US 5,140,703 A · Modulation distortion analyzer

Abstract

There is disclosed a method and apparatus for demodulating an R.F. modulated signal having a known carrier frequency Fo. A first reference signal of a frequency matching approximately the carrier frequency Fo, is mixed with the input R.F. modulated signal to produce the demodulated signal. The first reference signal is generated by a reference oscillator which provides a second reference signal of a second distinct reference frequency, a voltage control oscillator for generating the first reference signal dependent upon the amplitude of an error signal applied thereto, a selectively variable divider circuit for receiving and dividing the first frequency by a selected number to provide a third divided signal, and a comparator for comparing the phase of the second reference signal with that of the third divided signal to generate and apply the error signal to the voltage control oscillator. The demodulated signal is applied to the reference oscillator, whereby the second frequency varies in accordance with the phase of the demodulated signal and the first frequency is phase-locked to the carrier frequency Fo. A selected spectrum window of the demodulated signal is provided by generating a fourth reference signal of a frequency offset from the carrier frequency Fo and by multiplying that offset reference signal by the demodulated signal to provide a difference signal indicative of the difference therebetween centered about the offset frequency. The difference signal is filtered to pass only those frequencies below a selected first frequency to provide a spectrum window whose bandwidth is limited to that segment of the demodulated or base band signal to be examined.

Claims31

Dependent claims

Apparatus for examining a selected segment of a base band signal produced by demodulating an R.F. modulated signal having a carrier frequency Fo, said examining apparatus comprising:

a) homodyne means for demodulating the R.F. modulated signal to provide a base band signal to effectively remove therefrom said carrier frequency Fo;
b) means for generating a reference signal of a frequency offset from said carrier frequency Fo by a selected amount;
c) means for receiving and multiplying said reference signal and said demodulated baseband signal to provide a difference signal indicative of the frequency difference therebetween centered about said offset frequency;
and
d) first filter means for receiving said difference signal and for passing only those frequencies below a first predetermined frequency to provide a spectrum window thereof whose bandwidth is limited to that segment of said base band to be examined.

2 more elements

Apparatus for demodulating an R.F. modulated signal to provide a base band signal, said R.F. modulated signal having a carrier signal of known carrier frequency Fo, said demodulating apparatus comprising:

a) means for receiving said R.F. modulated signal;
b) variable oscillator means for generating a first reference signal dependent upon the amplitude of an error signal applied thereof;
c) a reference oscillator responsive to the phase of said demodulated signal for providing a second reference signal of a second reference frequency;
d) a selectively variable divider means for receiving said first reference signal and dividing said first reference signal by a selected number to provide a third divided signal whose third frequency is proportional to said first frequency divided by said selected number;
e) a comparative for comparing the phase of said second reference signal with that of said third divided signal to generate said error signal in accordance with the phase difference therebetween;
f) means for applying said error signal to said variable oscillator means;
g) means connected to said receiving means and said variable oscillator means for receiving respectively said received R.F. modulated signal and said first reference signal for demodulating said received R.F. modulated signal to produce said base band signal;
and
h) a feedback path for applying said base band signal indicative of any phase difference between said R.F. modulated signal and said first reference signal to said reference oscillator means, whereby said reference oscillator means is phaselocked to said R.F. modulated signal.

6 more elements

Apparatus for examining a selected segment of a base band signal produced by demodulating an R.F. modulated signal having a carrier frequency Fo and sidebands with distortion therein, said examining apparatus comprising:

a) means for demodulating said R.F. modulated signal to provide a base band signal;
b) means for generating a reference signal of a frequency offset from said carrier frequency Fo by a selected amount;
c) first filter means connected to said demodulating means for receiving said base band signal for removing therefrom those frequencies above a first predetermined frequency and for outputting a filtered signal representative of the distortion appearing in said sidebands and without any fundamental components of said base band;
d) means for receiving and multiplying said reference signal and said filtered signal to provide a difference signal indicative of the frequency difference therebetween centered about said frequency offset;
e) second filter means for receiving said difference signal and for passing only those frequencies below a second predetermined frequency to provide a spectrum window thereof whose bandwidth is limited to that segment of said base band signal to be examined;
f) switch means for selecting one of said base band signal, said filtered signal or said spectrum window;
and
g) display means for receiving said selected one signal for displaying an indication of it's amplitude.

5 more elements

a) means for receiving said R.F. modulated signal;
b) variable oscillator means for generating a first reference signal dependent upon the amplitude of an error signal applied thereto;
c) phase shift means for receiving and imparting to said first reference signal a phase shift of precisely 90.degree. to provide a phase-shifted reference signal in quadrature with said first reference signal;
d) a reference oscillator responsive to the phase of said base band signal for providing a second reference signal of a second reference frequency;
e) a selectively variable divider means for receiving said first reference signal and dividing said first reference signal by a selected number to provide a third divided signal whose third frequency is proportional to said first frequency divided by said selected number;
f) a comparator for comparing the phase of said second reference signal with that of said third divided signal to generate said error signal in accordance with the phase difference therebetween;
g) means for applying said error signal to said variable oscillator means;
h) an in-phase mixer and a quadrature mixer respectively coupled to said variable oscillator means and said phase shift means for receiving respectively said first reference signal and said phase-shifted reference signal and each connected to said receiving means for receiving said R.F. modulated signal for demodulating said received R.F. modulated signal to output respectively an in-phase base band signal and a quadrature base band signal;
and
i) a feedback path for applying said base band signal to said reference oscillator means, whereby said reference oscillator means is phaselocked to said R.F. modulated signal.

7 more elements

Apparatus for examining a selected segment of a base band signal produced by demodulating an R.F. modulated signal having a carrier frequency Fo and a sidebands with distortion therein, said examining apparatus comprising:

a) means for demodulating said R.F. modulated signal to provide a based band signal;
b) means for generating a reference signal of a frequency offset from said carrier frequency Fo by a selected amount;
c) first filter means connected to said demodulating means for receiving said base band signal for removing therefrom those frequencies above a first predetermined frequency and for outputting a filtered signal representative of the distortion appearing in said sidebands and without any fundamental components of said base band;
d) means for receiving and multiplying said reference signal and said filtered signal to provide a difference signal indicative of the frequency difference therebetween centered about said frequency offset;
and
e) second filter means for receiving said difference signal and for passing only those frequencies below a second predetermined frequency to provide a spectrum window thereof whose bandwidth is limited to that segment window thereof whose bandwidth is limited to that segment of said base band to be examined;
f) said second filter means comprises means for selectively varying said first predetermined frequency, whereby said bandwidth of said spectrum window may be varied accordingly.

4 more elements

Dependencies are inferred from the claim text.

Modulation distortion analyzer

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Modulation distortion analyzer

Life of this patent

Abstract

Claims31