VCO: Vackar 30Mhz-240Mhz

Vackar Oscillator

Dec 25, 2011

Vackar oscillator is famous for it's frequency stability, low phase noise, and low spurious.   Vackar is especially suitable  for BJT type oscillators.

Signal feedback from  collector use a LC π  bridge which provide a 180 degree phase shift.

•  C1 should use a big value capacitor for better performance, generally 6 times bigger tan C7.  it will reduce T1's junk capacitance influence.

•  shunt capacitor C7 with coupler C4 isolated tank from active device.

•  The parametric variables of the transistor (which depends by the bias current and temperature), are isolated from the resonator.

• The feedback division ratio is fixed (typical range for coupling ratio is 1:4 up to 1:9). Even if the VCO is tuned, the impedance divider is fixed, in this way increasing the stability.

• it might hard to start oscillating, then adjust C4, C7 to provide more feed back.

•  replace R4  as a RF choke might provide more performance : more wide tune range, more flat over frequency.

•  buffer is always recommended.

•  the above circuit oscillating from 30Mhz up to 240Mhz,  wider than Colpitts type VCO.

• tune range about 1.4:1, to get more, reduce C4, use large C5 and C1.

• here is the under test Vackar Oscillator.

Redraw sch add buffer

@2013 10/10