Spectrum analyzer with multiple operational modes

• US 4,054,785 A
• Filed: 09/16/1976
• Issued: 10/18/1977
• Est. Priority Date: 09/16/1976
• Status: Expired due to Term

• Current Assignee: Sangamo Weston, Inc.
• Original Assignee: Sangamo Weston, Inc.
• Inventors: Lehmann, Joseph L.
• Primary Examiner(s): Malzahn, David H.
• Application Number:
  US05/723,785
• Time in Patent Office: 397 Days
• Field of Search: 235/156, 235/152, 324/77 B, 324/77 D, 343/5 DP, 343/5 SA
• US Class Current: 708/405
• CPC Class Codes: G01R 23/16 Spectrum analysis; Fourier analysis

Abstract

Disclosed is a fully digital spectrum analyzer which includes a digital frequency translator that makes it possible to effectively analyze frequency bands that do not start at zero frequency. With the translator, the available spectral lines can be within the band of interest and not wasted in the range from zero frequency to the start of the band, and as a result the band is examined at optimal resolution. The translation takes place after the signal to be analyzed has been digitized, and the translator time-shares much of the conventional circuitry of the spectrum analyzer, to thereby minimize the cost of translation. Additionally, the spectrum analyzer includes an analog network that reduces the frequency of very fast signals so they can be analyzed by a spectrum analyzer having a relatively low upper frequency limit.

Claims

1. A spectrum analyzer including means operative in a first analyzer mode for processing digital samples representing the time history of an analog signal to derive power spectrum values for each of a plurality of spectral lines uniformly distributed over a frequency range starting at or about zero frequency, said spectrum analyzer further including means operative in a second analyzer mode for deriving power spectrum values for spectral lines distributed only within a frequency range that does not start at zero frequency, said means operative in the second analyzer mode comprising:

means for defining a finite frequency band of interest which starts at a non-zero frequency;

means for providing a sequence of digital words f(n), where n = 1, 2, ..., N, which correspond to samples of an analog signal at a sampling rate at least twice the upper limit of said band;

means for combining each of said digital words f(n) with a first and a second trigonometric function, said functions being in quadrature with respect to each other and having arguments determined by the center frequency of said band and the order of the digital word f(n) in the sequence, to form respective first and second sequences of resulting digital words g(n) and h(n);

means for digitally filtering the sequences of digital words g(n) and h(n) in accordance with a filter function determined by the width of said band to form respective sequences g(m) and h(m), where m = 1, 2, ..., M and M is less than N;

a discrete Fourier transform processor and means for applying thereto the digital word sequences g(m) and h(m) to form thereby a set of discrete Fourier transform terms RG'"'"' and IG'"'"' from each word g(m) and RH'"'"' and IH'"'"' from each word h(m), a set of terms being formed from each of the words g(m) and h(m) for each of a selected plurality of spectral lines, where R designates the real term and I the imaginary term of a discrete Fourier transform;

means for combining the like named terms for each subscript m to form respective discrete Fourier transform terms RG, IG, RH, and IH for each spectral line; and

means for algebraically combining the terms RG, IG, RH and IH with each other to form a set of terms RU and IU representing the discrete Fourier transform of the upper half of said band translated about zero center frequency and a set of terms RL and IL representing the discrete Fourier transform of the lower half of said band translated about zero center frequency, and means for algebraically processing said terms RU, IU, RL and IL to form power spectrum values for each of said spectral lines.

2. A spectrum analyzer as in claim 1 including means operative in a third analyzer mode and comprising:

means for receiving an analog signal having a frequency content exceeding a selected upper frequency limit;

means for heterodyning the analog signal with a heterodyning signal at a selected frequency to produce a resultant analog signal which has a center frequency below said upper frequency limit but above zero and extends over a selected frequency band starting above zero and ending below said upper limit; and

means for thereafter operating the spectrum analyzer in said second mode in which said means for defining a finite frequency band defines the frequency band of said resultant analog signal and said means for providing the sequence of words f(n) provides words f(n) which correspond to samples of said resultant analog signal.

3. A spectrum analyzer comprising:

means for defining a finite frequency band of interest which starts at a non-zero frequency;

means for providing a sequence of digital words f(n), where n = 1, 2, ..., N, which correspond to samples of an analog signal at a sampling rate at least twice the upper limit of said band;

means for combining each of said digital words f(n) with a first and a second trigonometric function, said functions being in quadrature with respect to each other and having arguments determined by the center frequency of said band and the order of the digital work f(n) in the sequence, to form respective first and second sequences of resulting digital words g(n) and h(n);

means for digitally filtering the sequences of digital words g(n) and h(n) in accordance with a filter function determined by the width of said band to form respective sequences g(m) and h(m), where m = 1, 2, ..., M and M is less than N;

a discrete Fourier transform processor and means for applying thereto the digital word sequences g(m) and h(m) to form thereby a set of discrete Fourier transform terms RG'"'"' and IG'"'"' from each word g(m) and RH'"'"' and IH'"'"' from each word h(m), a set of terms being formed from each of the words g(m) and h(m) for each of a selected plurality of spectral lines, where R designates the real term and I the imaginary term of a discrete Fourier transform;

means for combining the like named terms for each subscript m to form respective discrete Fourier transform terms RG, IG, RH and IH for each spectral line;

means for algebraically combining the terms RG, IG, RH and IH with each other to form a set of terms RU and IU representing the discrete Fourier transform of the upper half of said band translated about zero center frequency and a set of terms RL and IL representing the discrete Fourier transform of the lower half of said band translated about zero center frequency, and means for algebraically processing said terms RU, IU, RL and IL to form power spectrum values for each of said spectral lines.

