Device and method for measuring of the complex transfer function of an object
The invention is related to the field of measurement electronics and related to the measurement of complex transfer functions (e.g. of impedance), especially in the presence of large disturbances and for measuring of small changes of the characteristics of the object under test. The goals of the proposed invention are achieved by detuning of the excitation and reference signals, e.g. by adding incremental phase-shift to the excitation signal to generate the reference signal.
1. A device for measuring a complex transfer function of an object, comprising:
a generator comprising a first and a second direct digital synthesizers (DDSs), the first direct digital synthesizer (DDS) configured to generate an AC excitation signal to be introduced into an object and the second direct digital synthesizer (DDS) to generate an AC reference signal;
a multiplier configured to multiply a response signal from the object and the AC reference signal from the generator; and
a data processing unit connected to an output of the multiplier, wherein the generator is configured to generate the AC reference signal and the AC excitation signal during a measurement period, wherein the AC reference signal and the AC excitation signal coincide in phase at the beginning of said measurement period, and the AC reference signal has a discrete stepwise or a continuously increasing delay relative to the AC excitation signal during the measurement period until the AC excitation signal and the AC reference signal coincide again in phase at an end of the measurement period.
2. The device according to claim 1 , wherein said AC reference signal and said AC excitation signal have equal frequencies and in order to delay the AC reference signal with respect to the AC excitation signal, the device being configured to add a phase shift to the phase ΔΦ of the AC reference signal at least once at each predetermined period nT of the AC excitation signal, while n=1,2,3,4, . . . , where said phase ΔΦ=i* 360 /p, where p is the number of steps during which said phase ΔΦ becomes equal to 2πand i=0, 1, 2, . . . p−1.
3. The device according to claim 1 , wherein a frequency of the AC reference signal is selected to be different from frequency of the AC excitation signal in order to delay the AC reference signal with respect to the AC excitation signal.
4. The device according to claim 3 , wherein the frequency of the AC excitation signal is selected to be in a range of 10 kHz to 10 MHz, and the frequency of the AC reference signal differs from the frequency of the AC excitation signal by 300 Hz to 3 kHz.
5. The device according to claim 1 , wherein the AC reference signal is tuned away from the AC excitation signal during the measurement period until the AC excitation signal and the AC reference signals coincide again in phase at an end of the measurement period.
6. A method for measuring the complex transfer function of an object with the device of claim 1 , the method comprising the steps of:
generating an AC excitation signal by the first direct digital synthesizer (DDS) and feeding the signal to the object to be measured;
generating an AC reference signal by the second direct digital synthesizer (DDS);
receiving, by the multiplier, an AC response signal generated by the AC excitation signal from the object to be measured;
multiplying, by the multiplier, the AC response signal by the AC reference signal, wherein the AC reference signal is tuned with respect to the AC excitation signal so that during a measurement cycle the AC reference signal is increasingly delayed in discrete steps or continuously relative to the AC excitation signal so that at a beginning and at an end of the measurement cycle the AC excitation signal and the AC reference signal coincide.
7. The method according to claim 6 , wherein to delay the AC reference signal relative to the AC excitation signal, a phase shift is added by the device to the phase ΔΦ of the AC reference signal at least once for each predetermined period nT of the AC excitation signal, while n=1,2,3,4, . . . , where ΔΦ=i* 360/p and p is the number of steps during which ΔΦ becomes equal to 2π, while i=0. . . p−1.
8. The method according to claim 6 , wherein a frequency of the AC reference signal is selected to be different from a frequency of the AC excitation signal in order to delay the AC reference signal with respect to the AC excitation signal.
9. The method of claim 8 , wherein the frequency of the AC excitation signal is selected to be in a range of 10 kHz to 10 MHz, and the frequency of the AC reference signal is selected to be 300 Hz to 3 kHz different from the frequency of the AC excitation signal.