Microwave Synthesizers, RF Synthesizer - Selection Guide
Application Notes for Microwave Sources
Microwave Synthesizers - General
Kratos General Microwave (KGM) has developed a broad line of General Purpose RF and Microwave Synthesizers to be used in various applications. KGM has developed a line of high performance, broadband Fast Indirect Synthesizers (FIS) to provide a cost-effective solution to the requirements of new systems. Its high speed (as fast as 1 µsec) provides an economical alternative to direct synthesizers for many applications. Because of its low phase noise, it is an excellent alternate to the much slower and generally less reliable YIG-based rf synthesizer.
To provide optimum solutions for different requirements, Kratos General Microwave has developed a variety of Fast Indirect Synthesizers (FIS) with different parameter trade-offs: The standard FIS line for fastest tuning speed, the low phase noise line for ELINT applications and the compact FIS line for airborne small size applications. Some of the FIS rf microwave synthesizers can be configured (optionally) to operate as a dual mode source. In the synthesizer mode and in the high speed (1 µsec) DTO mode with modulation capability.
Microwave Synthesizers - Synthesizers Types
1.Direct Microwave Synthesizers
Direct Synthesizers are based on direct multiplications and summation of the reference oscillator. For example, if a 10 MHz reference oscillator is used, then by using multiplication factors of 1000 to 18000 a 10 GHz to 18 GHz signal source with 10 MHz step size could be generated.
- Very fast (down to 40 ns)
- Frequency and Phase Coherent
- Moderate Step Size
- Larger Package
- Moderate to High Cost
2. Indirect Microwave Synthesizers
Indirect Microwave Synthesizers are based on Phase Locked Loop (PLL) topology that is locked on the reference oscillator. Since PLL loops support both integer and fractional loops the step size could be fraction of the reference frequency.
- Fine Step Size (below 1 Hz)
- High Speed (better than 1 µs)
- Frequency Coherent
- Frequency Modulation
- Small to Moderate Package
- Low to Moderate Cost
Microwave Synthesizers - Coherency Definition
1. Phase Coherency
A frequency synthesizer is defined as phase coherent if for any chosen output frequency the phase of the signal stays the same no matter if the rf synthesizer has jumped to other frequencies and back to the original one or stayed the whole time in the same frequency.
2. Frequency Coherency
A frequency synthesizer is defined as frequency coherent if for any chosen output frequency the following relation is always being kept:
In other words, the output signal is always some multiplication of the reference signal.
Microwave Synthesizers - Frequency Modulation Definition
Some frequency sources have the ability to change their frequency in response to a change of the voltage on their modulation input pin. This is an FM modulation mechanism. Two important and sometime confusing parameters define this ability as detailed below:
1. Frequency Deviation Bandwidth
The peak-to-peak frequency deviation obtained for a given peak-to-peak voltage swing at the modulation port of a microwave rf synthesizer.
2. Modulation Bandwidth
The modulation frequency at which the frequency deviation bandwidth of a microwave rf synthesizer decreases by 3 dB relative to the deviation bandwidth at low frequencies.