Right value for low-pass filter capacitor ?

Discussion in 'Wireless & RF Design' started by dr.emp, Nov 10, 2018.

  1. dr.emp

    Thread Starter New Member

    Nov 13, 2017
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    I am using this RF receiver microcontroller I had from an old teardown of a rc car. The schematic of the IC is here:https://www.diodes.com/assets/Datasheets/PT8A977B978B978BL.pdf All values for the components are specified on page 9, except for C6.

    My question
    : Is R1 and C6 some kind of low-pass filter ? If yes, then what value should C6 be ? And can I calculate that value with the formula f= 1/2pi*RC ?
     
    Last edited: Nov 10, 2018
  2. ericgibbs

    Moderator

    Jan 29, 2010
    8,527
    1,713
    hi dr,
    The C6 value for tuning the frequency.
    E
    Woops incorrectly read the capacitor notation.!

    MOD: Clipped and posted the circuit, easier to read.E

    AA1 10-Nov-18 16.31.gif
     
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  3. dr.emp

    Thread Starter New Member

    Nov 13, 2017
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    Thanks for the answer. Well i asked around in other forums and they told me it was this or that, but they never came to a conclusion. So I applied the values of C3 (10pF) and L1(calculated by measuring the coil, can't remember the value of it) in the formula f= 1/2pi*square(LC), resulting a resonant frequency of about 25 MHZ, which I supposed that can be leveled up with the core of the inductor.

    Maybe I should have mentioned that I want it to receive in the range of 27 MHZ ?
     
  4. ericgibbs

    Moderator

    Jan 29, 2010
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    hi dr,
    Using C6 10pF this is LTSpice sim for that circuit, Fout =27MHz.
    Very rough sim be it suggests the 10pF is OK.
    E
     
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  5. dr.emp

    Thread Starter New Member

    Nov 13, 2017
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    Thank you very much :). It was the last component I had to solder to the PCB. I was thinking about using LTSpice like you did, but I didn't know how to make the antenna. I want to use the circuit to control a Lego Technic car.
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
    8,527
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    hi,
    Would be interested to hear how the project works out.:)
    E
     
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  7. Audioguru

    Expert

    Dec 20, 2007
    11,249
    1,349
    The radio receiver is a one transistor super regen circuit.
     
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  8. dr.emp

    Thread Starter New Member

    Nov 13, 2017
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    UPDATE : The circuit still doesn't work. I replaced C1 by a trimmer capacitor ( 5-15 pF ). I'll review the circuit, maybe I forgot to solder some of the Connections, who knows...
     
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