180 Radio Engineering for Wireless Communication and Sensor Applications
8.3 Oscillators
An oscillator is a circuit that transfers dc power to RF power [5–8]. The generated RF signal may be sinusoidal or distorted because of containing harmonic components. Important characteristics of an oscillator are its frequency and tuning range, output power, frequency stability, and spectral purity.
An oscillator may be modeled either as a feedback circuit providing a nonzero output voltage for zero-input voltage or as a one-port circuit having negative resistance. In this text we use the latter concept. An active element having a negative resistance may be a diode or a potentially unstable transistor (more about the stability of a transistor in Section 8.4). Figure 8.9 shows a simplified equivalent circuit of an oscillator: A passive load impedance Z L is connected to the input impedance Z in of an active component. The impedance of the active element depends on both current and frequency, that is,
Z in (I, f ) = R in (I, f ) + jX in (I, f ) |
(8.10) |
The load impedance depends on frequency:
Z L ( f ) = R L ( f ) + jX L ( f ) |
(8.11) |
Before an oscillation starts, the circuit must be in an unstable state, that is, R in + R L < 0. Because R L is always positive, R in must be negative. Then any disturbance or noise may cause oscillation at some frequency f . When the current increases due to oscillation, R in must change to less
Figure 8.9 A simplified equivalent circuit of an oscillator.
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negative. A properly designed oscillator settles down in a stable operation. Then, according to Kirchoff’s law (Z in + Z L ) I = 0, or
R in (I0 , f 0 ) + R L ( f 0 ) = 0 |
(8.12) |
X in (I0 , f 0 ) + X L ( f 0 ) = 0 |
(8.13) |
The final, stable oscillation frequency f 0 is usually different from the original frequency. In a stable condition, any disturbances of current or voltage are damped, and after a disturbance the oscillator rapidly returns into its stable state.
The load is a circuit with a high quality factor Q ; it is for example an LC circuit, quartz crystal, cavity, YIG, or dielectric resonator. Then X L changes fast as a function of frequency, and the reactance equation, (8.13), determines the oscillation frequency. In practice, R L is nearly independent of frequency. The higher the Q , the more stable the oscillation state. The oscillation may also be stabilized by using injection locking or phase locking. In the injection locking, a weak signal from an accurate frequency standard is fed into the oscillator. In the phase locking, the output frequency is compared to an accurate signal derived from a frequency standard (see Section 11.1). The frequency standard is usually a crystal-controlled oscillator, the frequency of which may be further stabilized by controlling the temperature of the quartz crystal.
Suitable diodes having a potential negative resistance for an oscillator are Gunn, IMPATT, and tunnel diodes. Only a proper bias voltage is needed to produce a negative resistance. A transistor oscillator may be based on a bipolar transistor in the common-emitter connection, or on an FET in the common-gate connection. Other configurations are also possible. A negative resistance is realized by connecting to the input port of the transistor a load, which makes the transistor unstable. Then the reflection coefficient seen toward the transistor in the output port is greater than unity; that is, the real part of the impedance is negative. The instability may be increased with feedback.
Bipolar transistors are used in oscillators up to about 20 GHz. HBT operates at higher frequencies, up to 50 GHz. These both have 10 dB to 15 dB lower phase noise, that is, a cleaner spectrum near the oscillation frequency, than an FET oscillator has. MESFET is suitable for oscillators up to 100 GHz and HEMT to 200 GHz. The Gunn oscillator operates in the fundamental mode to about 100 GHz, and in harmonic mode up to 200 GHz. IMPATT oscillators are used even at 300 GHz. The spectrum
182 Radio Engineering for Wireless Communication and Sensor Applications
of an IMPATT oscillator is rather noisy. The amplitude noise is especially strong; that is, the spectrum is noisy even far away from the oscillation frequency. Output powers available from semiconductor oscillators are presented in Figure 8.10. Tube oscillators provide much higher powers; for example, a klystron at 1 GHz or a gyrotron at 100 GHz may produce over 1 MW.
There are several alternative ways to tune the oscillation frequency. According to (8.13), the frequency f 0 depends on the reactance of both the active element and the load. Therefore the frequency can be tuned either mechanically or electrically. The resonance frequency of a cavity is tuned by changing its length, for example, by moving a short circuit in the end of the cavity. A Gunn oscillator may be tuned mechanically over an octave, and an IMPATT oscillator over ±10%. The resonance frequency of a dielectric resonator is tunable mechanically about ±2% by changing the distance of a metal plate or a screw above the resonator pill. With bias tuning the Gunn oscillator frequency changes about ±1%, and that of an IMPATT oscillator over ±5%. As mentioned before, a varactor is a voltage-dependent capacitor. With a varactor in the embedding circuit, a transistor oscillator may be tuned electrically about an octave. The varactor tuning is also often
Figure 8.10 Output powers available from semiconductor oscillators.
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used in oscillators stabilized with a DRO, but then the tuning range is small. A varactor tuning (Schottky varactor) is used up to 50 GHz. Such a voltagetuned oscillator is often called a voltage-controlled oscillator (VCO).
Figure 8.11 shows a 5.43-GHz transistor oscillator on a microstrip. A bipolar transistor is in the common-collector connection. The base impedance is made to be in an unstable region. The frequency is determined by a DRO and may be tuned electrically about ±1 MHz by changing the dc bias of a varactor.
Signal generators, especially sweepers, often utilize transistor oscillators, the frequency of which is tuned using a yttrium iron garnet (YIG) resonator. The YIG material is ferrite. In a static magnetic field the YIG resonator has a resonance at microwave frequencies. The YIG resonators are spherical, having a diameter of 0.2 mm to 2 mm. The resonator is coupled to the embedding network with a current loop around the sphere, as shown in Figure 8.12. The unloaded Q is about 10,000. The resonance frequency depends linearly on the magnetic field strength, which in turn depends linearly on the dc current of the magnet coil. The frequency tuning range can be 2–3 octaves, making it very useful for sweep generators. The YIG resonators are used up to 50 GHz.
The electric frequency tuning of an oscillator is also utilized in phase locking, frequency modulation, and demodulation. Frequency modulation (FM) (see Section 11.3) may be realized by modulating the control voltage of a VCO. Demodulation of an FM signal can be made by phase locking the frequency of a VCO to the received signal. The VCO control voltage is then the demodulated FM signal.
Figure 8.11 Microstrip layout of a transistor oscillator stabilized with a dielectric resonator and voltage tuned with a varactor.