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A PLL BANDWIDTH AUTOCALIBRATION METHOD

IP.com Disclosure Number: IPCOM000172275D
Publication Date: 2008-Jul-07
Document File: 5 page(s) / 45K

Publishing Venue

The IP.com Prior Art Database

Abstract

A method for automatically measuring and intelligently adjusting the loop gain parameter in a type I PLL is described, thereby minimizing bandwidth variations due to process, voltage, and temperature shifts and channel frequency.

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A PLL Bandwidth Autocalibration Method

Abstract

            A method for automatically measuring and intelligently adjusting the loop gain parameter in a type I PLL is described, thereby minimizing bandwidth variations due to process, voltage, and temperature shifts and channel frequency.

I. Motivation

The closed loop response of a PLL is given by:

where

·        KPD = phase detector gain

·        KV = VCO tuning sensitivity

·        N = feedback divide ratio

·        wpi = the ith pole of an all pole loop filter of order k

Inspection of this expression immediately shows that PLL bandwidth variation is dependent on

·        loop filter pole variations

·        loop gain variations

Many techniques exist for on-die calibration of RC time constants, addressing the first source of variation.  It can be seen that particularly in PLLs covering wide tuning ranges, there will be an inherent variation in the loop gain term in that N is varied to achieve tuning.  KV is a parameter that also tends to vary naturally with frequency:

where

  • L is the VCO tank inductance
  • CVAR is the analogue tuning varactor capacitance
  • VTune is the PLL tuning line voltage

Compensating control of KV is possible – for example by making the analogue tuning varactor out of an array of devices, the number of which connected to the tuning line can be digitally selected yielding the following expression for KV:

where a(w) is the number of varactor units switched into the analogue tuning port and if this is made according to an approximate inverse square relationship with frequency, KV/N variations are compensated to first order.

            It will be recognized that variations remain: KV is a complex function of temperature, process variations and amplitude of oscillation, and KPD may vary with process and mismatch too.  In one embodiment, that of an XOR phase detector,

where VDD is the supply voltage for the XOR and may, for example, be produced by a low noise regulator, which will exhibit some process and mismatch variations usually.  Therefore the loop gain term will suffer from undesired variations, thereby resulting in PLL bandwidth variations.

II. Proposed Solution

A diagram of the proposed calibration system is shown in fig. 1.  During autocalibration, a divider is employed on the referen...