Inductors for Colpitts Design AM Transmitter

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Discussion Overview

The discussion centers around the design of an AM transmitter using a Colpitts oscillator. Participants explore the selection of inductors suitable for this application, considering factors such as Q values, maximum current, self-resonant frequency (SRF), and DC resistance (DCR). The scope includes technical specifications and regulatory considerations related to broadcasting frequencies.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the appropriate type of inductor to use, noting the variety available and their differing Q values and maximum current ratings.
  • Another participant emphasizes the importance of frequency, DCR, and saturation current (Isat) in selecting inductors, suggesting that cost and size should also be considered.
  • A participant expresses a desire to broadcast over commercial AM frequencies (0.5 MHz to 1.6 MHz) and questions the appropriate SRF for inductors, indicating a lack of concern for FCC regulations due to the limited range of the broadcast.
  • Clarification is provided regarding DCR and Isat, with a suggestion that the SRF should be significantly higher than the operating frequency to ensure proper inductor function.
  • One participant proposes that an SRF value ten times greater than 1.6 MHz would be sufficient.

Areas of Agreement / Disagreement

Participants express varying levels of concern regarding FCC regulations, with some prioritizing technical specifications for inductor selection while others focus on practical aspects of the design. There is no consensus on the best type of inductor or the exact specifications needed.

Contextual Notes

Participants discuss the implications of SRF, DCR, and Isat without resolving the specific requirements for the inductor selection. The discussion reflects uncertainty regarding the exact values and specifications necessary for optimal performance in the transmitter design.

powzoom
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I'm trying to design a AM transmitter using a colpitts design. I see many different types of inductors available for sale, smt, torrid, dip, ferrite core, iron core, etc... Which one should I buy? I know they have different Q values and max current. What do you think the max current would be for my transmitter if it only broadcasts over a 100ft or so?
 
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powzoom said:
I'm trying to design a AM transmitter using a colpitts design. I see many different types of inductors available for sale, smt, torrid, dip, ferrite core, iron core, etc... Which one should I buy? I know they have different Q values and max current. What do you think the max current would be for my transmitter if it only broadcasts over a 100ft or so?

What frequency is your transmitter? Be careful just broadcasting a signal -- The FCC has rules about how much power you can put out in different bands without a license...

You will choose inductors based on the frequency (hence the SRF makes a difference), the DCR, and Isat. Beyond those things, you will usually choose the cheapest and smallest that meets those requirements.
 
I'd like to make it broadcast over commercial AM so that it can be picked up by a home radio. I'm not too concerned about FCC as it won't be broadcasting off my property and it's really only for learning purposes. So about 0.5Mhz to 1.6Mhz. If I understand correctly, I should pick a SRF value much greater than 1.6Mhz? What is DCR?

Thanks for the reply.
 
powzoom said:
I'd like to make it broadcast over commercial AM so that it can be picked up by a home radio. I'm not too concerned about FCC as it won't be broadcasting off my property and it's really only for learning purposes. So about 0.5Mhz to 1.6Mhz. If I understand correctly, I should pick a SRF value much greater than 1.6Mhz? What is DCR?

Thanks for the reply.

DCR is DC resistance. Isat is the saturation current (which you want to stay below). Yes, just choose the SRF well above your operating frequency to be sure it's still an inductor.
 
powzoom said:
If I understand correctly, I should pick a SRF value much greater than 1.6Mhz? ...
10X will do.
 

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