Multiloop PLL Frequency Synthesizer

- the multiloop PLL frequency synthesizer combines several loops to enable high levels of performance to be achieved

Many high performance synthesizers use several loops that incorporate both mixers and digital dividers.

By using these techniques it is possible to produce high performance wide range signal sources with very small step sizes. If only a single loop is used then there may be short falls in the level of performance.

There is a large variety of ways in which multi-loop synthesizers can be made, dependent upon the requirements of the individual system. However as an illustration a two loop system used as an example. This uses one loop to give the smaller steps and the second provides larger steps. This principle can be expanded to give wider ranges and smaller steps.

An example of a synthesizer using two loops

The first phase locked loop, PLL, has a digital divider and operates over the range 19 to 28 MHz. Having a reference frequency of 1 MHz it provides steps of 1 MHz. The signal from this loop is fed into the mixer of the second one. The second loop has division ratios of 10 to 19, but as the reference frequency has been divided by 10 to 100 kHz to give smaller steps.

The operation of the whole loop can be examined by looking at extremes of the frequency range. With the first loop set to its lowest value the divider is set to 19 and the output from the loop is at 19 MHz. This feeds into the second loop. Again this is set to the minimum value and the frequency after the mixer must be at 1.0 MHz. With the input from the first loop at 19 MHz this means that the VCO must operate at 20 MHz if the loop is to remain in lock.

At the other end of the range the divider of the first loop is set to 28, giving a frequency of 28 MHz. The second phase locked loop, PLL, has the divider set to 19, giving a frequency of 1.9 MHz between the mixer and divider. In turn this means that the frequency of the VCO must operate at 29.9 MHz. As the phase locked loops, PLLs, can be stepped independently it means that the whole synthesizer can move in steps of 100 kHz between the two extremes of frequency. As mentioned before this principle can be extended to give greater ranges and smaller steps, providing for the needs of modern receivers.

PLL frequency synthesizers are particularly flexible to use and to be controlled by digital commands. In addition to this they offer many distinct advantages over other forms of local oscillator for many applications both in radio communications equipment as well as test equipment, etc. As a result PLL frequency synthesizers are widely used for many RF applications. Even though other forms of generator including Direct Digital Synthesizers are available, the PLL based synthesizer is has many advantages, and can often be used in conjunction with other generators including DDS circuits.

By Ian Poole

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