Pulse or digital communications – Receivers – Automatic baseline or threshold adjustment
Reexamination Certificate
1998-05-26
2001-06-19
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Automatic baseline or threshold adjustment
Reexamination Certificate
active
06249552
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an audio frequency recovery-DC restorer circuit after demodulation.
2. Discussion of Related Art
For standard narrowband FM, FSK or PSK of cordless phones after demodulation, a conventional AC coupling capacitor method is used to remove the DC content off of the baseband signal (voice and/or data) before passing the signal to the microprocessor or to the AF amplifier. For a more sophisticated high data rate communication, as exemplified by Time Division Duplex (TDD), the following problems arise if such a conventional approach is employed, which consequently reduces quality performance of the cordless phone.
(a) Sensitivity
(b) Distorted demodulated AF signals
(c) Less accurate duty cycle of the replicated digital data.
(d) DC offsets due to frequency offsets between the transmitter and the receiver.
(e) DC offsets due to center frequency inaccuracies in a quadrature resonator.
(f) Time Division Duplex (TDD) burst DC ripple that is induced by transient frequency or DC voltage shifts.
The AC coupled circuit basically skews or distorts the demodulated signal. As a consequence, the signal becomes inefficient for use by the receiver systems that require precise baseband quality output. Due to the nature of a capacitor, a transient DC response arises to any time varying shifts in the input DC level. This leads to unstable bit duty cycles and thus increased bit jitter.
Conventional cordless phones employ 49 MHz narrowband and into use multiple stages of filters (passive or active) and transistor combinations. It would be desirable to avoid using such multiple stages of filters.
SUMMARY OF THE INVENTION
One aspect of the invention resides in an Audio Frequency (AF) recovery—DC restorer circuit after demodulation that filters received data without distorting, accurately and efficiently replicates digitally the transmitted data signal, and stabilizes the demodulated signal at a desired DC level which is critical for A/D conversion process or data acquisition to attain reliable bit duty cycles.
A demodulated baseband signal travels from a demodulated output terminal through a buffered low pass filter and then becomes coupled to an active peak clamper or DC restorer, which preferably includes a capacitor, an operational amplifier and a diode. The DC restorer removes all of the DC portion of the demodulated signal and replaces it with a fixed DC reference voltage. The demodulated signal, which is now ideally clamped at the fixed DC reference voltage that is stable, is then connected to a comparator or data slicer that converts the signal into an exact digital replica of the transmitted data.
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Caña John-Paul Pizaña
Smiley Michael Todd
Bayard Emmanuel
Cobrin & Gittes
Nortel Networks Limited
Pham Chi
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