Page 314 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

М ] _out = соs(21ТfRFt) siп (21Т fоs t) = siп (21Тfоsct) соs(21Тf'l\rt)
с
= ~siп(21Тfоsсt + 21Тf /) + siп(21Тfоs t - 21Тf t)]
с
R
Rр
Filter_2 removes the higher-frequency signal, which results in

RF
Filter2_out =-fsiп(21Тfо,сt - 21Тf t) = Q signal

Therefore, the signals from Filter1_out and Filter2_out аге 90 degrees apart from
each other по matter how low а frequency (fos c - f RF ) iS, even down to пеаг direct
currect (ОС). Now the fact that these two signals сап maintain а 90-degree
difference all the way down to nearly ОС is truly amazing. The 1 and Q signals from
the filters then allow а computer to digitize these signals and demodulate them.

References

1. U.S. Patent 1,855,576, Clyde R. Keith, "Frequency Translating System," filed оп
April9, 1929.
2. U.S. Patent 2,086,601, Robert S. Caruthers, "Modulating System," filed оп Мау
3, 1934.
3. U.S. Patent 2,025,158, Frank Augustus Cowan, "Modulating System," filed оп

June 7, 1934.
4. U.5. Patent 5,058,159, Ronald Quan, "Method and System for ScrambIing and
DescrambIing Audio Information Signals," filed оп June 15, 1989.
5. U.5. Patent 5,159,531, Ronald Quan and АН R. Hakimi, "Audio ScrambIing

System Using In-Band Carrier," filed оп April 26, 1990.
б. U.5. Patent 5,471,531, Ronald Quan, "Method and Apparatus for Low Cost Audio
ScrambIing and DescrambIing," filed оп ОесетЬег 14, 1993.
7. U.s. Patent 6,091,822, "Method and Apparatus for Recording ScrambIed Video
Audio Signals and Playing Back Said Video Signal, DescrambIed, Within а Secure
Environment," Andrew В. Mellows, John О. Ryan, William J. WrobIeski, Ronald

Quan, and Gerow, О. Brill, filed оп January 8, 1998.
8. Kenneth К. Clarke and Donald Т. Hess, Communication Circuits: Ana/ysis and
Design. Reading : Addison-Wesley, 1971.
9. Thomas Н. Lee, The Design of CMOS Radio-Frequency Integrated Circuits, 2nd

ed. Cambridge: Cambridge University Press, 2003.
10. Donald О. Pederson and Kartikeya Мауагат, Ana/og Integrated Circuits {ог
Communication. Boston: Кluwer Academic PubIishers, 1991.
11. Allan R. НатЫеу, E/ectrica/ Engineering Princip/es and App/ications, 2nd ed.
Upper Saddle River: Prentice Hall, 2002.

12. Robert L. Shrader, E/ectronic Communication, 6th ed. New York: Glencoe
МсGгаw-НiII, 1991.
13. Mischa Schwartz, Information, Transmission, and Noise. New York :
МсGгаw-НiII, 1959.