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This is a technique for generating sine waves of different frequencies using weighted capacitors, a recirculating shift register and an operational amplifier provided with a feedback loop.
English (United States)
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FSK Transmitter Using Weighted Capacitors
This is a technique for generating sine waves of different frequencies using
weighted capacitors, a recirculating shift register and an operational amplifier
provided with a feedback loop.
The capacitors C(M) = C(N-M) for M = 1, 2, ..., N/2, are designed with
capacitance values that are proportional to (sin(T1)-sin(T2)) with
The shift register is initialized with all 1 bits and then
shifted at a rate
with F being the mark or space
frequency. This will have the top plate of the capacitors C(1) to
C(N) switched a positive value to ground one at a time during the
first N shifts and from ground to the positive value during the next
The output sine wave will have the following characteristics:
- Frequency equal to the shift frequency divided by 2N
- Amplitude proportional to CTOT, with CTOT = (sigma) Ci.
To prevent a DC offset from developing on the feedback capacitor C(FB), phi
is brought high each time both bit positions 1 and N of the shift register contain 1.
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