OSCILLATOR CIRCUIT

Abstract

An oscillator circuit includes an oscillator transistor (Q1) having respective first, second, and control terminals, the oscillator transistor being arranged to generate a microwave oscillating signal at the first terminal. A surface integrated waveguide resonator (Y1) is connected to the second terminal of the oscillator transistor (Q1). An active bias circuit portion (202) including a negative feedback arrangement is between the first terminal of the oscillator transistor (Q1) and the control terminal of the oscillator transistor (Q1), the active bias circuit portion being arranged to supply a bias current to the control terminal of the oscillator transistor (Q1). The bias current is dependent on a voltage at the first terminal of the oscillator transistor (Q1) multiplied by a negative gain.

Claims

1. An oscillator circuit comprising: an oscillator transistor having respective first, second, and control terminals, said oscillator transistor being arranged to generate a microwave oscillating signal at the first terminal thereof; a surface integrated waveguide resonator connected to the second terminal of the oscillator transistor; and an active bias circuit portion comprising a negative feedback arrangement between the first terminal of the oscillator transistor and the control terminal of the oscillator transistor, the active bias circuit portion being arranged to supply a bias current to the control terminal of the oscillator transistor, wherein said bias current is dependent on a voltage at the first terminal of the oscillator transistor multiplied by a negative gain.

2. The oscillator circuit as claimed in claim 1, wherein the oscillator transistor comprises a bipolar junction transistor, wherein the first terminal of the oscillator transistor is a collector terminal, the second terminal of the oscillator transistor is an emitter terminal, and the control terminal of the oscillator transistor is a base terminal, optionally wherein the oscillator transistor comprises an npn BJT.

3. The oscillator circuit as claimed in claim 1, wherein the active bias circuit comprises a first feedback transistor having respective first, second, and control terminals, said first feedback transistor being arranged such that: the first terminal of the first feedback transistor is connected to the first terminal of the oscillator transistor via first feedback path; and the second terminal of the first feedback transistor is connected to the control terminal of the oscillator transistor via a second feedback path.

4. The oscillator circuit as claimed in claim 3, wherein the first feedback transistor comprises a bipolar junction transistor, wherein the first terminal of the first feedback transistor is an emitter terminal, the second terminal of the first feedback transistor is a collector terminal, and the control terminal of the first feedback transistor is a base terminal, optionally wherein the first feedback transistor comprises a pnp BJT.

5. The oscillator circuit as claimed in claim 3, wherein the first feedback path comprises first and second resistors arranged such that: a first terminal of the first resistor is connected to a supply voltage; a second terminal of the first resistor is connected to the first terminal of the first feedback transistor and to a first terminal of the second resistor; and a second terminal of the second resistor is connected to the first terminal of the oscillator transistor.

6. The oscillator circuit as claimed in claim 5, wherein a first capacitor is connected between the first terminal of the first resistor and ground; and/or a second capacitor is connected between the second terminal of the second resistor and ground.

7. The oscillator circuit as claimed in claim 3, wherein the second feedback path comprises third and fourth resistors arranged such that: a first terminal of the third resistor is connected to the second terminal of the first feedback transistor; a second terminal of the third resistor is connected to a first terminal of the fourth resistor and to the control terminal of the oscillator transistor; and a second terminal of the fourth resistor is connected to ground.

8. The oscillator circuit as claimed in claim 1, further comprising a second feedback transistor having respective first, second, and control terminals, said second feedback transistor being arranged such that: the first terminal of the second feedback transistor is connected to the supply voltage via a fifth resistor; the second terminal of the second feedback transistor is connected to the control terminal of the first feedback transistor; and the control terminal of the second feedback transistor is connected to the first terminal of the first feedback transistor via a third feedback path; optionally wherein the fifth resistor has a first terminal thereof connected to the first terminal of the second feedback transistor, and a second terminal thereof connected to the supply voltage and to the first terminal of the first resistor in the first feedback path.

