Solid State Microwave Oscillators Convert dc energy to microwave signals Can be used as generators in all communication systems, radars, electronic counter measures (ECM), etc Replacing more and more low power tubes such as small klystrons The main advantages of solid state oscillators are: Low voltage supply Reliability Small weight Simple in fabrication and low cost

Solid state oscillators are generally used as a Local Oscillator for mixing operation. The Local Oscillator is a pure sinusoidal signal source used to provide the mixing function in a mixer. It is fed into the mixer LO port, and is used for down conversions and up conversions. Oscillators are characterized by the following parameters: 1)Frequency 2)Frequency stability: - Frequency pushing (frequency change with poor supply voltage or current) - Frequency pulling (frequency change with load mismatch) - Temperature stability - Shock and vibration effect

3)Phase noise 4)Spurious Performance 5)Power and efficiency 6)Tuning: Mechanical Electronically Tuning sensitivity (Hz/V or Hz/ mA) Settling time (the time it takes to respond to frequency control signal) Post-tuning drift (drift after the oscillator reaches its desired frequency ) Oscillator parameters are mainly determined by the type of the resonance circuit or the cavity used

1.A resonant structure (is mainly characterized by its quality factor (Q) ). Quality factor is determine by the type of resonator at a specified frequency of operation. 2.A negative resistance system controlled by the power 3.A coupling system able to deliver power to the load. Oscillator is Composed of three parts:

Microwave Resonators generally used in microwave Oscillators: A typical microwave resonators are shown in the following Figure /2 Resonator Tran. power fofo ff Q=f o /  f Microstrip Dielectric Coaxial Lumped SAW

Active Devices used for microwave oscillators Microwave semiconductors used as the active oscillator devices are: BJT at the low end of the microwave band MESFETFET HEMT IMPATT Gunn devices at the high end of the microwave band Varactor multiplier

Mechanical tuned Resonators

Phase noise The phase noise is defined as the power in 1-Hz bandwidth at a frequency f m from the carrier, and measured in dB below the carrier power. The phase noise of microwave oscillator is 60 to 120 dB below the carrier power. 0 Phase noise (dB) 1 KHz

Effect of Phase Noise on Receiver Performance How much phase noise can be tolerated in a given system design ? IF Desired LO Phase noise Noisy LO Desired signal undesired signal

How phase noise affect false detection for radar applications Transmitted signal Main signalNoise sideband 40 dB Received signal Main signalNoise sideband 40 dB Airplane Mountain

Some Oscillator Configurations Varactor Tuned Oscillators:

Electronically Tuned

Some IMPATT Oscillators Mechanically Tuned

Transistors Based Oscillators Configurations

Harmonic Multiplier Varactor Duroid substrate LPF Matching Network

Frequency Synthesizers Precisely frequency controlled oscillator derived from a stable oscillator The stable reference source is usually a crystal controlled oscillator, Can be be housed in a temperature controlled environment for even greater stability. Eliminates the need for many independent crystal oscillators in a multichannl system (ex. GSM system) Can be easily implemented using a variety of available integrated circuits. There are three basic methods that can be used, the most popular method is the phase-locked loop.

Phase locked loops Loop Filter fofo Phase detector Loop amplifier VCO Frequency divider  N Reference Frequency Outpu t Nf o

PLL consists of the following circuits: Reference oscillator Phase detector produces an output voltage proportional to the difference in phase of the two inputs Loop amplifier and filter VCO operating at the desired output frequency Frequency divider In operation, the output of the voltage controlled oscillator is divided by N to match the frequency of the reference oscillator. The phase detector produces a voltage proportional to the difference in phase of these two signals and is used to make small corrections in the frequency of the the VCO in order to align the phase of the VCO with that of the reference source. There are