Oscillator coupling to reduce spurious signals in receiver circuits

A first receiver frequency reference is passively coupled to a second receiver by tapping a signal directly from the resonant element, such as a crystal, of an oscillator in the first receiver to drive the input of the second receiver. The sinusoidal signal from the resonant element is relatively free of harmonics and minimizes interference that could be caused by harmonics of a square wave signal coupling or an amplified signal. The oscillator of each receiver can be selectively enabled or disabled to allow the receiver to either generate or receive the frequency reference. This technique of coupling can be used to couple a frequency reference signal between integrated circuit receivers.

BACKGROUND

1. Field of the Invention

The present invention relates to oscillators and specifically to coupling a common reference between two oscillator circuits.

2. Prior Art

In receiver circuits, it is common practice to buffer the crystal oscillator signal of a phase-locked loop (PLL) of a first receiver circuit and use the buffered version of the signal as the reference for the phase-locked loop (PLL) of a second receiver circuit. The buffer used is commonly a saturated amplifier with its output signal being almost a square wave. This square wave output has a high number of harmonics present and some of these harmonics can end up present in the band of the receiver or the band of the mixer's output. The transmission of the harmonic-rich signal between receivers over circuit board traces provides an opportunity for the signal to radiate. The harmonics of the buffered signal can be an interferer and detrimental to the system similar to unwanted signals picked up from the antenna.

FIG. 1shows the prior art with the buffered oscillator signal being used as the reference for the second receiver circuit's PLL.

SUMMARY OF INVENTION

The crystal oscillator of a first phase-locked loop (PLL) of a first receiver circuit is coupled to a second PLL of a second receiver. This crystal oscillator is shared between the two receiver circuits without using a buffered oscillator signal as in the prior art. By avoiding the use of the buffer, the detrimental harmonics caused by the buffer are eliminated. The reference input buffer of the second receiver circuit is specified according to the amplitude available from the crystal circuit, providing a moderately high input impedance. The extra load capacitance introduced by the device being driven and by the board trace is taken in account during the design of the crystal oscillator circuit. The crystal oscillator (XO) can be of any topology, for example a Pierce crystal oscillator.

In the invention, the crystal oscillator of the second slave receiver is disabled. The oscillator is disabled in the slave device to avoid having the amplified version of the input signal radiating on the board which may cause similar detrimental harmonics as would the use of an output buffer in the prior art. The disabling of the oscillator can be achieved by using one P-type field-effect transistor (FET) and one N-type FET to couple power to the amplifier used in the crystal oscillators.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2shows a block diagram coupling an oscillator according to the present invention. Two receiver circuits are shown, master100and slave200. Master receiver100comprises oscillator circuit110, phase-locked loop (PLL) and filter circuit120, crystal oscillator (XO) input buffer115, voltage controlled oscillator (VCO)140, mixer150and low noise amplifier (LNA)160. The crystal oscillator output buffer130would be used in the prior art method of coupling the oscillator signal to a second receiver. Capacitors172provide the capacitance to achieve the necessary requirement that the overall loop gain has zero (or 360) degrees phase shift at the oscillation frequency. The crystal170, a resonant element, is connected such that it forms part of the feedback impedance of the inverting amplifier in the crystal oscillator circuit, which operates in a non-saturating linear or near linear mode.

Oscillator circuit110drives receiver200through capacitor180, which isolates the direct current bias of oscillator circuit210of the slave receiver200and passes only the oscillating signal. The signal is coupled between receivers passively, without the use of a buffer amplifier. The reference signal from oscillator circuit110drives input buffer215, an amplifier, to provide the reference signal to the PLL of slave receiver200. When used as a slave device, crystal oscillator circuit210is disabled and the reference signal is input at the disabled output terminal of the amplifier of oscillator circuit210where input buffer215is connected. Input buffer215is active and drives the reference signal in receiver200. A capacitor can optionally be connected to the input of oscillator circuit210to provide an AC ground at that terminal.

Output buffers130and230can be disabled when using the configuration of the present invention. Although not required in the present invention, the output buffer can be included in the receiver circuit to provide compatibility with other devices that do not use the present inventive configuration.

The present circuit and method passively couples a reference oscillating signal between two receivers with a substantially sinusoidal waveform, which avoids coupling and radiation of harmonics of the fundamental waveform that could occur in a signal path between the two receivers. Internal clock signals derived from the reference signal and used in the receivers can be non-sinusoidal or square. The receivers can be implemented as single-chip devices with short internal signal paths that reduce the opportunity for radiation of harmonics that can occur in a long signal path.

FIG. 3shows the topology of the oscillator110and210, using a Pierce topology. Other oscillator topologies can be used with this invention. The resonator used is preferably a piezoelectric crystal, and can alternatively be an inductor and capacitor circuit. In the master receiver circuit100, the crystal170is connected to the input303and output309of the active device, inverting amplifier300, and provides the feedback impedance in parallel with resistor301. In this configuration, the crystal is operating at or near its series resonant frequency and has low impedance.

The oscillator circuit may be disabled or enabled by the application of enable signal304. The enable signal304is connected to the gate of a P-type field effect transistor (PFET)306and to the gate of an N-type field effect transistor (NFET)307. When the enable signal is high, transistor306is active and power305is applied to the inverting amplifier300. When the enable signal304is low, transistor306is off and transistor307is active resulting in a voltage difference of zero being applied to the power inputs of amplifier300.

The oscillator circuit110of the master receiver must be enabled to drive the slave circuitry, while the slave circuit has its oscillator circuit210disabled. The selective enabling of the oscillator circuits of the present invention allows identically fabricated devices to be used in a multiple receiver system with programming to select master and slave receivers.

The method of coupling oscillator circuits of the present invention provides component saving in a multiple oscillator circuit while maintaining a spectrally pure reference signal for the oscillators. The transmission of a harmonic-rich non-sinusoidal waveform between receiver devices is avoided. The method and apparatus of connecting receiver frequency references of the present invention can be extended to any number of receivers.