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Frequency Shift Keyer

IP.com Disclosure Number: IPCOM000078786D
Original Publication Date: 1973-Mar-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Abbiate, JC: AUTHOR [+2]

Abstract

In frequency shift keying transmitters, the modulation operation is usually performed by controlling the frequency of a voltage-controlled oscillator (VCO) through use of a binary data voltage. Such an operation produces inter-symbol interferences in the FSK transmission. To avoid these interferences it is necessary to use, at the output of the oscillator, a filer which eliminates frequency components outside the useful frequency band. With such a circuit, the attenuation ratio of undesired frequencies is not sufficient for certain applications.

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Frequency Shift Keyer

In frequency shift keying transmitters, the modulation operation is usually performed by controlling the frequency of a voltage-controlled oscillator (VCO) through use of a binary data voltage. Such an operation produces inter-symbol interferences in the FSK transmission. To avoid these interferences it is necessary to use, at the output of the oscillator, a filer which eliminates frequency components outside the useful frequency band. With such a circuit, the attenuation ratio of undesired frequencies is not sufficient for certain applications.

With this circut it is possible to obtain a much higher attenuation ratio, by providing a nonlinear transfer function to the binary voltage controlling the VCO frequency. With the control voltage obtained, no filter is required at the VCO output.

The input data is fed to first and second stages connected in cascade. The binary data signal is fed to the first stage comprising a resistor R1 in series with an operational amplifier A1, the feedback loop of which includes a capacitor C in parallel with two diodes D1 and D2. The output of amplifier A1 is fed to a second stage similar to the first one, with the exception that capacitor C is replaced by resistor R3.

With this design, attenuation ratios in the range of 40 dB are easily obtained.

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