Browse Prior Art Database

THE USE OF RAMPED ATTENUATION TO REDUCE VOLTAGE CONTROLLED OSCILLATOR LOAD-PULLING EFFECTS DURING THE SWITCHING OF A PHASE-LOCKED LOOP

IP.com Disclosure Number: IPCOM000008134D
Original Publication Date: 1997-Jun-01
Included in the Prior Art Database: 2002-May-21
Document File: 3 page(s) / 161K

Publishing Venue

Motorola

Related People

Neil Turner: AUTHOR [+2]

Abstract

This paper is concerned with the use of a vari- important. One particular environment where the able attenuator to reduce the effects of voltage-con- principle may be applied is that of a frequency hop- trolled oscillator 'load-pull.' It is relevant to any ping system where two synthesisers are switched in system that switches a voltage controlled oscillator a 'ping-pang' arrangement. (VCO) in an environment where phase accuracy is vco ) In PLL Isolation Switch > output L Fig. 1 Consider the system in Figure 1. The voltage controlled oscillator signal is passed to a switch, and then to the output.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 40% of the total text.

Page 1 of 3

0 M

MOTOROLA Technical Developments

THE USE OF RAMPED ATTENUATION TO REDUCE VOLTAGE CONTROLLED OSCILLATOR LOAD-PULLING EFFECTS DURING THE SWITCHING OF A PHASE-LOCKED LOOP

by Neil Turner and Nigel Tolson

  This paper is concerned with the use of a vari- important. One particular environment where the able attenuator to reduce the effects of voltage-con- principle may be applied is that of a frequency hop- trolled oscillator 'load-pull.' It is relevant to any ping system where two synthesisers are switched in system that switches a voltage controlled oscillator a 'ping-pang' arrangement.
(VCO) in an environment where phase accuracy is

vco ) In PLL

Isolation Switch > output

L

Fig. 1

  Consider the system in Figure 1. The voltage controlled oscillator signal is passed to a switch, and then to the output.

  Under static conditions, the VCO is looking into a constant impedance. When the switch changes state, the instantaneous impedance that the VCO sees will change. Consider the example when the switch is a single-pole single-throw absorbtive device. Although the 'open' and 'closed' impedance may be identical, the return loss changes (its value is generally undefined) during the act of switching. If a gallium arsenide MMIC device is used, the switching time can be only a few nanoseconds in duration. This has the effect of creating an impulse which is applied to the output of the VCO. The phase-locked loop cannot track a disturbance of nanoseconds in duration, and the effect is that the VCO is 'pulled.' The phase of the oscillator is sub- ject to a disturbance, with the result that the loop

has to reacquire phase lock; the time this takes will depend upon the loop parameters. During this time, the signal is subject to phase error, and this may significantly impact the system that the signal is driving.

  The usual method to overcome load pulling is to insert sufficient wideband isolation between the VCO and the switch to attenuate any switching transients sufftciently so that they do not signifi- cantly impact upon the phase performance of the VCO. This can be achieved by the use of an isola- tor, or by insertion of amplifiers and attenuators to achieve the desired signal power at the switch. However, whilst this achieves the required isolation, it is inefficient if the signal is amplified and subse- quently attenuated merely in order to isolate two circuit elements. In addition, the gain line-up must generally be carefully chosen to avoid compressing the signal or driving it into the noise floor.

e Motmo,~ lot. ,997 34 June 1997

[This page contains 15 pictures or other non-text objects]

Page 2 of 3

0 M M-LA

Technical Developments

vco

In PLL

Switch

Control

Switch output

Fig. 2

  The system in Figure 2 is an alternative setup to achieve the required isolation. Assume that initially the PLL is locked, and that the switch is open (absorbtive) with the voltage-variable attenuator (VVA) giving a high insertion loss. The switch is then closed, which can...