FREQUENCY SPECTRUM ANALYZER WITH FFT COMPUTER

• US 3,634,760 A
• Filed: 06/22/1970
• Issued: 01/11/1972
• Est. Priority Date: 06/22/1970
• Status: Expired due to Term

• Current Assignee: Fernand R.C. Murtin, Owen Storey
• Original Assignee: Fernand R.C. Murtin, Owen Storey
• Inventors: Storey, Owen, Murtin, Fernand R. C.
• Primary Examiner(s): Kubasiewicz, Edward E.
• Application Number:
  US05/048,115
• Time in Patent Office: 568 Days
• Field of Search: 324/57,77 235/156
• US Class Current: 324/76.23
• CPC Class Codes: G01R 23/00 Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
• CPC Class Codes: G01R 23/165 using filters

Abstract

A broad band of signal frequencies, whose power spectrum is to be analyzed by the Fast Fourier Transform (FFT) technique, is sampled by heterodyning with various beat frequencies fb and passage of the modulation products through a low-pass filter of bandwidth b/2 where b represents the width of a subband substantially narrower than the overall band of width B. By concurrently performing the heterodyning operation in two parallel channels, with introduction of a 90* phase shift between beat and input frequencies in one of the channels, the two sidebands fb+fx and fb-fx (where fx represents any frequency within the selected subband b) can be separated in the outputs of the two channel filters. Frequency limit b/2 is selected in conformity with the capacity of an associated computer to handle the data from the FFT analysis of the subband spectrum. The sampling may be preceded by a transposition of the entire band B to a higher frequency range, in order to prevent any possible cluttering of the spectrum by harmonics of fb.

Claims

1. A method of analyzing physical phenomena, translatable into a broad band of electromagnetic signal frequencies, with the aid of a computer capable of performing mathematical power-spectrum analyses in a range of frequencies whose upper limit F is a fraction of the width B of said band of input frequencies, comprising the steps of:

subdividing said band of signal frequencies into a plurality of subbands of width b by selecting a plurality of beat frequencies fb each centered in the respective subband, with b/2 F;

simultaneously heterodyning said band with a selected beat frequency in two parallel channels with a shifting of the phase of the selected beat frequency with reference to the phases of said signal frequencies by substantially 90* as between said channels, thereby generating modulation products including a lower sideband and an upper sideband flipped into an overlapping position near the zero-frequency axis of an amplitude/frequency diagram;

filtering all frequencies above b/2 from the modulation products of each channel;

converting the remainder of said modulation products into digital data; and

feeding said digital data to the computer.

2. A method as defined in claim 1 wherein said band is transposed to a range of higher frequencies before heterodyning with said beat frequencies by modulation of said signal frequencies with a shift frequency fo>

B.

3. A method as defined in claim 2 wherein said band of signal frequencies is separately modulated with said shift frequency in each of said channels, said shifting of the phase being accomplished by transmitting to said channels two oscillations of said shift frequency fo in quadrature relationship with each other.

4. A method as defined in claim 1 wherein the step of heterodyning comprises interrupting the transmission of said band over said channels in the rhythm of two relatively dephased square waves having the selected beat frequency fb.

5. A method as defined in claim 1 wherein the modulation products of said channels are additively and subtractively combined, after a relative phase shift by 90*, to separate said lower and upper sidebands before conversion into digital data.

6. A system for analyzing physical phenomena, comprising:

a source of electromagnetic signal frequencies representative of the phenomena to be analyzed, said input frequencies ranging over a broad band of width B;

variable oscillator means for selectively generating a plurality of beat frequencies fb each centered in a respective subband of width b of said broad band;

computer means capable of performing mathematical power-spectrum analysis in a range of frequencies whose upper limit F is a fraction of said width B but at least equal to B/2;

a pair of parallel transmission channels leading from said source to said computer means, each of said channels including mixer means connected to said oscillator means for simultaneously heterodyning said band with a selected beat frequency bf;

phase-shifting means in circuit with at least one of said channels for displacing the phase of said selected beat frequency bf with reference to the phases of said signal frequencies by substantially 90* as between said channels, thereby generating modulation products including a lower sideband and an upper sideband flipped into an overlapping position ear the zero-frequency axis of an amplitude/frequency diagram;

filter means in said channels for eliminating all frequencies above b/2 from the modulation products transmitted thereover;

analog-to-digital converter means in said channels beyond said filter means; and

circuitry for feeding the outputs of said converter means to said computer.

7. A system as defined in claim 6, further comprising a generator of an oscillation of fixed shift frequency fo, said mixer means including two cascaded heterodyning stages in each of said channels, one of said stages in each channel being connected to said oscillator means, the other of said stages in each channel being connected to said generator.

8. A system as defined in claim 7 wherein said phase-shifting means is inserted between said generator and the associated heterodyning stage in one of said channels.

9. A system as defined in claim 7 wherein said filter means comprises a low-pass filter immediately behind each of said heterodyning stages.

10. A system as defined in claim 6 wherein said oscillator means comprises a square wave generator, said mixer means comprising a gate circuit in each of said channels controlled by said generator in the rhythm of its square wave.