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Voltage Controlled Oscillator

IP.com Disclosure Number: IPCOM000101642D
Original Publication Date: 1990-Aug-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 53K

Publishing Venue

IBM

Related People

Ebler, MB: AUTHOR [+4]

Abstract

A voltage controlled oscillator (VCO) is an important element of phase-locked loops. The figure shows a schematic of the VCO. In this circuit Q1 and Q2 form a cross-coupled pair of transistors. In a typical VCO, the multivibrator uses emitter followers to prevent Q1 and Q2 from saturation. In this application, the output voltage at 10 and 11 swings between 0.8 volts below the supply and 0.4 volts below the supply. The diodes D6 and D7 clamp the output at the low level. The high voltage level is generated by R7 and R9 and current sources Q5 and Q6. This high level of 0.4 volts below the supply prevents Q1 and Q2's bases from going too high and saturating. The capacitor, C1, is an on-chip oxide capacitor.

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Voltage Controlled Oscillator

       A voltage controlled oscillator (VCO) is an important
element of phase-locked loops.  The figure shows a schematic of the
VCO.  In this circuit Q1 and Q2 form a cross-coupled pair of
transistors.  In a typical VCO, the multivibrator uses emitter
followers to prevent Q1 and Q2 from saturation. In this application,
the output voltage at 10 and 11 swings between 0.8 volts below the
supply and 0.4 volts below the supply.  The diodes D6 and D7 clamp
the output at the low level.  The high voltage level is generated by
R7 and R9 and current sources Q5 and Q6.  This high level of 0.4
volts below the supply prevents Q1 and Q2's bases from going too high
and saturating.  The capacitor, C1, is an on-chip oxide capacitor.

      The VCO frequency is controlled by the current switched through
the capacitor.  The minimum frequency/current is set by an external
resistor connected from RMIN to ground.  The frequency gain of the
VCO is also set by an external resistor from RGAIN to ground.  The
control voltage of the VCO is clamped between two diode drops above
ground and four diode drops above ground by diodes D1-D5.  The
voltage at RGAIN can move between ground and two diode Vbe drops
above ground.  The current and the frequency changes as the voltage
moves.  The current from RGAIN and RMIN is mirrored through a PNP
mirror to Q15 to the multivibrator through Q3 and Q4.

      This VCO circuit is temperature-compensated based o...