The AM generator circuit using 455 KHz IFT.

Can any one give me the correct circuit for AM generator using BF494 transistor and 455 KHz IFT and its explanation with design or any reliable website. I don't know why, its not working for me. I don't know what is the problem . Is it related to old IFTs in the labs. :-(

-Devanand T

Having no details of your IF transformer, no one can offer you a ready to use design.
Find out if your IF transformer has two windings, one tuned to 455 kHz with an (internal?) capacitor, the other coupled, or the coupling made as a tap on the primary coil.
Then you can design an oscillator by connecting your IFT to the collector line, and the coupling winding into the base circuit. A simple two-resistor voltage divider can be used to bias the transistor. The coupling winding can be connected between the resistor mid-point and base. Use a 1 nF capacitor to RF "ground" the above mid-point. Connect the emitter to GND and measure transistor current. With BF494, do not exceed 10 mA at 6 VDC.
Use an oscilloscope to see if the circuit oscillates (connect the probe to transistor base). If it does not, invert the coupling winding wires.

jiripolivka said:

Having no details of your IF transformer, no one can offer you a ready to use design.

Click to expand...

The IFT we have in the lab is having 5 legs and one screw to tune (455khz). Can you give the circuit please?

Many IFTs have five legs. Can you use an ohmmeter to find about the windings? Can you tell if the IFT includes the capacitor to resonate with one coil at 455 kHz?

jiripolivka said:

Many IFTs have five legs. Can you use an ohmmeter to find about the windings? Can you tell if the IFT includes the capacitor to resonate with one coil at 455 kHz?

Click to expand...

Yes the IFT includes the capacitor to resonate with one coil at 455KHz.

A 5 legged IFT, uses a tap across the tuned circuit for the collector or Vcc connection. This is to keep a high working Q. This would be a good place to connect your collector. So the base winding will be the lowest resistance, then the collector end to the tap (Vcc) will be the next highest resistance, then the tap to the open circuit (tuned end) will be the highest resistance.
Frank