9. The oscillator circuit as claimed in claim 8, wherein the second feedback transistor comprises a bipolar junction transistor, wherein the first terminal of the second feedback transistor is an emitter terminal, the second terminal of the second feedback transistor is a collector terminal, and the control terminal of the second feedback transistor is a base terminal, optionally wherein the second feedback transistor comprises a pnp BJT.

10. The oscillator circuit as claimed in claim 1, further comprising a switching transistor having respective first, second, and control terminals, said switching transistor being arranged such that a control signal applied to the control terminal of said switching transistor varies a current through the first and second terminals of said switching transistor, wherein the current supplied to the control terminal of the oscillator transistor is dependent on the current through the first and second terminals of said switching transistor.

11. The oscillator circuit as claimed in claim 1, further comprising first, second, and third microstrip lines, arranged such that: the first microstrip line has a first end thereof connected to the control terminal of the oscillator transistor, and a second end thereof is open-ended; the second microstrip line has a first end thereof connected to the first end of the first microstrip line and the control terminal of the oscillator transistor; and the third microstrip line has a first end thereof connected to a second end of the second microstrip line, and a second end thereof is open-ended; wherein the first end of the third microstrip line and second end of the second microstrip line are connected to the active bias circuit.

12. The oscillator circuit as claimed in claim 1, further comprising fourth and fifth microstrip lines, arranged such that: the fourth microstrip line has a first end thereof connected to the first terminal of the oscillator transistor; and the fifth microstrip line has a first end thereof connected to a second end of the fourth microstrip line, and a second end thereof is open-ended.

13. The oscillator circuit as claimed in claim 1, further comprising a bandpass filter connected between the first terminal of the oscillator transistor and an output terminal of the oscillator circuit; optionally further comprising a sixth microstrip line having a first end thereof connected to an input of the bandpass filter and to the first terminal of the oscillator transistor, and a second end thereof is open ended; and/or optionally further comprising a seventh microstrip line having a first end thereof connected to an output of the bandpass filter and to an output terminal of the oscillator circuit, and a second end thereof is open ended.

14. The oscillator circuit as claimed in claim 1, further comprising a ninth microstrip line connected between the first terminal of the oscillator transistor and an output terminal of the oscillator circuit.

15. An impulse radar motion sensor comprising the oscillator circuit as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Certain embodiments of the present disclosure will now be described with reference to the accompanying drawings, in which:

[0053] FIG. 1 is a circuit diagram of an oscillator in accordance with an embodiment of the present invention in which a single-stage feedback arrangement is used to provide the negative collector-base parallel feedback;

[0054] FIG. 2 is a circuit diagram of an oscillator in accordance with a further embodiment of the present invention in which a two-stage feedback arrangement is used to provide the negative collector-base parallel feedback;

[0055] FIG. 3 is a circuit diagram of an oscillator in accordance with a further embodiment of the present invention in which the bandpass filter is replaced with a microstrip line;

[0056] FIG. 4 is a graph showing phase noise as a function of frequency for different oscillator circuits;

[0057] FIG. 5 is a table showing various parameters relating to different oscillator circuits;

[0058] FIG. 6 is a plot showing a microwave pulse;

[0059] FIG. 7 is a microwave circuit layout for the oscillator circuit of FIG. 2; and

[0060] FIG. 8 is a microwave circuit layout for the oscillator circuit of FIG. 3.

DETAILED DESCRIPTION

[0061] FIG. 1 is a circuit diagram of an oscillator circuit 100 in accordance with an embodiment of the present invention in which a single-stage feedback arrangement is used to provide the negative collector-base parallel feedback.

[0062] The oscillator circuit 100 comprises an oscillator transistor Q1 which, in this particular embodiment, is an npn BJT. This oscillator transistor Q1 has respective collector (first), emitter (second), and base (control) terminals. The oscillator transistor Q1 is arranged to generate a microwave oscillating signal at its collector terminal, where this microwave oscillating signal is, subject to further processing steps outlined below, provided as an output. In particular, this output signal is a high frequency microwave signal (e.g. approximately 10 GHz) suitable for use in a radar motion sensor.

