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RF Electronics Chapter 6: Oscillators Page 173 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Chapter 6 Oscillators Principles of Oscillators Requirements 1 The LO must be able to supply required output power specified. For example +7 dBm to drive a level 7 mixer. 2 The LO must have a sufficient low Phase and amplitude noise for the application. For example, FM radio has a 75 kHz deviation. For 60 dB SNR at the receiver output, a LO with < 75 Hz phase noise when measured in the 50Hz to 15 KHz band is needed. 3 The harmonics of the LO must be sufficiently small not to cause interference to other users or other parts of the receiver. In particular, harmonic mixing products must be very small and not cause any unwanted spurious components (spurii). Oscillator Types There are two basic oscillator types: Oscillator using positive feedback: This is the most common oscillator design technique for common lower frequencies and can be used at any frequency with a variety of different topologies (see below). Oscillator using negative Resistance: This relies on S11 of an amplifier in a circuit being >1 at some frequencies, resulting in oscillation. The amplifier is unstable at those frequencies where S11 >1, causing the amplifier to oscillate. This design technique is predominantly used for microwave operating frequencies. The design of oscillators using negative resistance is outside the scope of this book. Positive Feedback Oscillators The oscillator consists of an amplifier and a Frequency Selective Network. The amplifier output is applied to the frequency selective network, the output of which is applied to the amplifier input. For the analysis, a switch can be imagined to open the closed loop signal path, as shown in figure 6.1. If V2 is exactly the same as V1, both in amplitude and in phase then steady state oscillations result. If the magnitude of V2 > V1 and the phase is the same then the oscillations will grow. If V2 < V1, then the oscillations will decay. Figure. 6.1. Feedback oscillator block diagram. RF Electronics: Design and Simulation 173 www.cadence.com/go/awr

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