Hand-made coil works where mini-wire wound SM inductor fails - why?

Discussion in 'Wireless & RF Design' started by Spottymaldoon, Feb 5, 2016.

  1. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
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    1
    I have been trying to use a mini wire-wound air core SM inductor (100nH - digikey 732-1820-1-ND) as a component to match stages of a nominal 100MHz amplifier (not actually for radio use) and I found it simply didn't perform at all: but when I replaced it with a hand-made coil (4 tight turns of number 20 on 0.25" diameter) everything worked perfectly.

    Please explain why?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Can you sweep the two components on your network analyzer?
    My guess would be L/R ratio between the two, or different capacitative parasitics.
     
  3. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
    All I'm able to do is try to get it in series resonance with an 8-30pF trimmer capacitor with 100MHz applied and watch the RF voltage out - with the mini I get no resonance - with the hand-made coil it's fine. These mini coils seem to work fine as chokes/loads however and they look nicely made.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    Well if you can't even measure the inductance, what hope do you have of debugging the situation? Have you considered renting the proper tools to do your job, or are you just some kind of low budget tinkerer? For all we know the mini might not even be an inductor at that frequency.
     
    Last edited: Feb 5, 2016
  5. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
    Papabravo - I tend to accept the value and tolerance stated by the manufacturer who makes them by the ten-millions, just as we do for resistors and capacitors, don't you? Also, I can see the actual coils which look to be geometrically in-line with the claimed inductance. The meat of my question was actually "Can we expect these little SM inductors to perform as resonant components in place of the more common hand-made coils?"
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    The problem you are faced with in applying a general purpose component to a specific application is that you really don't know what you are getting. As I am sure you are aware the inductance of any inductor is not constant. If you have ever swept an inductor you would see what I am talking about. In some frequency ranges it actually has the impedance characteristics of a cappacitor. I know that is strange, but it is entirely true. Perhaps the datasheet will have some clues about the appropriate frequencies where this SMD mini might be applied. Other than that I can only tell you what a professional RF engineer with access to the proper tools would do. If you don't have access to the tools then you are well and truly up against it.

    Your mini has a resistance of 0.18Ω which seems kinda high. What is the DC resistance of the hand wound coil?
     
    Last edited: Feb 5, 2016
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