[0063] The oscillator circuit 100 also comprises: an active bias circuit 102 (the components of which are explained in further detail below); a surface integrated waveguide (SIW) resonator Y.sub.1; a bandpass filter Y.sub.2; and a number of microstrip lines MSL.sub.1-8.

[0064] The SIW resonator Y.sub.1 is connected to the emitter terminal of the oscillator transistor Q.sub.1 and thus connects the emitter to ground, where MSL.sub.8 is connected between the emitter terminal of the oscillator transistor Q.sub.1 and the SIW resonator Y.sub.1.

[0065] The active bias circuit portion 102 provides a negative feedback arrangement between the collector and base terminals of the oscillator transistor Q.sub.1. As explained in more detail below, this negative feedback arrangement is arranged to supply a biasing base current I.sub.b to the base terminal of the oscillator transistor Q.sub.1, where the base current I.sub.b is dependent on a voltage Vc at the collector terminal of the oscillator transistor Q.sub.1, multiplied by a negative gain. In other words, the active bias circuit portion 102 acts as a negative transconductance amplifier connected between the collector and base terminals of the oscillator transistor Q.sub.1.

[0066] In order to provide this negative collector-base parallel feedback, the active bias circuit portion 102 comprises a feedback transistor Q.sub.2, which in this embodiment is a pnp BJT having respective emitter (first), collector (second), and base (control) terminals.

[0067] This feedback transistor Q.sub.2 is arranged such that its emitter terminal is connected to the collector terminal of the oscillator transistor Q.sub.1 via first feedback path constructed from a pair of resistors Rc.sub.1, Rc.sub.2; a pair of capacitors Cc.sub.1, Cc.sub.2; and two of the microstrip lines MSL.sub.4 and MSL.sub.5.

[0068] The first terminal of the first of these resistors Rc.sub.1 is connected to the supply voltage Vcc, while the other terminal of that resistor Rc.sub.1 is connected to the emitter terminal of the feedback transistor Q.sub.2 and to a first terminal of the second of the resistors Rc.sub.2. The other terminal of the second resistor Rc.sub.2 is connected to the collector terminal of the oscillator transistor Q.sub.1.

[0069] One of the capacitor Cc.sub.1 is connected between the first terminal of the first resistor Rc.sub.1 and ground. The other capacitor Cc.sub.2 is connected between the second terminal of the second resistor Rc.sub.2 and ground. The first capacitor Cc.sub.1 is a decoupling capacitor. The second capacitor Cc.sub.2 in combination with the resistors Rc.sub.2 and Rb.sub.4 provide a low pass filter.

[0070] The collector terminal of the feedback transistor Q.sub.2 is connected to the base terminal of the oscillator transistor Q.sub.1 via a second feedback path constructed from a further pair of resistors R.sub.b4, R.sub.b6 and three of the microstrip lines MSL.sub.1-3.

[0071] A first of these resistors R.sub.b4 is arranged such that one of its terminals is connected to the collector terminal of the first feedback transistor Q.sub.2. The other terminal of this resistor R.sub.b4 is connected to a first terminal of the other resistor R.sub.b6 and to the base terminal of the oscillator transistor Q.sub.1. Thus this second feedback path ‘closes the loop’ back from the collector terminal of Q.sub.1 to the base terminal of Q.sub.1. The other terminal of the second resistor R.sub.b6 in the second feedback path is connected to ground.

[0072] The low-pass filter comprising the capacitor C.sub.c2 and the resistors R.sub.c2 and R.sub.b4 results in the signal at the collector terminal of the oscillator transistor Q.sub.2 being subjected to filtering across the closed feedback loop before being applied to the control terminal of the oscillator transistor Q.sub.1.

[0073] Thus the feedback transistor Q.sub.2 and the feedback path circuitry outlined above provides negative collector-base parallel feedback to the oscillator transistor Q.sub.1. This helps to stabilize the DC voltage and current variations of the oscillator transistor Q.sub.1, to minimise degradation of the oscillator's parameters, and to reduce phase noise.

