Colpitts VCO Problem.
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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No ideas yet..?
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.
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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