VFO: hybrid feedback oscillator

created @ 2014/7/1

 qrp-gaijin  suggest me reading the vladn work  on the theradioboard , which given a novel  idea about improve regen receiver's performance,  which use hybrid feedback method to keep the amplitude stable on a very wide band rang( about 3:1).  which means on whole regen band, amost does not  need change the regen knob. this amazing.

hybrid feedback oscillator 

i don't understand the whole theory now,  but for me, i treat this idea as a  feedback loop which had a  flat amplitude response over frequency.   when building the Regen: High Performance Rig  i found the armstrong oscillator will hard to start oscillation at it's low band,   the major reason is, when on bottom of the band,  the tickler coil, L2 will given minimal  feedback  because  low frequency( XL= 2*pi*f*L), then you might need change L2 to make whole range can start oscillation.

vladn notice another type of oscillator, which  reduce amplitude  when frequency increase,  like vackar oscillator.  so,  vladn suggest use a hybrid feedback method, add tickler in the  vackar oscillator .  ( vladn  great experimental and theory analysis,  )   

I do get some experience when play with  VCO: Vackar 30Mhz-240Mhz , but this time i notice another thing:  for a given active component and coil,  use vackar is far more easy to start oscillation than armstrong in the whole band the coil/variable capacitor can cover.  this might because the vackar almost feedback all signal from output to input. and i omit the capacitor mark with * in my prototype. 

anyway, with  this configuration, this oscillator cover 4.5Mhz to 12Mhz, and get a almost flat amplitude response( in  middle of band, there are some bump).   vladn had tune this circuit to get "zero tilt"  performance, i did not play with yet.