My First receiver:Mixer & IF

[froten]

Hello all,

I have starting building the proto type of my receiver circuit. I started by the mixer+IF stage. Attached is the schematic, PCB (before modifications) and transformers details. The transformers are rewond of some old toko coils I have on hand.

The resulting circuis caused me many nightmares, it is oscillating, noisy and refuses to work for me.

Attached also is a picture of the output at detector D1. When I disconnect C13 & C19 oscillation stops, also if I lower the gain by adding un-bypassed resistor at the emitters the oscillation stops too.

Checked in many text books for solutions and techniques, found nuetralization techniques but none of them worked. For example, I tried to connecte a capacitors (ranging from 2pf to 47pf) from the pin 1 on T6, T5 & T3 to the base but still unstable, tried to use pin 3 for the same thing but still the same.

When I disconnect the stage of Q3, the oscillation stops.

What is going wrong with my design? This simple circuit I was thinking I will build during the week end, and it is now the fourth week without any success.

Any help is appreciated.

 

[JimB]

If I understand your PCB layout, you have:

1/ a single sided board

2/ a thin trace for the 0v/ground line

This is most likely the source of your problems.
The ground line needs ot be low impedance at RF, a thin pcb trace is anything but low impedance.
My choice would be to use a double sided board and leave the top surface as a solid ground plane, obviously there will be holes and the coper will be removed for a millimetre or so around the hole where the wire passes through to the wiring side of the board.
Make all the ground connections directly to the top surface.
This will have much greater chances of stability than what you have now.

I also think that you are going to have other design issues with this circuit.

I guess that you are trying to receive AM signals in the 27Mhz range.
This is a 455khz IF, you will almost certainly have IF image interference problems.
Have you considered the need for some AGC?

JimB

 

[froten]

thanks for the help. AGC will added later when the circuit is finalzed, in the PCB it is there already.

You are correct, the green line is for the GND and the red line is the VCC (+12), while the brown is the AGC (not connected for now). I'll consider another layout for the PCB.

What about the transformers, is the turns ratio ok (about 20)? inductance?