# Colpitts oscillator help

#### latino

Joined Oct 4, 2017
1
Hello!

I have to make a Colpitts oscillator working on 1.1 GHz, and another Colpitts oscillator on 1.5-2 GHz.
Here is the circuit:

Now, the C1 and C2 capacitors are 1 pF, L1= 5 nH, Q1=BFR92P
The frequency is about 880 MHz, and i've already tried a lot of different values for L1, C1,C2 but i can't increase the frequency to 1.1 GHz. My professor said that there is no problem with the circuit, so let's try again and again, but i've no more ideas. I've already calculate(Thomson formula) that if C1=C2=1 pF and L1=5 nH, the circuit has to work on 3,1 GHz(??).

So, what do you think? What is wrong, what should i do?

Thank you

#### RichardO

Joined May 4, 2013
2,271
Did you look at the data sheet for the BFR92P? It shows junction capacitances of a good fraction of a picofarad.
These capacitances are large relative to your 1pF capacitors.

I suggest you simulate the circuit with a model for the actual BFR92P and see what you get.

I am guessing that another problem is that you have a significant amount of stray capacitance in your oscillator circuit. Again, significant, is just a fraction of 1 pf.

#### Alec_t

Joined Sep 17, 2013
12,746
Have you allowed for the inductance of the connecting wires?
Is the circuit built on a breadboard?