Linear phase demodulator including a phase locked loop with auxiliary feedback loop

• US 3,997,848 A
• Filed: 11/26/1975
• Issued: 12/14/1976
• Est. Priority Date: 11/26/1975
• Status: Expired due to Term

• Current Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Rippy, Ronnie R.
• Primary Examiner(s): Grimm, Siegfried H.
• Application Number:
  US05/635,531
• Time in Patent Office: 384 Days
• Field of Search: 329/50, 329/122, 329/124, 329/125, 332/30 V, 331/23, 331/36 C, 331/177 V
• US Class Current: 329/346
• CPC Class Codes: H03D 3/24 Modifications of demodulators to reject or remove amplitude variations by means of locked-in oscillator circuits
• CPC Class Codes: H04L 2027/0044 Control loops for carrier regulation
• CPC Class Codes: H04L 2027/0051 Harmonic tracking
• CPC Class Codes: H04L 2027/0067 Phase error detectors
• CPC Class Codes: H04L 27/2273 associated with quadrature demodulation, e.g. Costas loop

Abstract

A phase modulated wave that may have no carrier power is demodulated by a phase locked loop including a phase detector for deriving an A.C. data output signal having a magnitude and a phase indicative of the phase of the modulated wave. A feedback loop responsive to the data output signal restores power to the carrier frequency component to the loop. In one embodiment, the feedback loop includes a phase modulator responsive to the phase modulated wave and the data output signal. In a second embodiment, carrier frequency power is restored by differentiating the data output signal and supplying the differentiated signal to an input of a voltage controlled oscillator included in the phase locked loop. Preferably, the phase modulator is linear and includes a series inductance-capacitance network, wherein the capacitor is a voltage controlled varactor. To compensate for nonlinearities in the varactor reactance vs. voltage characteristic, the series circuit is shunted by a properly selected inductance.

Claims

1. Apparatus for demodulating a phase modulated wave comprising a phase locked loop responsive to the phase modulated wave, said loop including:

a voltage controlled oscillator, a phase detector responsive to the oscillator and the phase modulated wave for deriving an A.C. data output signal having a magnitude indicative of the phase of the phase modulated wave and a phase indicative of the phase of the modulated wave and for deriving an error signal indicative of the phase deviation between an input carrier wave component of the phase modulated wave and the voltage controlled oscillator, and a loop filter responsive to the error signal for deriving a control signal for the oscillator, means for controlling the oscillator phase in proportion to an integral of the error signal; and

a feedback loop responsive to the data output signal for increasing the power of the carrier frequency component in the phase locked loop.

2. The apparatus of claim 1 wherein the feedback loop includes a phase modulator having a modulation input signal responsive to a replica of the data output signal and another input responsive to the phase modulated wave, said modulator deriving the wave replica to which the phase locked loop is responsive.

3. The apparatus of claim 2 wherein the phase modulator is linear to derive an output wave having a phase directly proportional to the phase of the phase modulated wave minus an amount directly proportional to the amplitude of the data output signal.

4. The apparatus of claim 3 wherein the phase modulator includes a parallel substantially all reactive network having a variable reactance of a first type connected in series circuit with a first reactance of a second type opposite from the first type, the value of the variable reactance being controlled by the modulating input signal amplitude such that the reactance value is a non-linear function of the modulation input signal amplitude while the series circuit is approximately at resonance, a second reactance of the second type shunting the series circuit, said first and second reactances having values such that the series circuit has a reactance of the first type that is approximately in parallel resonance with the second reactance to provide compensation for the non-linear relationship between the value of the variable reactance and the modulation input signal;

and means for coupling energy from the phase modulated carrier wave to the parallel circuit and for coupling reflected energy from the parallel circuit to the phase detector without coupling any substantial energy from the parallel circuit back to the phase modulated wave source.

5. The apparatus of claim 4 wherein the variable reactance includes a varactor diode.

6. The apparatus of claim 5 wherein the variable reactance further includes a pair of series connected varactor diodes having like electrodes connected together to be responsive to a voltage that is a replica of the modulation input signal.

7. The apparatus of claim 2 wherein the wave is quaternary phase modulated and further comprising another phase modulator responsive to the phase modulated wave, another phase detector responsive to replicas of the another phase modulator and the voltage controlled oscillator, the input to the another phase detector from the voltage controlled oscillator being displaced 90°

from the input of the phase detector of the phase locked loop, said another phase detector deriving a data output signal for another channel of phase modulated data on the modulated carrier, and means for feeding the data output signal for the another channel as a modulating input for the another phase modulator.

8. The apparatus of claim 1 wherein the feedback loop includes differentiator means responsive to a replica of the data output signal having an amplitude indicative of the rate of change of the output signal, and means for controlling the oscillator phase by an amount proportional to the integral of the amplitude of the another wave in a sense opposite to that of the loop filter control signal.

9. A circuit for linearly phase modulating an r.f. input in response to a modulation input signal comprising a parallel substantially all reactive network including:

a variable reactance of a first type connected in series circuit with a first reactance of a second type opposite from the first type, the value of the variable reactance being controlled by the modulation input signal amplitude such that the reactance value is a non-linear function of the modulation input signal amplitude while the series circuit is approximately at resonance, a second reactance of the second type shunting the series circuit, said first and second reactances having values such that the series circuit has a reactance of the first type that is approximately in parallel resonance with the second reactance to provide compensation for the non-linear relationship between the value of the variable reactance and the modulation input signal; and

means for coupling energy from the r.f. input to the parallel circuit and for coupling reflected r.f. energy from the parallel circuit to a load without coupling any substantial r.f. energy back to the r.f. input.

10. The apparatus of claim 9 wherein the variable reactance includes a varactor diode.

11. The apparatus of claim 9 wherein the variable reactance includes a pair of series connected varactor diodes having like electrodes connected together to be responsive to a voltage that is a replica of modulation input signal.