Hutchinson, B. H., Jr.

DSpace@MIT

VII. MODULATION THEORY AND SYSTEMSProf. E. J. Baghdady A. F. Ghais R. B. C. MartinsW. M. Beery B. H. Hutchinson, Jr. C. B. MetzadourH. G. Fritz R. E. Maloney R. A. MetzgerA. C. MarshallA. FEEDFORWARD ACROSS THE LIMITERThis report completes the summary of a Master's thesis on the feedforward tech-nique submitted by the author (1). The thesis reports in detail experimental work thathad been described previously (2) and gives a few additional experimental results.The most important new experimental result was the completion of a set of measure -ments of the demodulator output spectrum of the experimental feedforward receiver whileit was capturing the weaker of two cochannel sinusoidally modulated signals. The inher-ent distortion on captured weaker-signal modulation seems to result from a very high-order interaction between the stronger- and weaker-signal modulating frequencies;frequency components appear at a frequency fo that is equal to the highest common factorof the two modulating frequencies and at all harmonics of fo in the audio range. Theweaker-signal modulation component was clearly predominant over all distortion com-ponents.Work has been started on the design of a laboratory -model feedforward receiverthat should give improved performance and be free of many of the secondary circuitdifficulties and adjustments of the earlier models.B. H. Hutchinson, Jr.References1. B. H. Hutchinson, Jr., Interference Suppression Performance of Several FMReceivers Using Feedforward, S. M. Thesis, Department of Electrical Engineering,M.I.T., Jan. 18, 1961.2. B. H. Hutchinson, Jr., Feedforward across the limiter, Quarterly ProgressReport No. 60, Research Laboratory of Electronics, M. I. T., Jan. 15, 1961, p. 113.111

Modulation Theory and Systems

IX. MODULATION THEORY AND SYSTEMSProf. E. J. BaghdadyProf. J. B. WiesnerJ. M. GutweinA. L. HelgessonB. H. Hutchinson, Jr.E. M. KittredgeD. D. WeinerA. FM RECEPTION WITH COHERENT INTERFERENCE AND RANDOM NOISEA preliminary experimental investigation was conducted to examine the behavior ofa feedforward signal-cancellation system under conditions of interference from randomnoise and interference from noise plus a coherent signal. The main purpose was todetermine whether or not interference from a weaker or a stronger coherent cochannelsignal in an FM system could be suppressed with a feedforward circuit without increasingthe vulnerability of the system to random noise.The capture performance of the experimental receiver is illustrated in Fig. IX-1.Measurements were made by establishing a fixed signal input level to the system,injecting random noise of controllable level, and observing the effects of different inputS/N ratios on the demodulator output for various circuit adjustments.When the feedforward circuit was adjusted for best capture of the stronger of twoFig. IX-1. (a) Detected stronger-signal modulation with interference from weakersignal, with no feedforward. (b) Detected stronger-signal modulation,with feedforward adjusted for stronger-signal capture. (c) Detectedweaker-signal modulation, with feedforward adjusted for weaker-signalcapture (a=0. 7). (d), (e), and (f) Detected weaker-signal modulation(a=0. 6) after being passed through: 750-sec de-emphasis filter (d);750-Lsec de-emphasis filter plus a 3-kc lowpass filter (e); and 7. 5-Lsecde-emphasis filter (f). Notice details of the capture-transition distor-tion in (f).111 _ I-..(IX. MODULATION THEORY AND SYSTEMS)cochannel coherent signals, the random-noise performance of the system was not notice-ably affected by the introduction of the feedforward amplifier. This result agrees withHelgesson's theoretical results (1).When the feedforward circuit was adjusted for best capture of the weaker of twocochannel coherent signals, the random-noise performance of the system was not notice-ably affected by the introduction of the feedforward amplifier, although theoretical pre-dictions indicate that deterioration could be expected if the input noise level issufficiently high. This experiment has shown conclusively that an FM signal can becaptured in the presence of a stronger cochannel interfering signal plus a moderateamount of random noise, even when the input weaker-signal is at or below the random-noise threshold.B. H. Hutchinson, Jr.References1. A. L. Helgesson, Signal-to-noise ratios in a limiter with feedforward, QuarterlyProgress Report No. 56, Research Laboratory of Electronics, M. I. T., Jan. 15, 1960,p. 113.

http://dspace.mit.edu/bitstream/1721.1/53392/1/RLE_QPR_057_IX.pdf

Modulation Theory and Systems

Abstract

Similar works

Full text

Available Versions

DSpace@MIT

DSpace@MIT