[0074] The oscillator circuit also includes a switching transistor Q.sub.4—in this case an npn BJT—having respective collector, emitter, and base terminals. This switching transistor Q.sub.4 is arranged to receive a control signal V.sub.ctr—i.e. a control voltage—at its base terminal, as outlined in further detail below.

[0075] The switching transistor Q.sub.4 is arranged such that it is connected to the rest of the oscillator circuit 100 via a pair of resistors R.sub.b1, R.sub.b2. The first of these resistors R.sub.b1 is connected such that one terminal of the resistor R.sub.b1 is connected to the supply voltage V.sub.cc—specifically at the node at which the first resistor R.sub.c1 and first capacitor C.sub.c1 of the first feedback path are connected to the supply voltage V.sub.cc. The other terminal of the resistor R.sub.b1 is connected to the base terminal of the feedback transistor Q.sub.2 and to one of the terminals of the other resistor R.sub.b2, the other end of which is connected to the collector terminal of the switching transistor Q.sub.4. The emitter terminal of the switching transistor Q.sub.4 is connected to ground.

[0076] Varying the control signal V.sub.ctr varies a the collector-emitter current through the switching transistor Q.sub.4. Due to its connection to the feedback transistor Q.sub.2, inhibiting the collector-emitter current through the switching transistor Q.sub.4 also inhibits the collector-emitter current through the feedback transistor Q.sub.2. Due to the base current I.sub.b supplied to the base terminal of the oscillator transistor Q.sub.1 being dependent on the feedback loop through the feedback transistor Q.sub.2, inhibiting the collector-emitter current through Q.sub.2 also inhibits the base current I.sub.b.

[0077] Thus, the base current I.sub.b supplied to the base terminal of the oscillator transistor Q.sub.1 is ultimately dependent on the current through the switching transistor Q.sub.4. As a result, the switching transistor Q.sub.4 provides a simple way to switch the entire oscillator circuit 100. This provides for relatively fast start-up because the active bias circuit 102 itself can be switched, thereby switching the current to the base terminal of the oscillator transistor Q.sub.1. As outlined previously, having a fast start-up time makes the arrangement of the present invention particularly well-suited to high frequency pulse operation.

[0078] While the switching transistor Q.sub.4 may be part of the active bias circuit portion 102 as shown in FIG. 1, it will be appreciated that the switching transistor Q.sub.4 may not necessarily be an standalone components and may, for example, form part of an external controlling circuit, e.g. a microprocessor (not shown).

[0079] The MSL.sub.1-8 may be used to ‘tune’ the characteristics of the microwave circuit, i.e. the circuitry surrounding the oscillator transistor Q.sub.1. These MSL.sub.1-8 act together to set the resonant frequency of the microwave circuit and to filter out unwanted frequencies.

[0080] The first MSL.sub.1 is connected at one end to the base terminal of the oscillator transistor Q.sub.1, while it's other end is open-ended (i.e. unconnected). The second MSL.sub.2 has one end connected to the first end of MSL.sub.1 and the base terminal of the oscillator transistor Q.sub.1. The third MSL.sub.3 is connected such that one end is connected to the second end of MSL.sub.2 (i.e. to the end that is not connected to MSL.sub.1) while the other end of MSL.sub.3 is open-ended. The node connecting the first end of MSL.sub.3 and the second end of MSL.sub.2 is further connected to the active bias circuit 102, and specifically to the node connecting R.sub.b4 and R.sub.b6. Thus MSL.sub.1-3 can be seen as forming part of the second feedback path, i.e. they sit between the collector terminal of the feedback transistor Q.sub.2 and the base terminal of the oscillator transistor Q.sub.1.

[0081] As MSL.sub.1 is open-ended, its dimensions significantly influence the microwave frequency. Generally, MSL.sub.1 may be a quarter wavelength (λ.sub.g/4) long, where the wavelength (λ.sub.g) is the wavelength of the microwave signal of interest, i.e. the signal generated by the oscillator transistor Q.sub.1, in the substrate. It will also be appreciated that MSL.sub.2 and MSL.sub.3 provide a band-stop filter transfer function, acting to isolate microwave frequency components, preventing them flowing through the bias circuit.

