Microwave Synthesizers

Herley General Microwave has been a leader in the field of microwave PIN diode control components for more than 30 years. A natural extension to its product line, microwave oscillators, was launched in 1989. In a relatively short period of time, General Microwave has established itself as an industry leader in manufacturing microwave oscillators as well. Its oscillator engineering staff has been recognized as a dynamic, innovative force that is willing and quite able to take on and solve today’s most demanding problems.

Principle of Operation

An oscillator has at least one active device, such as a transistor, that acts as an amplifier. When power is first applied, random noise is generated within the active device, and then amplified. This noise is fed back positively through frequency-selective circuits to the input, where it is amplified again. Ultimately a state of equilibrium is reached: the losses in the circuit are balanced by consuming power from the power supply, and the amount of positive feedback to sustain oscillation, as well as the frequency of oscillation, is determined by the external components. These external components may be inductors and capacitors, as in an LC circuit, or a crystal, in a crystal oscillator.

Modern microwave oscillators utilize a solid-state device, such as a transistor or diode, together with a resonant circuit and matching network, to convert DC power to microwave power at a specified microwave frequency. By appropriate choice of these elements, oscillators may be designed for an extremely wide range of applications. Signals produced by oscillators are extremely precise and can be used in a variety of consumer electronic products. In addition, low frequency digital and analog control circuitry may be incorporated to provide further flexibility.

Types of Oscillators

Herley General Microwave has a complete line of high-performance voltage-controlled oscillators (VCOs) and digitally-tuned oscillators (DTOs).

A voltage-controlled oscillator provides a signal whose frequency is controllable using an analog voltage signal. When employing an LC oscillator, for example, there are high costs and other difficulties encountered when employing quality variable capacitors. This often makes VCOs an extremely attractive alternative. This is because diodes, when they have a reverse voltage applied, exhibit the characteristics of a capacitor. Altering the voltage alters the capacitance. This is where voltage-controlled oscillators enter the picture.

VCOs can be generally categorized into two groups based on the type of waveform produced:

1. harmonic oscillators, which generate a sinusoidal waveform, and
2. relaxation oscillators, which can generate a sawtooth or triangular waveform.

VCOs are used in:

• Electronic jamming equipment. For example, by sending out radio waves along the same frequencies that a radio or cellular phone uses, this causes enough interference with the communication between the radio or cell phone and the transmission tower to render the phone or radio unusable.
• Function generators, which generate either repetitive or single-shot (trigger) waveforms
• Electronic music, to generate variable tones,
• Phase-locked loops, which generate an output signal whose phase is matched with that of an input reference signal
• Frequency synthesizers used in communication equipment.

A digitally-tuned oscillator was designed as an attempt to increase stability in tuning the frequency of a VCO having a quartz crystal. The VCO is synchronized by an external frequency reference. The reference in this case is reset pulses produced by a digital counter—digital tuning input. The DTO thus provides the desired output frequency in response to a digital control signal.

The DTO features also a modulator circuit that produces a digitally-modulated output signal by noise shaping or oversampling a multi-bit input signal. The modulator circuit includes an input for the multi-bit input signal and an output that has a lower number of bits than the input. A DTO can also include a tuning capacitor to further tune the frequency of the crystal oscillator circuit.

Herley offers a line of single-band DTOs, employing a single VCO, or multiband, employing a range of VCOs. Multiband DTOs achieve broadband frequency coverage and improve settling speed. A schematic of a multiband DTO is shown in Figure 1.

Figure 1: Multi Band DTO Block Diagram

DTOs are used in:

• Electronic Warfare / Signals intelligence (EW/SIGINT)
• Radar Test Equipment
• Microwave Radio
• Stabilizing signals for navigation and communications electronics
• Providing signals for clocks used in data processing equipment.

General Microwave offers a broad line of high-performance VCOs, DTOs and Microwave Synthesizers in the microwave frequency range. The VCOs and DTOs have the following features:

• Fast settling time
• Low post-tuning drift
• Low phase noise.

In addition to General Microwave’s standard catalog products, a wide variety of custom oscillators have been developed for demanding applications such as:

• Airborne receivers,
• Jamming, and
• Simulator applications.

This site is proof of General Microwave’s success. It includes expanded versions of catalog oscillator products and highlights many of the custom oscillators, both military and commercial, that have been successfully developed and manufactured. If your system requirements demand a device which cannot be found in this brochure, do not hesitate to contact General Microwave directly. A sales engineer will be happy to discuss your specific needs.

Definitions of Parameters

Frequency Settling Time/Post-Tuning Drift:

When applying a step voltage to a VCO, it changes its frequency from an initial value F1 to a final value, F2. The frequency F2 will settle to a stabilized value after some time. Frequency Settling/Post-Tuning Drift is the maximum deviation in frequency at a given time, following a change in tuning command, relative to the frequency one second after the change in tuning command. The worst-case condition usually occurs for frequency steps from one end of the band to the other.

Settling time

Settling time is the response up to several hundred microseconds, while post-tuning-drift usually refers to the variation from several hundred microseconds to as long as several hours. Figure 2 illustrates Short-Term Post Tuning Drift (settling time) and Long-Term Post Tuning Drift, in response to the step voltage V2-V1:

Figure 2: Illustration of Post Tuning Drift

Results of a typical measurement of settling time are shown in Figure 3:

Figure 3: Measured Settling Time of 8-12 GHz VCO

Modulation Sensitivity Ratio:

Modulation Sensitivity Ratio is the ratio between the maximum and minimum slopes of the frequency vs. voltage tuning curve of a VCO over its frequency band. (For a DTO, this is defined at the FM modulation port.)

Frequency Deviation Bandwidth:

Frequency deviation bandwidth the peak-to-peak frequency deviation obtained for a given peak-to-peak voltage swing at the modulation port of a VCO or DTO.

Modulation Bandwidth:

Modulation Bandwidth is the modulation frequency at which the frequency deviation bandwidth of a VCO or DTO decreases by 3 dB relative to the deviation bandwidth at low frequencies.

Phase Noise:

Phase Noise is the sideband noise level at a given deviation, FM, from the oscillator frequency, relative to the carrier power level and normalized to a bandwidth of I Hz. Typical measured Dielectric Resonator Oscillator (DRO) phase noise versus frequency deviation is shown in Fig. 2. From 10 kHz to 100 kHz, the phase noise of a VCO or DRO has a nominal 1/fm(3) dependence. Thus, as shown in the figure, the phase noise at 100 kHz is approximately 30 dB lower than that at 10 kHz.

Residual FM:

Residual FM is the peak-to-peak frequency deviation of an oscillator at its -3 dBc points, when measured on a spectrum analyzer with a resolution bandwidth of 1 kHz. (See Figure 4)

Figure 4: Residual FM

Temperature Stability:

Temperature Stability is defined as the total oscillator frequency variation over the rated operating temperature, usually expressed in ppm/°C.

Pulling:

Pulling is the maximum variation in oscillator frequency relative to its frequency when operating with a matched load, when the output load is rotated through a full 360° phase change. The peak-to-peak variation in oscillator frequency is approximately twice the pulling figure defined above.

By using the following approximate formula, the pulling figure may be scaled as a function of the VSWR (Voltage Standing Wave Ratio):

Where f_o is the oscillator frequency, Q_EXT is the external Q (=Quality factor, which describes the damping of an oscillator; a high Q indicates a lower rate of energy loss relative to the stored energy of the oscillator), and S is the load VSWR.

Pushing:

Pushing is the incremental change in oscillator frequency that results from an incremental change in power supply voltage.