Dear George
Hi again
I'm completely agree with BradtheRad ( i learned many things for simplifying each design from him )( such as some of oscillators ) .
And about the oscillator that i introduced ( i learned this one from one other of my best friends )
As you probably understood it is a common base amplifier . ( i designed it for frequencies around 180MHZ successfully )
Thus , because it is a CB amplifier the positive feed back will produce by a capacitor between emitter and collector .
and as you probably understood , the out put frequency will select by that LC network in collector (L1 and C3)
So Fr will given by Fr=1/(2*pi*sqrt(LC)) and about the feed back capacitor = it's value will be nearby 1/2*c3 ( don't forget that the internally capacitor of collector emitter junction will have effect on it's value ( those are in parallel together ) and you should consider it's value at high frequencies as well .
and about R3 and R2 , you can use a simple potentiometer with a series resistance to prevent short circuiting , and then you can calibrate it for highest out put amplitude . ( it will be the best way that you can use , but if you don't want use a potentiometer , you should design a CB amplifier , independently , and i won't prefer it for these circuits ! )
and the out put will be from collector that you should use a coupling capacitor to remove DC signal from your out put . in this oscillator you can achieve the out puts , that are multiplied by twice the vcc ( vpp) , which is good ! ( i earned 80vp at 5MHZ) ! and about bypass capacitor of base it should has low impedance instead of frequency of oscillation ( about 10 ohms , instead frequency of operation ( approx )) . and about emitter resistor , you'd better to select it with low values ! ( in practice it should be between 20 ohms and 47 ohms ) and be sure that these considerations will work for all of the frequencies that you need . and about the input potentiometer that i talked about it , at past , it's value can be between 20 k ohms until 1 mega ohms .
and about the duty of the bypass capacitor in emitter , it should has an impedance around 75 ohms instead of operating frequency . as you probably know the Av at CB amplifier will be : Zc/ZE . so if you decrease the ZE the out put voltage will increase but you can't decrease it lower than 75 ohms because thus the feed back voltage will connect to the ground . it will bypass the high frequency components that we want them ( those components maybe create because of a bit nonlinearity ! ) ( each oscillator should have terms of linearity and can be a bit nonlinear )
By the way , the out put impedance of this oscillator will be as high as 200 K ohms and if you need to get current from it's out put you'll need a buffer with transistor or ....... . and about inductors : if you create enough gap space in the core of them , or select them with air core it will be very good , because with gap space your inductor will be pretty linear . ( the nonlinearity of inductor can be cause of damped oscillations and then oscillation will stop !) ( i tested it for many times ) .
And at practice and in board ( not bread board ) and at high frequencies you'd better to use a capacitor in parallel with supply ( pretty nearby your circuit ) , because at high frequencies the current will be high frequency and it will destroy on equivalent circuit of wire ( low value capacitor )
( i have same and good ways for other oscillators too ! )
Try to design , and ask again if you couldn't earn success .
Good luck
Goldsmith
---------- Post added at 03:14 ---------- Previous post was at 03:11 ----------
( by the way , i forgot to say ! try to design , for 10 MHZ , if you couldn't , i can design one of them and bring you the design example ! )