[0082] MSL.sub.4 and MSL.sub.5 similarly form part of the first feedback path. Specifically, MSL.sub.4 is connected at one end to the collector terminal of the oscillator transistor Q.sub.1, and at the other end to a first end of MSL.sub.5. The other end of MSL.sub.5 is open-ended.

[0083] As outlined above, the oscillator circuit 100 generates the microwave oscillating signal at the collector terminal of the oscillator transistor Q.sub.1. The output matching circuit, comprising MSL.sub.6-7 and the bandpass filter Y.sub.2, is connected between the collector of the oscillator transistor Q.sub.1 and the output terminal of the oscillator circuit 100. The bandpass filter Y.sub.2 is connected between the collector of the oscillator transistor Q.sub.1 and the output terminal of the oscillator circuit 100 and acts to attenuate signals having a frequency outside of a particular ‘pass band’ range. As can be seen in FIG. 1, the node at which MSL.sub.4 and MSL.sub.6 are connected is further connected to the input of the bandpass filter Y.sub.2. MSL.sub.6 has one end connected to the first end of MSL.sub.4 (i.e. the end of MSL.sub.4 that is not connected to MSL.sub.5) and to the collector terminal of the oscillator transistor Q.sub.1. The other end of MSL.sub.6 is open-ended. MSL.sub.7 is connected such that one end of MSL.sub.7 is connected to the output terminal of the oscillator circuit 100 and to the output of the bandpass filter Y.sub.2, and the other end of MSL.sub.7 is open-ended.

[0084] The output matching circuit, comprising MSL.sub.6-7 and the bandpass filter Y.sub.2, transforms the load (output) impedance to a value suitable for the collector terminal of the oscillator transistor Q.sub.1 to provide conjugate matching. The bandpass filter Y.sub.2 filters out both higher harmonics and DC components from the microwave oscillation signal generated by Q.sub.1.

[0085] The connection between the emitter of the oscillator transistor Q.sub.1 and the SIW Y1 could be direct, however in this particular embodiment MSL.sub.8 is connected between these. MSL.sub.8 transforms the input impedance of the SIW resonator Y.sub.1 to certain values at the Q.sub.1 emitter in order to obtain a negative resistance at the Q.sub.1 collector at the frequency of interest.

[0086] In accordance with Leeson's equation, the power spectral density of the oscillator's phase noise may be described as (D. B. Leeson “A simple model of feedback oscillator noise spectrum” Proceedings of the IEEE, Volume: 54, Issue: 2, Pages: 329-330, February 1966):

[00001]$S_{\varphi }\left({\omega }_m\right)=\left[\frac{a}{{\omega }_m}+\frac{\mathrm{FkT}}{P_s}\right].Math.\left[1+{\left(\frac{{\omega }_0}{2Q_L{\omega }_m}\right)}^2\right]$

[0087] where α—a constant determined by the magnitude of l/f variations (flicker noise) of an active device;

[0088] .sub.L—loaded quality factor of a resonator in an oscillator;

[0089] ω.sub.m—carrier offset radian frequency, rad/sec;

[0090] ω.sub.0—carrier radian frequency, rad/sec;

[0091] P.sub.s—the signal level at an oscillator active element, W;

[0092] F—noise factor of the oscillator transistor;

[0093] k—Boltzman constant; and

[0094] T—temperature, K.

[0095] From the equation above it follows that the main contributors to the phase noise of the oscillator are the resonator's loaded quality factor and the flicker noise of the active device.

[0096] The l/f flicker noise in BJT may be represented by the current noise spectrum density that is expressed as (J. L. Plumb and E. R. Chenette, “Flicker Noise in Transistors,” IEEE Trans. Electron Devices, vol. ED-10, pp. 304-308, September 1963)

[00002]$S_{\mathrm{IB}}=\frac{K{I}_B^n}{f}$

[0097] where I.sub.B—base current, A;

[0098] K—a constant depending on transistor's technology;

[0099] n—a constant usually from 1 to 2; and

[0100] f—offset frequency, (Hz).

