Browse Prior Art Database

CONTROL NETWORK TO PREVENT RAILING OF PLL WITH LOSS OF REFERENCE FREQUENCY

IP.com Disclosure Number: IPCOM000007740D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-18
Document File: 2 page(s) / 76K

Publishing Venue

Motorola

Related People

Dave Neperud: AUTHOR

Abstract

In the iDEN base station, a voltage controlled crystal oscillator is phase locked to a high stability external frequency reference. In the event of the loss ofthis reference frequency to the station, the crystal oscillator's control voltage will go to a railed state. This results in the radio transmitting on a wrong frequency (channel) and the inability to receive a signal, forcing the radio to be immediately shut down, causing a loss of calls. This is especially a problem when the external reference is momentarily broken to add another station to the system.

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Technical Developments

CONTROL NETWORK TO PREVENT RAILING OF PLL WITH LOSS OF REFERENCE FREQUENCY

by Dave Neperud

BACKGROUND

  In the iDEN base station, a voltage controlled crystal oscillator is phase locked to a high stability external frequency reference. In the event of the loss ofthis reference frequency to the station, the crystal oscillator's control voltage will go to a railed state. This results in the radio transmitting on a wrong frequency (channel) and the inability to receive a signal, forcing the radio to be immediately shut down, causing a loss of calls. This is especially a problem when the external reference is momentarily broken to add another station to the system.

PRINCIPLE OF OPERATION

  Figure 1 shows the control network that needs to be added to a conventional PLL block diagram in order to prevent railing of the VCXO (Voltage Con- trolled Crystal Oscillator) control line. Essentially it is a multivibrator with its input monitoring the ref- erence frequency, outputting a digital "1" when a signal is present and a "0" when it is missing. Adjust- ments for different reference frequencies can be made

by varying the integrator stage (Ri and Ci).

  This digital output has a dual purpose. One, it controls a gate on the oscillator's control line between the phase detector/charge pump output and the loop filter, opening it when the reference signal is lost. Second, it serves as an alarm to the controlling microprocessor telling it that the reference is go...