am radio detector

Discussion in 'Wireless & RF Design' started by sampinoy, Feb 6, 2005.

  1. sampinoy

    Thread Starter Active Member

    Sep 24, 2004
    31
    0
    hello everybody,

    i have been trying to study circuit diagrams of am radios and this what i have observed:

    1. in the 1st circuit that i have build the 1n4148 was connected this way. the kathode was connected to the secondary of the black IFT and the anode as the output.

    2. in another schem. the diode has its anode connected to the secondary of the black IFT and this time the kathode as the output.

    3. yet in another schem. i dont see any diode.

    can anybody explain to this high skol kid why is there different ways in connecting the diode? :huh: am terribly confused :blink:

    whats d main purpose of this diode? :unsure:

    thanx

    sam
     
  2. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    In the AM detector circuit the diode rectifies the RF carrier voltage from the last IF stage of the receiver. The diode thus "detects" the audio information, pulls it off the RF carrier and amplifies it through the audio stages and ultimately outputs it to the speaker. 1N4148 is a common small signal diode for that purpose. As far as the different connections, I'd have to see the schematic.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    For AM detector purposes, it probably doesn't make any difference which way the diode is connected. It would be significant only if the audio amp following the detector stage had to have a certain polarity signal. Most likely, though, the audio stages are connected with blocking capacitors, so the detected signal's polarity won't make any difference.
     
  4. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi sam,

    here's some more info on why there are diffrent ways of connecting the detector. have also attached circuits of different connections.

    for figure 1 when NPNs are used as mixer-converter transistor & IF signal amplifier

    for figure 2 when PNP are used as mixer-converter & IF signal amplifier

    for figure 3 like figure 2 an PNP transistor is used as detector, with its base connected to the secondary winding of the last IFT (black) and its collector connected to the volume control and the base of the voltage amplifier of the power amp;

    for figure 4 like figure 1 an NPN transistor is used as detector, with its base connected to the secondary winding of the last IFT (black) and its collector connected to the volume control and the base of the voltage amplifier of the power amp

    hope this clears up your confusion :D
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Hi mozikluv,
    Just a small point. You left out C3 in the first two diagrams.
     
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1

    hi pebe,

    thanks for pointing it out, it was an oversight :eek: :unsure: , however those circuits are for demo purposes on how the detector are connected. the C3 is an internal cap of the IFT. the values shown are not meant to be used as best values. :)
     
  7. David Bridgen

    Senior Member

    Feb 10, 2005
    278
    0

    What's the (only) purpose of this diode?

    This may be of interest:

    Mixers
     
  8. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi sync,

    let me add a different approach as to the purpose of the detector diode as "n9xv" has stated.

    in a step by step process, radio freq. signal (RF signal) that is picked up by the antenna is being tuned-in by the tuning capacitor which in turn is converted by the mixer/converter transistor into intermediate freq. signal (IF signal). this IF signal is converted by the detector into audio freq. signal (AF).

    technically the mixer/converter is the 1st detector and the diode is the 2nd detector.

    now why do we have to convert the IF to AF? ans. for the simple reason that we can't hear IF signal, but we can hear AF signals becoz the audio range is from 20hz to 20khz, while the IF signal of an AM radio is at 455khz although this freq is still in the radio freq. range but it is beyond out hearing range. :)
     
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