[0101] Therefore, it is highly important to provide the oscillator circuit with high-Q resonator and feedback techniques, which reduce flicker noise (l/f noise) and stabilize DC current in order to achieve good phase noise signal performance.

[0102] FIG. 2 is a circuit diagram of an oscillator circuit 200 in accordance with a further embodiment of the present invention in which a two-stage feedback arrangement is used to provide the negative collector-base parallel feedback. An example of a suitable microwave circuit layout for the oscillator circuit 200 is shown in FIG. 7.

[0103] The oscillator circuit 200 of FIG. 2 is similar in structure and function to the oscillator circuit 100 of FIG. 1. As such, for ease of reference, components having a reference numeral appended starting with a ‘2’ are alike in structure and function to those components having the same reference numeral starting with a ‘1’ described previously with reference to FIG. 1, unless context dictates otherwise. Thus a component having reference numeral ‘2xx’ in FIG. 2 should be assumed to be alike in form and function to the component having reference numeral ‘1xx’ in FIG. 1, unless otherwise specified.

[0104] The oscillator circuit 200 of FIG. 2 provides further improvements in the negative feedback arrangement. In addition to the components of the oscillator circuit 100 of FIG. 1, the oscillator circuit 200 of FIG. 2 further comprises a second feedback transistor Q3, which in this embodiment is a pnp BJT having respective emitter (first), collector (second), and base (control) terminals.

[0105] The emitter terminal of the second feedback transistor Q.sub.3 is connected to the supply voltage via resistor R.sub.b3. The collector terminal of the second feedback transistor Q.sub.3 is connected to the base terminal of the first feedback transistor Q.sub.2, and the base terminal of the second feedback transistor Q.sub.3 is connected to the emitter terminal of the first feedback transistor Q.sub.2 via a third feedback path, as outlined in further detail below.

[0106] This second feedback transistor Q.sub.3 advantageously increases the negative feedback to achieve higher dc stability of the oscillator transistor Q.sub.1 and to further reduce phase noise. Furthermore, the second feedback transistor Q.sub.3 provides significant improvements to temperature stability of the oscillator transistor Q.sub.1.

[0107] The third feedback path comprises an additional resistor R.sub.b5 connected between the base terminal of the second feedback transistor Q.sub.3 and the emitter terminal of the first feedback transistor Q.sub.2, at the node that connects R.sub.c1 and R.sub.c2 in the first feedback path. Thus, the voltage at the emitter terminal of the first feedback transistor Q.sub.2 is applied to the base terminal of the second feedback transistor Q.sub.3 via R.sub.b5.

[0108] R.sub.b1, described previously, is now connected to between R.sub.b3 and the node at which the collector terminal of the second feedback transistor Q.sub.3 and the control terminal of the first feedback transistor Q.sub.2 are connected.

[0109] A further resistor R.sub.b3 is connected between the emitter terminal of the second feedback transistor Q.sub.3 and the supply voltage Vcc at the node that connected R.sub.c1 and C.sub.c1 in the first feedback path. Thus the voltage at the emitter terminal of the first feedback transistor Q.sub.2 is applied to the base terminal of the second feedback transistor Q.sub.3 via R.sub.b5.

[0110] Thus due to this arrangement, the second feedback transistor Q.sub.3 drives the base current of the first feedback transistor Q.sub.2, and the first feedback transistor Q.sub.2 drives the base current I.sub.b of the oscillator transistor Q.sub.1 and, consequently, controls the collector current of the oscillator transistor Q.sub.1.

[0111] FIG. 3 is a circuit diagram of an oscillator in accordance with a further embodiment of the present invention in which the bandpass filter Y.sub.2 is replaced with an additional microstrip line MSL.sub.9. An example of a suitable microwave circuit layout for the oscillator circuit 300 is shown in FIG. 8.

