Transfer function analysis

Claim 3

A method according to claim 1, wherein said two signals of the lower frequency WL differ in phase by 90*.

Claim 4

A method according to claim 1, wherein said step of generating a third signal comprises: generating a further signal of said high frequency WH differing in phase from said first signal by 90*; multiplying said first and said further signals of frequency WH by a respective one of said second and further signals of said low frequency WL to provide two multiplied signals; and adding said multiplied signals to provide said third signal.

Claim 5

A method according to claim 3, wherein said second and further signals of frequency WL which multiply said first and further signals of frequency WH differ in phase by 90*.

Claim 6

A method according to claim 1, wherein said third signal is applied to the system under test.

Claim 7

Apparatus for testing the response of a system to an input signal, said apparatus comprising: a first generator for generating at an output thereof, a first signal of a high frequency WH; a second generator for generating at two outputs thereof, two signals of relatively low frequency WL and differing in phase by 90*; a third generator responsive to said first and second generator outputs for generating at an output thereof, a third signal having a frequency equal to one of the set comprising the sum and difference of said WH frequency signal and said WL frequency signal, means for coupling a preselected one of said first and third generator outputs to a system to be tested; a multiplier having a first input coupled to the other of said first and third generator outputs and a second input for coupling to the output of the system to be tested, said multiplier being responsive to signals coupled to said first and second inputs to provide a multiplied signal at an output thereof; and a correlator capable of operation at relatively low frequencies and coupled to the output of said multiplier for correlating said multiplied signal with said two signals of the lower frequency WL which differ in phase by 90* so as to provide an output signal indicative of the response of the system to an input signal of the frequency WH.

Claim 8

Apparatus for testing the response of a system to an input signal, said apparatus comprising: a first generator for generating at an output thereof, a first signal of a high frequency WH; a second generator for generating at two outputs thereof, two signals of relatively low frequency WL and differing in phase by 90*; a third generator responsive to said first and second generator outputs for generating at an output thereof, a third signal having a frequency equal to one of the set comprising the sum and difference of said WH frequency signal and said WL frequency signal, means for coupling a preselected one of said first and third generator outputs to a system to be tested; a multiplier having a first input coupled to the other of said first and third generator outputs and a second input for coupling to the output of the system to be tested, said multiplier being responsive to signals coupled to said first and second inputs to provide a multiplied signal at an output thereof; and a correlator capable of operation at relatively low frequencies and coupled to the output of said multiplier for correlating said multiplied signal with said two signals of the lower frequency WL which differ in phase by 90* so as to provide an output signal indicative of the response of the system to an input signal of the frequency WH.

Claim 9

Apparatus according to claim 6, wherein the frequency of said first generator is locked to the frequency of said second generator, so that the frequency ratio of the high frequency signal to the low frequency signal is thereby maintained at a predetermined factor.

Claim 10

Apparatus according to claim 6, wherein the first said multiplier comprises a multiplicative modulator.

Claim 11

Apparatus according to claim 7 and further comprising: a frequency divider having an input coupled to an output of said first generator and responsive to provide an output signal corresponding in frequency to said signal WH divided by said predetermined factor; a multiplier responsive to the output of said divider and to an output signal from said second generator for providing a signal indicative of the frequency difference between said second generator signal and said divider output signal; and means coupling said multiplier output signal to said first generator to provide a signal to lock the frequency of said first generator to a predetermined multiple of said second generator frequency, thereby to provide the frequency ratio of said high frequency signal to said low frequency signal at said predetermined factor.