Variactor when to use it/frequency range

Discussion in 'Analog & Mixed-Signal Design' started by Leo Silver, Oct 3, 2016.

  1. Leo Silver

    Thread Starter New Member

    Apr 27, 2016
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    Hello everyone,
    once again I will need your help. I have never used a variactor before but a colleague of mine insists to use one for an application. I see that mostly if not always it is used in transmitters and receivers. I am wondering though if I can use it in lower frequencies kHz range.. I see that they lack stability as they vary with voltage but I need it as a compensating component. Do you think I could use it in the 100kHz frequency? Could you recommend me one from RS. I would really like to know more about this interesting use of a reversed biased diode.
    Thank you
     
  2. #12

    Expert

    Nov 30, 2010
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    That's the main use of a varactor. This one can vary from about 35pf to 100pf.
    If you want stability, use a capacitor.
    If you want a tuning capacitor, the tank circuit must be designed so the range available in the varactor suits the range of frequencies you want.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    The largest varactor diode I found in a quick search was this, designed for AM radio tuning.
    It's available here.
    You'll have to determine if that capacitance tuning range will work for your application.
    Here's some info on varicaps (varactors).
     
  4. Leo Silver

    Thread Starter New Member

    Apr 27, 2016
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    Thank you for your replies. Unfortunately, I want to use a transformer to provide the voltage in my circuit and I only detect problems with the use of variactor. 1MHz frequency is too high and I am afraid it will increase any parasitic capacitancies in my topology and also transformer working at such high frequency...is it possible? Also I see that the applied ac voltage at the varactor has to stay in low levels (some mV) to avoid changes in the capacitance, noise and harmonic distortion.

    If I understand correctly they are used in such high frequencies in order to give a more steady capacitance. I feel that if it was in lower frequencies the time that would take for the voltage to rise and fall, it would have a significant effect in the capacitance value and if I wanted to use it for tuning it would be impossible as the reference capacitance would vary a lot?

    I just need someone expert as you to clear these things out for me. As I said, I have never used these before and I want to be sure that these overall interesting components are not suitable for my application.
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You are not making sense. You can use a varactor at any frequency you like from DC to daylight, but its usability will very much depend on what you expect from it and how it is connected in the circuit.
    Without posting a schematic and some description why and how you want to use it we can only guess.
     
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  6. #12

    Expert

    Nov 30, 2010
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    This makes no sense without a schematic of your circuit.
    For instance, using a variable capacitor to avoid changes in capacitance seems crazy without any more information about the circuit.

    and...kubeek got here 30 seconds ahead of me.:(
     
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  7. Leo Silver

    Thread Starter New Member

    Apr 27, 2016
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    OK I m sorry if I confused you!! I need it for balancing a bridge! So the capacitance has to vary only with the applied reverse biased DC voltage and not with the frequency of the applied AC voltage at its edges? Have I got it completely wrong? Isnt it the idea that you reverse bias the diode with a dc voltage to form a capacitance and then this capacitance you can use in a circuit and according to kubeek at any frequency? In wiki it says that the applied ac voltage has to remain low so you wont mess a lot with the dc biased voltage that would lead to a different capacitance! Does it make sense now what I meant? Of course I want to vary the capacitance to balance and tune my circuit but only using the DC voltage across it... I dont want the capacitance to be affected by the ac voltage of the circuit!
     
  8. OBW0549

    Well-Known Member

    Mar 2, 2015
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    It will be affected by the AC voltage in the circuit, unless the amplitude of the AC voltage is small compared to the DC control voltage so that there is little capacitance modulation by the signal.
     
  9. Leo Silver

    Thread Starter New Member

    Apr 27, 2016
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    Yes this is what I wrote in my previous post "Also I see that the applied ac voltage at the varactor has to stay in low levels (some mV) to avoid changes in the capacitance, noise and harmonic distortion". And I believe that the frequency range will also have a significant effect when you go lower to kHz range as the voltage will remain high for a longer time it will mess with the capacitance. So I think when kubeek referred to it as "from dc to daylight" he was referring to a different case.
     
  10. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Neither the varactor's capacitance nor the amount of capacitance modulation by the signal voltage depends on frequency-- not even a little bit. The only thing that affects capacitance modulation is the relative magnitudes of the AC and DC voltages. Frequency is irrelevant.
     
  11. Leo Silver

    Thread Starter New Member

    Apr 27, 2016
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    There must be a reason that varactors are used in MHz frequency. Even in datasheets they only include characteristic curves/performance etc for 1MHz usually. Frequency should play some role. I haven't seen an application in kHz range. Maybe it has to do with the width of the depletion region? If the electric field is so slow the drift velocity would be enough to make capacitor conducting..? Just a guess! dont shoot me :)
     
    Last edited: Oct 4, 2016
  12. OBW0549

    Well-Known Member

    Mar 2, 2015
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    There's nothing magic or mystical about that 1 MHz frequency you see on the datasheet; it's merely the frequency at which their capacitance measurements were made.

    As for these devices being used in the MHz range and not in the kHz range, that is merely a reflection of the fact that most varactors don't have very much capacitance to begin with, and thus are usually not terribly useful at lower frequencies. But in principal, they can be used at any frequency provided their capacitance and capacitance adjustment range meet the application's requirements.
     
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  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Varactors have relatively low capacitance. Therefore they are not generally used in low frequency circuits, where relatively large capacitances are the norm.
     
  14. Leo Silver

    Thread Starter New Member

    Apr 27, 2016
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    Alright, thank you very much for your replies you 've been very helpful.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    How much capacitance change do you need?
     
  16. Leo Silver

    Thread Starter New Member

    Apr 27, 2016
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    Hi crutshow sorry for the late reply. The capacitance I look for is in the pF range 100-10pF. Do you think I could find one for kHz frequency?
     
  17. bertus

    Administrator

    Apr 5, 2008
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