Browse Prior Art Database

A/FC Loop for Doppler Correction

IP.com Disclosure Number: IPCOM000096612D
Original Publication Date: 1963-Aug-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

Rutz, EM: AUTHOR

Abstract

The automatic frequency control corrects for the Doppler effect in a microwave synchronous communication system. An unmodulated continuous wave (CW) signal is transmitted to satellite 18. The CW signal is amplitude modulated (AM) in 18 and reflected back to the receiver. Because 18 is moving with respect to the receiver, the received signal is shifted in frequency by the Doppler effect.

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A/FC Loop for Doppler Correction

The automatic frequency control corrects for the Doppler effect in a microwave synchronous communication system. An unmodulated continuous wave (CW) signal is transmitted to satellite 18. The CW signal is amplitude modulated (AM) in 18 and reflected back to the receiver. Because 18 is moving with respect to the receiver, the received signal is shifted in frequency by the Doppler effect.

The received signal is a suppressed carrier signal. The receiver, therefore, must restore the carrier between two Doppler shifted sidebands. The Doppler shift frequency is derived in the receiver and is used to supply the modulation voltage to a single-sideband modulator 64. The input signal to 64 is the CW carrier from the transmitter. The shifted frequency output of 64 is used as the re- inserted carrier in the receiver. In the system shown, both transmitter and receiver operate via common antenna 10. In the transmitter portion, crystal oscillator 12 generates a signal frequency which is multiplied by multiplier 14 up, to the carrier frequency Fc. The carrier is amplified by klystron amplifier 16 and transmitted via antenna 10 to satellite 18. This continuous wave (CW) signal enters 18, is amplitude modulated, and reflected in the direction of origin. Antenna 10 receives the amplitude modulated (AM) signal. Transmitter feed- through is reduced by band rejection filter 22, which is centered around Fc, and nulling system 24. The transmitter signal is superimposed on the received AM signal. Both are amplified by parametric amplifier 26 and traveling-wave-tube amplifier 28. This amplified signal drives two synchronous detectors 30 and 32. The local oscillator inputs 34 and 36 to detectors 30 and 32 are placed in phase quadrature by phase shifter 38. The outputs of detectors 30 and 32 are passed through low pass filters 40 and 42 and amplifiers 44 and 46, respectively.

The information for automatic frequency correction is obtained from the branch containing detector 30. Output voltage 48 of amplifier 44 is the vector sum of the AM signal and the transmitter feed-through signal. This output 48 drives a square-law detector 50 whose output passes through low pass filter 52. Thus, a f...