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 Field of Search ............. .. 455/202, 208, 256, 260, 455/265; 375/77, 81, 97, 119, 120; 329/50, 122, 124
 References Cited
U.S. PATENT DOCUMENTS
3,710,261 l/1973 Low et al. ......................... .. 455/260 3,745,255 7/1973 Fletcher et al. 455/260 3,768,030 10/1973 Brown et al. 455/260
3,806,815 4/1974 Fletcher et al. 375/81 3,984,778 10/1976 Bhopale .............................. .. 375/77
Primary Examiner—Jin F. Ng -
Attorney, Agent, or Fi'rm—Marvin F. Matthews; John R.
 Appl. No.: 147,695  ABSTRAC1‘ _ A receiver for tracking and demodulating several types  Flledi May 7' 1980 of phase modulated communications, either analog or digital data, for either residual carrier or suppressed Related "-8- Aiificafie we f.‘l’.§“}Z§1g$‘§‘i.iZ’i.‘l.f,‘§i.‘Zl‘”T.ii*giZ§‘l?l*ifi%'3 .?Lo‘§.Z’fi’3%6’~i  Continuation of Ser. No. 737,975, Nov. 2, 1976, aban- A carrier component signal and a converted series sidedOued- - band component signal are obtained from the incoming p signal and summed to produce a carrier tracking signal,  In; (119 ______ _, H043 1/26 permitting more accurate tracking of the above types of  U.S. Cl. .................................. .. 455/202;; 455/202; phase modulated data455/265; 455/260; 375/77; 375/81; 375/ 120; 329/ 124 21 Claims, 3 Drawing Figures 54 66 V 7” =9? 1-¢ 42>" $6 /P5,)"/DUAL . Q A Q .400/= . Q‘ om?/£/7 F/L 75/? Q‘ HZ 755, V60 /-'/Z75/? “IE . .. _ ‘*1 k Er ~ ' § E “> e .54 V r . 1 ~ N L‘ Q | A 52 /.I >4 . 61 “' / ._ I _ , in . F
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METHOD AND APPARATUS FOR RECEIVING AND TRACKING PHASE MODULATED SIGNALS
The invention described herein was made by employees of the United States Govemment and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This is a continuation, of application Ser. No. 737,975 filed Nov. 2, 1976, now abandoned.
BACKGROUND OF THE INVENTION:
l. FIELD OF INVENTION: '
The present invention relates to receiving and tracking phase modulated signals.
2. DESCRIPTION OF PRIOR ART:
Prior art phase modulation receivers have in the past used various forms of tracking loops, such as phaselocked loops, squaring tracking loops and Costas loops to track the incoming signal carrier frequency. In each of these tracking loops, a voltage controlled oscillator (V CO) was driven through a loop filter by an error signal derived from a multiplication product of two signals. In ‘phase-locked loops and squaring tracking loops, the output of the VCO was multiplied with the incoming signal to form the error signal. In the Costas loop, two error signals were formed. The first Costas loop error signal was formed by multiplying the VCO output, termed an I signal, with the incoming signal. The second Costas loop error signal was formed by multiplying a 90° phase shifted I signal, termed a Q signal, with the incoming signal. Examples of such tracking loop receivers are in U.S. Pat. Nos. 3,289,082; 3,546,703; 3,550,131; 3,701,948; 3,710,257; and 3,736,507.
Receivers using these tracking loops were, however, limited to quadriphase or lower (e.g., biphase) modulation and to digital data. Also, these receivers used only the error signal portion of the incoming signal in tracking.
Briefly, the present invention provides a communication system receiver for receiving either residual carrier or suppressed carrier phase modulated communication signals, and further capable of accepting N-level (higher than 4) phase modulation. The multi-level phase modulation may represent digital data or may take the form of analog phasemodulated data. The receiver of the present invention is capable of operating for low incoming signal power levels and at low signal-to-noise (S/N) ratios because residual carrier components may not be multiplied by the demodulated signal and the carrier tracking noise bandwidth can be made narrower. The term residual carrier refers to the residual component when the data-bearing signal is modulated on the carrier in such a way that a residual component at carrier frequency exists in the overall signal power spectrum.
In the receiver of the present invention, a phase detector detects the phase of the incoming signal. The detected phase is reversed and multiplied with the incoming signal to form a converted series sideband component carrier signal. The residual carrier component signal and the converted series sideband signals are then summed to form a tracking carrier signal which is multi
plied with the output from the voltage controlled oscillator in the tracking loop to obtain an error signal which drives the voltage controlled oscillator in the tracking loop.
By controlling the tracking loop with a tracking signal having both carrier and converted series sideband component carrier signals, a stronger tracking signal for more numerous phase angles is obtained. The tracking signal is further less susceptible to carrier interference.
It is an object of the present invention to provide a new and improved receiver for phase modulated signals.
BRIEF DESCRIPTION OF THE DRAWINGS:
FIG. 1 is a schematic electrical circuit diagram of a prior art Costas loop tracking circuit;
FIG. 2 is a schematic electrical circuit diagramof a receiver tracking 0° to 180° phase shift according to the present invention; and
FIG. 3 is a schematic electrical circuit diagram of another receiver tracking 0° to 360° phase shift according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT:
At the outset, it is considered helpful to briefly discuss prior art demodulation techniques as exemplified by a Costas loop (FIG. 1). In the prior art loop, a voltagecontrolled oscillator (VCO) 10 furnished a reference signal based on an error signal fumished thereto by a loop filter 12 and a multiplier 14. The multiplier 14 received input quadrature signals known as I and Q signals from filters 16 and 18 and formed a product signal containing as a component thereof an error signal which was used to control the oscillator 10 in tracking the incoming signal. The I signal was formed by multiplying the output of the oscillator 10 with the incoming signal in the multiplier 20 after a -90° phase shift in the phase shift circuit 22, while the Q signal was formed by multiplying the output of the oscillator 10 with the incoming signal in the multiplier 24.
Although suitable for certain purposes, the prior art tracking circuits, including Costas loops, phase lock loops, and squaring tracking loops, suffered from several shortcomings. For example, these circuits were not adapted for analog or higher than four phase digital modulation without unique circuit modifications for each modulation type. Further, the circuits were not suitable for use with input signals at low power levels or at low signal-to-noise ratios because of a greater tracking noise bandwidth and different modulation types. Also, the circuits required suppressed modulation carrier transmission signals and were not suited for use with modulated communication signals having a residual carrier frequency component.
Considering now a receiver R (FIG. 2) according to the present invention, and incoming biphase modulation of 0° to 180° phase shift communication signal from a suitable communication link is multiplied in a first multiplier 24 with a local reference signal from a voltagecontrolled oscillator 26. The first multiplier 24 forms a product signal, containing a signal component, termed a residual carrier signal and sideband components, representing the data modulation components and the amount of signal power in the intermediate frequency component present in the incoming signal. The product signal formed in the multiplier 24 is fumished to a sum