4. A spectrum analyzer as in claim 3 including means for receiving an analog signal having a frequency content exceeding a selected upper limit, means for heterodyning the last recited analog signal with a heterodyning signal which is at a selected frequency to derive a resultant signal which is within a range above zero but below said upper limit frequency, and means for causing said means for defining said finite band to define said resultant signal and said means for providing words f(n) to provide words f(n) which are samples of said resultant signal.

5. A spectrum analyzer as in claim 3 where said means for digitally filtering the sequences of digital words g(n) and h(n) comprises a cascade of digital finite impulse response filters and including means for directing to the discrete Fourier transform processor the output of a single one of said filters selected on the basis of a characteristic of said finite frequency band.

6. A method of carrying out spectrum analysis in a selected one of a first and a second analysis mode, said first mode including processing digital samples representing the time history of an analog signal to derive power spectrum values for each of a plurality of spectral lines uniformly distributed over a frequency range starting at or about zero frequency, and said second mode including deriving power spectrum values for spectral lines distributed only within a frequency range that does not start at zero frequency, said second mode comprising the steps of:

defining a finite frequency band of interest which starts at a non-zero frequency;

providing a sequence of digital words f(n), where n = 1, 2, ..., N, which correspond to samples of an analog signal at a sampling rate taking into account the upper limit of said band;

combining each of said digital words f(n) with a first and a second trigonometric function, said functions being in quadrature with respect to each other and having arguments determined by the center frequency of said band and the order of the digital word f(n) in the sequence, to form respective first and second sequences of resulting digital words g(n) and h(n);

digitally filtering the sequences of digital words g(n) and h(n) in accordance with a filter function determined by the width of said band to form respective sequences g(m) and h(m), where m = 1, 1, ..., M and M is less than N;

performing discrete Fourier transform processing of the digital word sequences g(m) and h(m) to form thereby a set of discrete Fourier transform terms RG'"'"' and IG'"'"' from each word g(m) and RH'"'"' and IH'"'"' from each word h(m), a set of terms being formed from each of the words g(m) and h(m) for each of a selected plurality of spectral lines, where R designates the real term and I the imaginary term of a discrete Fourier transform;

combining the like named terms for each subscript m to form respective discrete Fourier transform terms RG, IG, RH, and IH for each spectral line; and

algebraically combining the terms RG, IG, RH and IH with each other to form a set of terms RU and IU representing the discrete Fourier transform of the upper half of said band translated about zero center frequency and a set of terms RL and IL representing the discrete Fourier transform of the lower half of said band translated about zero center frequency, and algebraically processing said terms RU, IU, RL and IL to form power spectrum values for each of said spectral lines.

7. A method as in claim 6 including selectively carrying out a third analysis mode comprising the steps of:

receiving an analog signal having a frequency content exceeding a selected upper frequency limit;

heterodyning the analog signal with a heterodyning signal which is at a selected frequency to produce a resultant analog signal which is within a frequency band starting above zero and ending below said upper limit; and

thereafter carrying out said second analysis mode in which said step of defining a finite frequency band includes defining the frequency range of said resultant analog signal and said step of providing the sequence of words f(n) includes providing words f(n) which correspond to samples of said resultant analog signal.

9. A method as in claim 6 including the steps of receiving an analog signal having a frequency content exceeding a selected upper limit, heterodyning the last recited analog signal with a heterodyning signal which is at a selected frequency to derive a resultant signal which is within a range above zero but below said upper limit frequency, and in said defining step defining said finite band to include the frequency range of said resultant signal and in said providing step providing words f(n) which are samples of said resultant signal.

10. A method as in claim 6 where said step of digitally filtering the sequences of digital words g(nm) and h(n) comprises successive filtering at a cascade of digital finite impulse response filters and directing for discrete Fourier transform processing the output of a single one of said filters selected on the basis of a characteristic of said finite frequency band.

8. A spectrum analysis method comprising the steps of:

defining a finite frequency band of interest which starts at a non-zero frequency;

providing a sequence of digital words f(n), where n = 1, 2, ..., N, which correspond to samples of an analog signal at a sampling rate at least twice the upper limit of said band;

combining each of said digital words f(n) with a first and a second trigonometric function, said functions being in quadrature with respect to each other and having arguments determined by the center frequency of said band and the order of the digital word f(n) in the sequence, to form respective first and second sequences of resulting digital words g(n) and h(n);

digitally filtering the sequences of digital words g(n) and h(n) in accordance with a filter function determined by the width of said band to form respective sequences g(m) and h(m), where m = 1, 2, ..., M and M is less than N;

carrying out discrete Fourier transform processing of the digital word sequences g(m) and h(m) to form thereby a set of discrete Fourier transform terms RG'"'"' and IG'"'"' from each word g(m) and RH'"'"' and IH'"'"' from each word h(m), a set of terms being formed from each of the words g(m) and h(m) for each of a selected plurality of spectral lines, where R designates the real term and I the imaginary term of a discrete Fourier transform;

combining the like named terms for each subscript m to form respective discrete Fourier transform terms RG, IG, RH and IH for each spectral line;

algebraically combining the terms RG, IG, RH and IH with each other to form a set of terms RU and IU representing the discrete Fourier transform of the upper half of said band translated about zero center frequency and a set of terms RL and IL representing the discrete Fourier transform of the lower half of said band translated about zero center frequency, and algebraically processing said terms RU, IU, RL and IL to form power spectrum values for each of said spectral lines.