Oscillators – Solid state active element oscillator – Transistors
Patent
1998-05-04
2000-12-05
Kinkead, Arnold
Oscillators
Solid state active element oscillator
Transistors
331 96, 331107DP, 331117FE, 3332191, H03B 518
Patent
active
061572691
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to noise rejection circuitry and, particularly but not exclusively, to noise rejection techniques for use in local oscillator circuits contained within low noise blocks (LNBs) for use in satellite receiving equipment.
Due to the introduction of digital transmission standards for future satellite broadcasts, there is a greater requirement to have improved levels of phase noise in local oscillators contained within LNBs, such as that disclosed in applicant's co-pending published application PCT/GB92/01065 published as WO 92/22938.
The local oscillator is part of the general circuitry used in the LNB and its function is to create a signal which drives a mixer which also receives amplified microwave signals from the satellite. The output of the mixer is UHF (down converted) signals which contain the relevant video and audio information. It is very important that the noise from the local oscillator is minimised, otherwise this will end up on the UHF signal with a consequent reduction in video and audio quality especially for digital transmissions.
The prior art circuit is shown in FIG. 1 of the drawings. This circuit uses a field effect transistor (FET) or high electron mobility transistor (HEMT) as its principal oscillating element. The circuit shown in FIG. 1 has a ceramic resonator which acts as a filter coupled to the gate transmission line and controls the oscillation frequency. With this arrangement the output of the FET is d.c. coupled directly to ground via an inductor on the printed circuit board. This arrangement is the standard technique for a series feedback oscillator design.
However, with this arrangement there is no protection against current surges within the device which can result in fluctuations in the oscillator frequency. The result of this is an increase in phase noise leading to reduced signal quality.
EP 0 202 652 discloses a microwave oscillator circuit which uses a resistor in the d.c. path of the circuit to set the d.c. self-biasing of the FET. At high frequencies the resistor is effectively connected to an open circuit.
The proceedings of the 19th European Microwave Conference, 1989, Sep. 4, 1989, pages 549 to 554, XP000067323, Kazuo Imai et al: "A 22 Ghz Band Low Noise Down Converter for Satellite Broadcast Receivers" discloses a dielectric resonator oscillator circuit which also has a resistor in the d.c. path of the circuit. This resistor is decoupled by a capacitor and so is effectively short circuited.
An object of the present invention is to provide an improved local oscillator noise rejection circuit which obviates or mitigates the aforementioned disadvantage.
This is achieved by providing resistance between the output of the MES FET/HEMT and ground. This resistance has the effect of introducing a self-biasing arrangement to the oscillator circuit which means that when there is any increase in current through the MES FET/HEMT device, the voltage drop across the resistance also increases. Since the gate is tied to ground via a low resistance (51 .OMEGA.) this leads to an increased negative gate to source voltage which causes the device to "pinch-off" slightly which, in turn, gives a reduction in the current level through the device. This arrangement therefore has the effect of reducing any current surges within the device and produces an improvement in phase noise.
In a preferred arrangement a 30 Ohm resistor is located between the source of the FET/HEMT and ground, the resistor being located in series within the printed circuit inductor and the output line.
With the aforementioned modified circuit, an improvement in the phase noise performance has been measured from the standard circuit which gave a figure of -64 dBc. at 10 KHz. After modification using the above mentioned circuit a figure of -76 dBc. at 10 KHz. was obtained which is approximately a 16 times improvement. Simultaneously, the level of oscillator sidebands generated by an interfering 200 KHz. signal on the d.c. supply line were improved from -50 dBc. to -70 dBc. which is an improveme
REFERENCES:
patent: 4590443 (1986-05-01), Ishigaki et al.
patent: 4609884 (1986-09-01), KinDinger et al.
patent: 5157357 (1992-10-01), Katoh
P. Horowits and W. Hill, "The Circuit Engineering Art," Peace Publishers, 1983, vol. 1, p. 117.
G.S. Tyskin, "Amplifying Devices," Communication Publishers, Moscow, pp. 37-39, 283, 285, 289.
Cambridge Industries Limited
Kinkead Arnold
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