My First receiver:Mixer & IF

Discussion in 'Wireless & RF Design' started by froten_140, Oct 16, 2010.

  1. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    30
    4
    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.
     
    RRITESH KAKKAR likes this.
  2. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    30
    4
    any help on this.
     
    RRITESH KAKKAR likes this.
  3. bertus

    Administrator

    Apr 5, 2008
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    RRITESH KAKKAR likes this.
  4. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    30
    4
    The building blocks I used are similar to those in that link in the other thread.

    When I disconnect the base of Q3 the oscillation stops, but during oscillation Q2 is not stable too, or affected by Q3, I'm not sure.
     
    RRITESH KAKKAR likes this.
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you make some pictures of the PCB trace and component side?
    Do the IF transformers have metal cans?
    If so ,are these cans grounded?

    Bertus
     
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  6. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    30
    4
    The IF transformers have cans and they are grounded. The pictures are taken by the mopile phone so quality is not very good. My digital camera is not working.

    The PCB in general as attached above.
     
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  7. Tesla23

    Active Member

    May 10, 2009
    318
    67
    My guess is that the layout is causing your problems. I know it's only 455kHz, but the devices have lots of gain.

    Just one example, the ac component of the collector current of Q3 will be set by its Vbe and this is the voltage across the secondary of T5 plus any voltage between the cold ends of C14 and C19. If you look at the PCB layout you find that these are connected by a thin, long meandering earth track which has lots of other signal curents flowing in it. These can cause potential drops to add to Vbe. There is lots of scope here for feedback.

    My suggestion would be to redo the pcb leaving all the copper on the component side as a groundplane. Solder the decoupling capacitors and other ground connections directly to the groundplane.
     
  8. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    30
    4
    Changing the ground connections stopped the oscillation and the circuit is performing better now altough it needs more modifications.

    Thanks alot for all the help provided in this thread.
     
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