequency signal generating means, for synchronizing said visual display means with said swept frequency.

Claim 15

The apparatus according to claim 11 further including: means for automatically maintaining constant the level of the first and second test signals applied to the input by said detector means even in the presence of gain variations in the device under test.

Claim 16

The apparatus according to claim 13 wherein said level maintaining means comprises: means for inserting a selectively variable level of attenuation in the output signal from said first, second and third signal combining means; and control means, connected to and driven by the output signal of frequency δ from said bandpass filter, for controlling the attenuation inserted by said inserting means so that the level of the first and second test signals applied to the input of the device under test is altered, in an offsetting manner, as the level of the signal of frequency δ from the bandpass filter rises and falls in accordance with corresponding changes in the gain of the device under test.