FM Receiver - TDA7000 Project / Varicap Problem

Discussion in 'Wireless & RF Design' started by johncena, Aug 6, 2011.

  1. johncena

    Thread Starter Member

    Aug 15, 2010
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    I know that TDA7000 is obsolete nowadays, but we managed to bought somewhere near in our place. We need to construct a FM Receiver, capable of receiving signals up to 5 or more stations, ranging from 88Mhz to 108Mhz.

    All of the components were bought (TDA7000, capacitors, resistors) but two of them are hard to find. The most problematic to us is the Varicap (Voltage Variable Capacitance Diode)

    We used the circuit diagram here --> http://electronics-diy.com/TDA7000_FM_Receiver.php

    However, the only available Varicap here in our place were the ff:

    MV104 -(Dual Voltage Variable Capacitance Diode, 37-42pF (range cap))
    MV2107 -Nominal Cap 22pF
    MV1650 -Nominal Cap 100pF

    IF we are going to use MV2107 as a substitute for MV2105 (wherein, the MV2105 has a Capacitance: 2 - 16pF ).. What are we going to do if we want to make that MV2107 suitable for the job? Should we change the Value of the capacitors nearby the varicap? or should we put another capacitor in series/parallel? , modify the coil? or change the value of resistor? in order for us to produce a smooth capacitance increments for tuning.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you see this in the partlist?

    Bertus
     
  3. johncena

    Thread Starter Member

    Aug 15, 2010
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    Yes i read about those, and both BB105 and BB205 are unavailable here also..

    But i wonder if we can use MV2107 as a substitute?. Although the Min and Max pF are not the same..

    [​IMG]
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The other varicap can be used.
    Only the coils need to be smaller in that case.
    (spreading the coil may already be enough).

    Bertus
     
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  5. johncena

    Thread Starter Member

    Aug 15, 2010
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    thanks again Bertus, that clear things up :D

    i'll update this post as soon as i build the circuit.
     
  6. johncena

    Thread Starter Member

    Aug 15, 2010
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    ok another problem...

    MV1650 and MV104 Varicap are the only available components here. (mV2107 out of stock >.<)

    If i'm going to use the MV1650 (min of 90pF to max of 110pF). what will i do to get that 88Mhz to 108Mhz frequency?

    An example was using 106nH inductor and a 41.4pF to 20.5pF Varicap will yield an exact 76 to 108Mhz, which covers all the FM frequency band.

    if i will use MV1650 (neglecting MV104 since it has 37pf - 42pf), is there anyway to achieve that Fm band?
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You could try to change the 1 nF capacitor to a lower value like 100 pF and see what happens.
    The total value will be smaller than in case of the 1 nF capacitor. (series connection of capacitors).

    Bertus
     
  8. johncena

    Thread Starter Member

    Aug 15, 2010
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    hmm..

    90pf in series with 100pF = 47.37pf
    and
    110pf in series with 100pf = 52.38pf

    and 47.37pf with 70nH will result in 87.7Mhz
    and 52.38pf with 70nH will result in 83.4Mhz
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You should try to find a varicap with a comparable Cmax / Cmin ratio.
    The ratio of the MV1650 is to small to cover the FM band.

    What other varicaps can be found at your place?

    Bertus
     
  10. johncena

    Thread Starter Member

    Aug 15, 2010
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    only MV104 and MV1650 are available.


    I do have old non working TV here, maybe i can find Varicap diodes there..
    I'll check it later..
     
  11. radiohead

    Active Member

    May 28, 2009
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