VCO: Improved Series E VCO

Oct 22 2016

A 55-120Mhz VCO intended to driven DBM

the buffer is just the simple amplifier with light couple to VCO.  to minimize the harmonics, a 105Mhz LPF is connected to output, intended to reduce the harmonics of band 60Mhz-105Mhz. but it's obviously, while VCO tuned to 60 Mhz the 2nd harmonics could not be well eliminated.

the following schematic is proved for better harmonics clean, tune the trim-capacitor to better harmonics clean. 

a Reference topology:

The simple Amplifier, 2 cascaded stage

Insert Gain: 10 dB up to at least 100 Mhz.

BJT:  2sc9018, 800 Mhz Ft 

Output transformer: 6T:2T on TV core

low output:(VCC 8V ~ 11V)

      bias 6.8k ,1k or  8.2k 1.2k

              47 ohm degeneration

5 order 105Mhz chebyshev LPF

mistake made: varactor bias resistor too high(88k, leading to voltage can not reach zero while sweep fly-back), 12k is good. <6k will stop the osci