LOW VOLTAGE VCO WITH DIFFERENTIAL CONTROL VOLTAGE FOR WIDEBAND TUNING

Publishing Venue

Motorola

Related People

Abstract

Normally in a wideband VCO, in order to keep the oscillator noise to a reasonable level, the gain (MHz/V) of the VCO must be kept within reasonable value consistent with the hum and noise performance. In order to achieve that, the available tuning voltage must be as large as possible. This can be achieved by either a large positive control voltage and or by using a negative voltage convertor which has to be programmable as the fre- quency selection is made.

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MOlVROLA Technical Developments Volume 9 August 1989

LOW VOLTAGE VCO WITH DIFFERENTIAL CONTROL VOLTAGE FOR WIDEBAND TUNING

by Joey L.H. Ooi

   Normally in a wideband VCO, in order to keep the oscillator noise to a reasonable level, the gain (MHz/V) of the VCO must be kept within reasonable value consistent with the hum and noise performance. In order to achieve that, the available tuning voltage must be as large as possible. This can be achieved by either a large positive control voltage and or by using a negative voltage convertor which has to be programmable as the fre- quency selection is made.

A technique for extending the VCO frequency range without the aid of a microprocessor or a programmable negative supply is shown in Figure 1.

The negative power supply to the operational amplifiers need not be programmable.

Ul, U2 are low noise op amps

The output of Ul is V'c = + B+ ROY%3 + 3

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The output of U2 is V'% = - B+

ROYi + 3

Therefore the total voltage dropped across the varactor is

2

RF+B;3 + ;vc

1

If the control voltage is now allowed to from Vcl to VC2 the voltage dropped across the varactor is now:

= g nV where nV = Vc2 - Vcl

We now have the ability to scale the tuning voltage across the VCO varactor.

   Consider the application is a low voltage application where the control voltage is only allowed to move 1 to3V. If afixed -5V negativeconvertoris used forthecaseof Rl = R2 then the allowed tuningvoltagechange across Cv is then 4 volts.

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0 Motorola, Inc. 1989...