FM Receiver Resonator Question

Discussion in 'Wireless & RF Design' started by johncena, Sep 2, 2011.

  1. johncena

    Thread Starter Member

    Aug 15, 2010
    32
    0
    http://imageshack.us/photo/my-images/12/tda7000a.gif/

    From the schematic above.. at pin 6 of the TDA7000, that is the local oscillator based on the user's website.

    by calculating the values, the

    frequency when that PVC is at 2pF = 1/[2*pi*squreroot(LxC) ]
    where C = (2pf||56pf) + 33pf
    where L = 56nH

    = 113Mhz

    frequency when that PVC is at 60pF = 1/[2*pi*squreroot(LxC) ]
    where C = (60pf||56pf) + 33pf
    where L = 56nH

    =85Mhz

    Pretty much, it covers the 88Mhz to 110Mhz FM Stations

    1st question:

    is my calculation correct? because when we built the circuit, the station were moved, wherein if we moved the PVC to it's Maximum, the station is at 98Mhz or something (It cannot reach more than 100Mhz stations). What we did is to reduce that 33pf to a lower value (10pF) to adjust the stations (The coil was adjusted few times before we did the replacement of cap). So, what was the culprit then? Does the copper at the pcb's board can add some extra capacitance to the local oscillatorr? since our PVC was located a few mm from the inductor and other cap.

    2nd question

    if that pin 6 is the local oscillator... what was that coil and two capacitors at pin 13 and 14 are used for? some circuit doesn't have that. what was the pros and cons of not having that in that circuit?
     
    Last edited: Sep 2, 2011
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The TDA7000 is obsolete and has not been made for a long time. Its performance was horrible.
    Today you can buy an FM "radio" at The Dollar Store for only $1.00. It uses the newer TDA7088 IC and its performance is also horrible.

    The tuned circuit at pin 13 and pin 14 of the TDA7000 are a bandpass filter for 88MHz to 108MHz to reduce interference from other frequencies.

    You cannot calculate capacitor and inductor values for a circuit that operates at the very high frequency of 100MHz because of stray capacitance and stray inductance in the wiring.
     
    johncena likes this.
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