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Fast-Hopping RF Synthesiser using Switched Charge-Pump Current for Optimum Channel-Dependent Noise Performance

IP.com Disclosure Number: IPCOM000021051D
Original Publication Date: 2003-Dec-18
Included in the Prior Art Database: 2003-Dec-18
Document File: 6 page(s) / 63K

Publishing Venue

Motorola

Related People

Neil Turner: AUTHOR

Abstract

The current state of the art in fast frequency-hopping synthesiser is to use a fractional-N core using a phase alignment system, switching to dual modulus after frequency and phase acquisition. This gives very rapid frequency change without the associated fractional-N spurious signals, by using a conventional dual-modulus system when locked. This paper shows how the use of a channel-dependent phase detector frequency can be implemented at no cost, to reduce in-band noise whilst still allowing the use of a simple, low-order loop filter. The key improvement is that the charge pump current can be set to optimise the locked performance. The phase detector frequency should be increased to as high a value as is feasible whilst still retaining an integer-N divide ratio. This dramatically improves the noise performance and is a significant improvement to the current state of the art.

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Fast-Hopping RF Synthesiser using Switched Charge-Pump Current for Optimum Channel-Dependent Noise Performance

 

Neil Turner

Abstract

The current state of the art in fast frequency-hopping synthesiser is to use a fractional-N core using a phase alignment system, switching to dual modulus after frequency and phase acquisition. This gives very rapid frequency change without the associated fractional-N spurious signals, by using a conventional dual-modulus system when locked. This paper shows how the use of a channel-dependent phase detector frequency can be implemented at no cost, to reduce in-band noise whilst still allowing the use of a simple, low-order loop filter. The key improvement is that the charge pump current can be set to optimise the locked performance. The phase detector frequency should be increased to as high a value as is feasible whilst still retaining an integer-N divide ratio. This dramatically improves the noise performance and is a significant improvement to the current state of the art.

Introduction

Frequency-hopping rf synthesiser design generally involves a trade-off between lock-time and spurious rejection. A narrow loop filter can be used to filter off reference sidebands and noise, but at the expense of a longer lock-time. Widening the loop bandwidth decreases the lock-time, but spurious rejection is decreased. The aim for the designer is to create a synthesiser that balances both demands and meets the system requirements.

However, one of the elements of the balancing act (locktime) can be removed, by using an open-loop technique unrelated to loop filter bandwidth. This involves a fast-hopping system that breaks the loop and turns on the charge pump for rapid frequency change. The phase-frequency detector is used to detect when the correct frequency is met, then the loop is re-closed to enable traditional closed-loop control operation again. The technique uses the method of resetting the divide-by-N counter to enable frequency comparison and then phase synchronisation.

The system design for a fast-hopping synthesiser can then become one of optimised loop filter design for spurious rejection, probably using a standard dual-modulus system. It is assumed that the designer can meet the minimum system requirements for, say, a GSM synthesiser using this architecture, but is there an enhancement that can be applied for enhanced rf performance ?

Problem to be Solved

Flexibility can be achieved in the loop filter design since there are now no constraints on locktime. Assume the designer’s aims are to optimise noise performance whilst locked, whilst using the minimum number of components in the loop filter to reduce material costs.

Let us progress on the assumption that the system spec can be met using a dual-modulus design with a phase detector comparison frequency equal to the channel spacing. In GSM, for example this would be 200 kHz.

Assume in the GSM synthesiser it is required to generate a local oscillator signal of 800....