Colpitts VCO Problem.

[xitronics]

Hello friends,

I designed a common base Colpitts osc. with a load of 1k which is to dissipate around 50mW of power at 100 MHz freq.

The expected o/p voltage is 10V. However I get only 1V o/p during testing. This makes the power dissipation to just about 0.5mW.. Whats the problem..?

I have posted my ckt diag. Pls reply if anyone has any suggestion

Regards Prasanna.
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[xitronics]

No ideas yet..??

 

[biff44]

Try playing around with R1 and R3 while watching the actual voltages at the Base, collector and emitter. You might simply have a bad DC bias point for the transistor. If R3 was larger, it would provide more DC negative feedback and make the DC biasing more stable.

 

[xitronics]

Colpitts VCO : BIGTIME PROBLEM..!

The problems seem to be too many..

1) I get FM reception at 3-4 frequencies, even when i dont connect any antenna or load to the oscillator, i.e no Rload.

2) If Cvar = 22pF, and L = 30nH. Then considering the transistor parasitic capacitances Cbe = 10pF and Cbc = 5 pF, the oscillating frequency Fosc must be around 150-160 MHz.
But with this configuration, I get FM reception at frequencies of 97 MHz, 101 MHz and 107.5 Mhz, WITHOUT ANY ANTENNA CONNECTED.

Approx Ranges were :
10-20 meters at 107.5 MHz
0.5 - 1 metre at 97 MHz and 101 MHz.

Inclusion of 75 cm wire antenna makes absolutely no diff. in the ranges.

3) Even if i enclose the entire ckt in a fully conducting and grouded metallic body, the Tx. continues to transmit at these frequencies with the same ranges.


Observations made are simply inconclusive. I tried some basic things, but as I am clueless now, i need some help.!

Pls reply if anyone has any suggestion.

Regards Prasanna.
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