Getting this VHF band II VCO to work

[alexander3133]

I constructed this VHF back II VCO to work with FM. I simulated this circuit in MultiSIM with initial condition set to zero and it works. Once I implemented into hardware, it does not work and refuse to give any oscillation, any idea what went wrong with this design?

Both C9 and C5 are trimmer capacitor.

 

[kchriste]

You've go no power supply decoupling caps. The bias circuit for Q2 is really poorly designed and beta/temperature dependent. 2n3904's don't have much gain above 100Mhz, but will marginally work in those circuits.

 

[alexander3133]

You've go no power supply decoupling caps.

Could you further explain the statement?

I happened to get the circuit to oscillate but it oscillate at a frequency of around 18.4MHz. I guess something is wrong with my LC tank or design?

 

[kchriste]

Could you further explain the statement?

You need some capacitors (10uF, 0.1uF & 1000pF) across each battery, but physically located as close as possible to the oscillator and amplifier. (You can get away without the 10uF for now, but it helps bypass noise at audio frequencies.)
Did you build the circuit on a PCB and keep the wiring short? It won't work properly on a breadboard.

 

[alexander3133]

You need some capacitors (10uF, 0.1uF & 1000pF) across each battery, but physically located as close as possible to the oscillator and amplifier. (You can get away without the 10uF for now, but it helps bypass noise at audio frequencies.)
Did you build the circuit on a PCB and keep the wiring short? It won't work properly on a breadboard.

I built this circuit on a stripboard and I already cut the extra legs for the components to make the wiring short. Images attached are front and back of the circuit. Although I know my soldering skill not really good, but I doubt it still acceptable. The varicap is soldered on the back of the stripboard as it is a surface mount component.

 

[kchriste]

Instead of low Q chokes in your tuned circuits, you should be using high Q air wound inductors. Mount the inductors that are closest to each other so they are rotated 90 degrees from one another. This will minimize inductive coupling. Though it is hard to tell from the pictures, make sure that any PCB tracks are cut off short and are not acting as long antennas. ie: Don't let the PCB traces be any longer than needed.