[0112] The oscillator circuit 300 of FIG. 3 is similar in structure and function to the oscillator circuits 100, 200 of FIGS. 1 and 2. As such, for ease of reference, components having a reference numeral appended starting with a ‘3’ are alike in structure and function to those components having the same reference numeral starting with a ‘1’ described previously with reference to FIG. 1 and/or starting with a ‘2’ described previously with reference to FIG. 2, unless context dictates otherwise. Thus a component having reference numeral ‘3xx’ in FIG. 3 should be assumed to be alike in form and function to the component having reference numeral ‘1xx’ in FIG. 1 and/or reference numeral ‘2xx’ in FIG. 2, unless otherwise specified.

[0113] As outlined above, in this particular embodiment, the bandpass filter Y.sub.2 used in the oscillator circuits 100, 200 of FIGS. 1 and 2 respectively is replaced with an additional microstrip line MSL.sub.9. This MSL.sub.9 is dimensioned such that it possesses the desired wave impedance and electrical length and transforms the impedance at the point MSL.sub.7 is connected to necessary values at the collector terminal of the oscillator transistor Q.sub.1 in order to match the output impedances of the oscillator output and the collector of the oscillator transistor Q.sub.1.

[0114] FIG. 4 is a graph showing phase noise spectral density as a function of frequency for different oscillator circuits. Specifically, FIG. 4 provides a comparison of the performance of a conventional oscillator circuit with ‘passive’ bias control (not shown) shown as plot a), the oscillator circuit 100 with ‘one-stage active bias control’ of FIG. 1 shown as plot b), and the oscillator circuit 200 with ‘two-stage active bias control’ of FIG. 2 shown as plot c). A further comparison of the performance of these circuits can be seen in the table of FIG. 5.

[0115] As can be seen from the plots, the ‘one-stage’ oscillator circuit 100 of FIG. 1 provides improvements over the prior art example. Specifically, it can be seen from comparing plot a) with plot b) that around a 7 dB improvement is achieved in terms of phase noise by implementing the oscillator circuit 100 of FIG. 1. Thus, this provides for lower phase noise and faster switching than is achievable with the prior art circuit.

[0116] The ‘two-stage’ oscillator circuit 200 of FIG. 2 provides yet further improvements in phase noise. By comparing plot c) with plots a) and b), it can be seen that the oscillator circuit 200 of FIG. 2 provides a further 3 dB reduction in phase noise than the one-stage oscillator circuit 100 of FIG. 1, and an overall 10 dB reduction compared to the prior art passive circuit.

[0117] As can be seen from the table of FIG. 5, the oscillator circuit 100 of FIG. 1 provides improvements over the prior art passive circuit in terms of DC current drop and microwave power drop for a given drop in supply voltage V.sub.cc, which may correspond to a drop in battery voltage over time as outlined previously.

[0118] The data shown in the table of FIG. 5 also demonstrates that the oscillator circuit 200 of FIG. 2 is even more stable over the same supply voltage drop, providing twice lower reduction of DC current and microwave output power than the prior art example.

[0119] Thus it will be appreciated that FIG. 4 demonstrates that both the oscillator circuit 100 of FIG. 1 and the oscillator circuit 200 of FIG. 2 perform better than a conventional oscillator circuit, known in the art per se. While not shown in FIGS. 5 and 6, the oscillator circuit 300 of FIG. 3 would exhibit similar performance to the circuit 200 of FIG. 2.

[0120] FIG. 6 is a plot showing a microwave pulse. In particular, FIG. 6 shows a 5.5 ns microwave pulse. A control pulse at the switch on/off input (i.e. at the base terminal of Q.sub.4) of the active bias circuit 200 of FIG. 2 is 10 ns. Consequently, turn-on/off time is approximately 2.3 ns. This faster switching makes the oscillator circuits of the present invention particularly well-suited to impulse operation.

[0121] Thus it will be appreciated that aspects of the present disclosure provide an improved bias circuit for an oscillator, as well as an improved oscillator circuit comprising such a bias circuit and high quality resonator, that exhibits improvements in terms of phase noise, output power, and stability than existing ‘passive’ bias circuits.

[0122] While specific examples of the disclosure have been described in detail, it will be appreciated by those skilled in the art that the examples described in detail are not limiting on the scope of the disclosure.