Simple Regen. Receiver Questions

Discussion in 'Wireless & RF Design' started by miguelpedroso, Dec 26, 2012.

  1. miguelpedroso

    Thread Starter New Member

    Aug 13, 2012
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    Hey guys!

    I'm very interested in learning more about RF electronics (I'm more of a "digital guy"). I've seen a circuit on a magazine I bought the other day, it is a wideband regenerative receiver. They explained the circuit but they were not very clear in some parts so can you guys check if I got it right?


    - They say that the input cap shorts the audio signals in the input to ground via the coil... Now lets take a 200Hz signal at the input. Its a 10uF cap so we have a reactance of 79ohms, so the signal that arrives to the base of the transistor is attenuated. On the other hand if we take a 500KHz RF signal at the input, the reactance we get is around 0.031 ohms, so it arrives at the base of the transistor with almost no attenuation to be amplified, correct?

    - Now, at the collector of the transistor, we got the amplified signals, with a mix of both RF high frequency signals, and also the demodulated audio, so we wanna short out the high frequencies to ground, and wanna keep the audio frequencies as an output... They use that 10n cap to short out the RF component... They use a 10nF and not a 10uF, so that the audio is not shorted out too, because at 10nF capacitance an audio 200Hz signal would get 79577 ohms and at 10uF it would be 79ohms. But then they use that 10uF cap, I guess its only to block DC?

    - And what about the 10k resistor and the 1M negative feedback resistor, how does one pick those values?

    Thank you guys, happy new year!
    Miguel
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Your transistor circuit is just a simple RF amplifier, except the 10nF output capacitor kills the output signal. It is not a regen receiver.
    It might produce such high distortion that the distortion demodulates an AM signal.

    The BC547B transistor has an hFE of about 240 then for 0.6mA in the 10k collector resistor so the collector voltage is 3.0V there is about 0.6mA/240= 2.5uA of base current in the 1M base resistor. Then the collector voltage can swing up and down with the signal.

    A regen receiver regenerates the signal with positive feedback so that its gain is extremely high. This transistor does not have positive feedback.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    He meant to say this circuit does not have positive feedback.
    As ag says, this is not a regen receiver.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    A regenerative receiver works by almost (but not quite) being an oscillator. If hit by an oscillation at its resonant frequency it oscillates in sympathy, which makes it much more sensitive.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Here is the schematic of a super-regen FM receiver.
    C3 causes positive feedback that causes the first transistor have very high gain if a station is received and the gain builds up higher and higher. Then the first transistor begins to oscillate at the tuned frequency. C5 and R3 cause the oscillation to stop and the gain to drop then it builds up higher and higher again. This happens over and over at a frequency higher than you can hear.

    The receiver can pickup AM transmissions from aircraft or their control towers or from FM broadcast stations. It is tuned to one side of an FM station so that the amplitude changes when the station has FM and is called "slope detection" of the audio.
     
  6. miguelpedroso

    Thread Starter New Member

    Aug 13, 2012
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    Thank you guys, I'm starting to finally get it now.

    Just two questions:
    - In the first circuit, if the bc547 was a "perfect" transistor and didn't distort the RF so much, how could I extract de audio? Should I connect a 10n cap to block dc directly from the connector, and then connect that cap to a diode (to cut the negative half-wave) and then use a low-pass filter work to get the AF?

    - In Audiogurus circuit, the RF stage is not directly coupled to the audio amplifier stage connected to the loudspeaker, so how does the right part of the circuit gets the audio from the left, RF, part?

    Thank you all again!
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Yes, that would be a standard AM detector.

    The input of the right audio amplifier transistor is DIRECTLY connected to the collector of the left RF transistor through the inductor L1. The collector voltage of the super-regen RF transistor is determined by the signal strength it picks up. Changes in signal strength are caused by Amplitude Modulation.
     
  8. miguelpedroso

    Thread Starter New Member

    Aug 13, 2012
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    Oh all right I see it now, thought it was directly connected to V+ by mistake, thank you very much!
     
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