VFO:  ultra-audion  LF to VHF

Meet the popular Oscillator

@2012/5/21

recently I need a simple AM transmitter to test my superhet receiver. google the web, there are may type transmitter form AM to FM, and the simple one always look like this:   from few hundred kHz to around top VHF even to UHF,

This one is copy from Andy Collinson: http://www.zen22142.zen.co.uk/Circuits/rf/amtx.htm ) 

I  made a FM bug long time ago, everything is same except the LC tank tune to FM range, that one works well. but this time i failed to make this one start oscillation. I try Q2 2sc9011/2sc9018, Ft 150Mhz/800Mhz, i'm very sure the transistor's amplification function is very enough to work in AM band. but why am failed?


What exactly is this oscillator 

you will found so may bugs/transmitter  AM/FM schematic on WWW,  and all the oscillator is actually  same like this one.  

same oscillator for 90% schematics!

there are so many circuit for hobbies use this type oscillator.  what it it? Colpitts? is not very like that.  and In <<Practical Oscillator Hand book >> by  Irving M. Gottlieb - Newnes (1997),   mention this as  ultra-audion oscillator , it's another form of Colpitts:

As its name implies, its life span go all the way back to electron-tube technology. It tends to be a sure-fire oscillator, providing the active device has

adequate high-frequency capability. my 9018 has enough for that.  it characterized by the capacitive voltage divider in the feedback is on internal capacitance of the active device.

And if the amplifier is connect as Common base topology,  we get the favorite form of the bugs/transmitters on the Internet.

 The internal emitter to base capacitance constitutes the second capacitor of the Colpitts capacitive voltage divider.

Why does it not starting

   Now, back to my failure case:

i try to make it be a real Colpitts,  paralleled a capacitor with R1.

active device is ok, 9018 has 800Mhz Ft and I want 1Mhz, very enough. But as Irving  mentioned above, the second Colpitts capacitor  exploit the Cbe, which is very small for high Ft device. it's very seems like I pick up a too much better performance device for AM band.

Fig. 8

NOTE:  I realy want it to be a pure Ultra-audition.  Add cap to Cbe is one of the choice,  but because BJT base is AC connected to ground, actually,  this would make it same as Fig.8, and not be a ultra-audition anymore.

Conclusion:

     ultra-audion oscillator is very popular Oscillator for Beginners and simple 'toy' like application.  and it's almost start oscillation at very time, sure fire, but that's is in the top of HF and the VHF.  for AM band oscillator, please use a Colpitts to provide efficient feed back to make it fired. 


How simple it's going to be?

look at this, it's extremely simple by exploit all the internal capacitance in an active device.

simplicity gain something and losing might even more:

1)  loose control the feedback ratio in a flexible way.

2)  internal capacitance is out of control, so frequency stability is vary from device to device. vary at temperature, load etc. 

3)  suitable for high frequency, not good for low frequency ( except you had a 'bad' performance transistor had large internal capacitance).


I  have no 2N2222A when built this one,  that might be why my circuit does not work at first place.  2N222A is a 300Mhz Ft, Cibo about 25pF,   the 9011/9018 transistor only had a Cibo abot 1-2pF.