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Where is the bandpass filter in this circuit?

  • #1
4
0

Homework Statement


I am doing a project on the Elenco AM/FM superheterodyne radio kit. I have been stuck for ages trying to figure out how the two common emitter amplifiers are tuned to filter out anything that is not around 455 kHz. I tried to find a bandpass filter, or a high-pass and a low-pass filter, but to no avail.

qxnci8.jpg

Here's the entire circuit for the radio kit. I am interested in the AM part of the circuit (the lower part).

2. Equations
Not any for now...

The Attempt at a Solution


I know that transistors Q8 and Q9 form two common emitter amplifiers. I think that the capacitors C34 and C36 might have something to do with the selectivity of the amplifiers. Transformer T7 could be playing a role in it as well, I have no idea. What I do know is that the instructions that came with the kit say the following:
Untitled_4.jpg

To be honest, as a beginner, I don't understand all of what is explained. I do understand that T7 is probably the most important component to provide selectivity. That is all I know.

Please, can someone help me figure out how the amplifiers are setup to filter anything but frequencies around 455 kHz?
 

Answers and Replies

  • #2
tech99
Gold Member
1,783
616

Homework Statement


I am doing a project on the Elenco AM/FM superheterodyne radio kit. I have been stuck for ages trying to figure out how the two common emitter amplifiers are tuned to filter out anything that is not around 455 kHz. I tried to find a bandpass filter, or a high-pass and a low-pass filter, but to no avail.

qxnci8.jpg

Here's the entire circuit for the radio kit. I am interested in the AM part of the circuit (the lower part).

2. Equations
Not any for now...

The Attempt at a Solution


I know that transistors Q8 and Q9 form two common emitter amplifiers. I think that the capacitors C34 and C36 might have something to do with the selectivity of the amplifiers. Transformer T7 could be playing a role in it as well, I have no idea. What I do know is that the instructions that came with the kit say the following:
Untitled_4.jpg

To be honest, as a beginner, I don't understand all of what is explained. I do understand that T7 is probably the most important component to provide selectivity. That is all I know.

Please, can someone help me figure out how the amplifiers are setup to filter anything but frequencies around 455 kHz?
T6, T7 and T8 are transformers tuned to the 455 kHz intermediate frequency. They have capacitors connected across the primary but these are not shown because they will be inside the screened transformer can as purchased. For information, Q7 is a self oscillating mixer, using L5 as the LO, and Q8 and Q9 are common emitter IF amplifiers. Older equipment used transformers with both primary and secondary tuned, which gave a flat topped pass band.
 
  • #3
4
0
T6, T7 and T8 are transformers tuned to the 455 kHz intermediate frequency. They have capacitors connected across the primary but these are not shown because they will be inside the screened transformer can as purchased. For information, Q7 is a self oscillating mixer, using L5 as the LO, and Q8 and Q9 are common emitter IF amplifiers. Older equipment used transformers with both primary and secondary tuned, which gave a flat topped pass band.
Thank you so much! Now, it makes sense! Thanks again!
 

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