Oscillators – Ring oscillators
Reexamination Certificate
2000-01-21
2001-05-29
Lam, Tuan T. (Department: 2816)
Oscillators
Ring oscillators
C331S10800D, C331S10800D, C327S238000, C327S254000
Reexamination Certificate
active
06239661
ABSTRACT:
The present invention relates to a transmitter, particularly but not exclusively a zero IF transmitter, and to a quadrature oscillator network generating FSK modulating signals for use in a transmitter.
U.S. Pat. No. 5,489,878 discloses in its preamble an oscillator comprising a non-inverting integrator followed by an inverting integrator, the output of which is coupled back to the input of the non-inverting integrator to form a closed loop. Typically the integrators are implemented as op-amps having a capacitance connected between the output and input of the associated op-amp. The oscillator comprises first and second gm/C integrator stages having common mode biasing means. Each of the gm/C stages comprises a long tail pair circuit in which the tail current is used to tune the oscillator. If such an oscillator is used as the modulating signal in a zero IF transmitter in which the phase of the modulating frequency is switched from positive to negative, an abrupt change will occur in the modulated signal causing the carrier to be spread well beyond its allowable band.
It is an object of the present invention to avoid spreading the carrier when switching the phase of such a type of oscillator.
According to a first aspect of the present invention there is provided a quadrature oscillator comprising a first integratable oscillator comprising a first non-inverting transconductance amplifier having an output connected to a first inverting transconductance amplifier, a second integratable oscillator comprising a second inverting transconductance amplifier having an output connected to a second non-inverting transconductance amplifier, a first capacitance coupled between the outputs of the first non-inverting and second inverting amplifiers and a reference voltage line, a second capacitance coupled between outputs of the first inverting and second non-inverting amplifiers and the reference voltage line, a feedback coupling between the outputs of the first inverting and second non-inverting amplifiers and inputs to the first non-inverting and second inverting amplifiers, a phase rotating circuit comprising means for switching-off one or other of the first and second integratable oscillators, and means coupled to the phase rotating circuit for slew rate limiting a phase setting data input signal applied to the phase rotating circuit.
According to a second aspect of the present invention there is provided a transmitter comprising a quadrature oscillator having outputs for in-phase and quadrature phase signals, first and second mixers, each mixer having a first input coupled to a respective one of the outputs of said oscillator and a second input for a local oscillator signal, means for combining outputs from the first and second mixers and means for propagating the combined signal, wherein the quadrature oscillator comprises a first integratable oscillator comprising a first non-inverting transconductance amplifier having an output connected to a first inverting transconductance amplifier, a second integratable oscillator comprising a second inverting transconductance amplifier having an output connected to a second non-inverting transconductance amplifier, a first capacitance coupled between the outputs of the first non-inverting and second inverting amplifiers and a reference voltage line, a second capacitance coupled between outputs of the first inverting and second non-inverting amplifiers and the reference voltage line, a feedback coupling between the outputs of the first inverting and second non-inverting amplifiers and inputs to the first non-inverting and second inverting amplifiers, a phase rotating circuit comprising means for switching-off one or other of the first and second integratable oscillators, and means coupled to the phase rotating circuit for slew rate limiting a phase setting data input signal applied to the phase rotating circuit.
According to a third aspect of the present invention there is provided an integrated transmitter having the features in accordance with the second aspect of the present invention.
By slew rate limiting the phase setting data input signal, the pairs of transconductance amplifiers which form oscillators of opposite direction of rotation are switched softly from one direction of phase rotation to the other thereby minimising the out-of-band emissions whenever the outputs are mixed with a carrier to generate a frequency modulated signal.
In an embodiment of the present invention the phase rotating circuit comprises a long tail pair circuit having a low transconductance which facilitates the soft switching operation. The low transconductance was achieved by using MOS devices with a low W/L (width to length) ratio. An advantage of the MOS devices being p-channel devices is that first and second current mirror arrangements used for mirroring current from the long tail pair circuit can be arranged at the bottom so that the analogue ground can be set as low as possible for low voltage battery operation. The respective first and second current mirror arrangements duplicate the control currents present in outputs of the long tail pair circuit and ensure that overall to respective first and second integratable oscillators have equal and opposite transconductances.
REFERENCES:
patent: 5446417 (1995-08-01), Korhonen et al.
patent: 6008701 (1999-12-01), Gilbert
patent: 6137370 (2000-10-01), Yamamoto
Lam Tuan T.
Slobod Jack D.
U.S. Philips Corporation
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