Browse Prior Art Database

ELECTRONIC TUNING OF TX VCOs

IP.com Disclosure Number: IPCOM000007017D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2002-Feb-18
Document File: 1 page(s) / 59K

Publishing Venue

Motorola

Related People

Derek Cairns: AUTHOR

Abstract

The schematic below shows the Transmit VCO components for a GP300 radio The coil L3 is tuned in the factory to "centre" the VCO and compensate for component variation between different VCOs. The varactor CR1 is pro- vided with a variable voltage from the synthesiser chip which alters the frequency of the VCO. An AC signal supplied by the synthesiser to CR2 allows the VCO to be modulated.

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MOTOROLA INC. Technical Developments Volume20 October 1993

ELECTRONIC TUNING OF TX VCOs

by Derek Cairns

The schematic below shows the Transmit VCO components for a GP300 radio

  The coil L3 is tuned in the factory to "centre" the VCO and compensate for component variation between different VCOs. The varactor CR1 is pro- vided with a variable voltage from the synthesiser chip which alters the frequency of the VCO. An AC signal supplied by the synthesiser to CR2 allows the VCO to be modulated.

  The idea presented here is to also supply a DC bias signal from the synthesiser chip to the varactor CR2 (this would require a DAC to be added to the synthesiser chip). This would allow CR2 to alter the frequency of the VCO and could be used to "cen- tre" the VCO. Once this level was determined for a particular set of VCO components (i.e., during fac- tory tuning) the DAC value could be fixed/stored. This would allow a smaller and cheaper (non- adjustable) inductor to be used for L3 and factory tuning of the VCO could be carried out by com- puter instead of requiring an operator to physically tune the coil.

  The coils currently used are relatively large items and the ferrite cored versions used on VHF radios are quite fragile and cause more production prob- lems than other components. (By adding another varactor to the RX VCO and using the same approach tts own tuneable coil could also be elim- inated. The tuneable coil in the RX VCO is often the cause ofmicrophonics in some rad...