Voltage controlled oscillator
BACKGROUND OF THE INVENTION 
     The present invention relates to voltage-controlled oscillators. 
     In particular, although not exclusively, the invention is concerned with broadband voltage-controlled oscillators for electronically settable channel selection for radio transmission systems operating in the 37 to 39.5 GHz frequency band. Such oscillators need to be designed to have a tuning bandwidth of, say, 2 GHz, to allow for production tolerances, to cover an upper or lower half-band of 1.25 GHz bandwidth. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention a voltage-controlled oscillator comprises a strip-line resonator, a negative resistance device mounted adjacent the centre of said resonator, respective variable capacitance devices mounted adjacent the ends of said resonator, high impedance means coupling said resonator to an output path, and means to apply bias signals to said negative resistance device and to said variable capacitance devices. 
     According to another aspect of the present invention a strip-line oscillator for operation at wavelengths of the order of millimetres comprises a length of strip-line, a negative resistance device mounted adjacent the centre of said length of strip-line, respective variable capacitance devices mounted adjacent the ends of said length of strip-line, high impedance means coupling said length of strip-line to an output path, and means to apply bias signals to said negative resistance device and to said variable capacitance devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A voltage controlled oscillator in accordance with the present invention will now be described with reference to the accompanying drawing of which: 
     FIG. 1 shows diagrammatically a plan view of the oscillator, and 
     FIG. 2 shows part of FIG. 1 on a larger scale. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing the oscillator, for operation in the frequency band from 37 GHz to 39.5 GHz, is formed in microstrip on a glass fibre reinforced PTFE substrate 1 which may be 0.25 mm thick, with continuous sheet metallization on its underside forming a ground plane and metallization in strip-line form on its upper surface defining the circuit elements of the oscillator. The metallization is each case may comprise 17.5 microns of copper covered by 5 microns of gold. 
     The oscillator comprises a negative resistance device 2, in the present case a Gunn diode, which is connected underneath the substrate 1 such that the top cap of the diode is flush with the upper metallization, and a resonant circuit comprising a centre strip 3, a pair of varactor diodes 4 and a pair of 30 degree radial stubs 5. Power is coupled out of the resonant circuit by way of a high impedance tap 6 and is launched into an output waveguide (not shown) by way of a printed E-plane transition 7. 
     A bias current for the Gunn diode 2 is applied by way of a band-stop filter 8, and a control signal voltage for the variable capacitance varactor diodes 4 is applied by way of a pair of band-stop filters 9. 
     The Gunn diode 2, operating with a bias current of several amps, may exhibit a resistance of the order of minus ten ohms, and the resonant circuit is arranged to present a positive resistance of the same order, the centre strip 3 being 1 mm wide and 3.17 mm long and the stubs 5 constituting low-Q quarter wave stubs. 
     Referring particularly to FIG. 2, the high impedance tap 6 tapers to a narrow entry 10 from a strip-line output conductor 11, the entry 10 being some 0.2 mm wide and having a characteristic impedance of some 100-120 ohms and the conductor 11 being 0.75 m wide and having a characteristic impedance of 50 ohms. It will be appreciated therefore that the tap 6 presents to the resonant circuit an impedance an order of magnitude higher than the impedance of the resonant circuit. 
     The strip-line conductors of the bandstop filters 8 and 9 are also some 0.2 mm wide, while the quarter wave stubs of these filters are high-Q 90 degree stubs.