Hi!

I need to design and build a tank (LC) circuit. Reading around the internet, and basing it off of previous ones I have, it is fairly easy.

I however, really want to optimize the efficiency of the tank circuit for amplifying a specific frequency, specifically 8 MHz. I do want a fairly high Q (narrow frequency of amplification).

When shopping for inductors at digi-key, there are specs I do not quite understand, such as "frequency self resonant". Do I want to use an inductor that is self resonant at 8 MHz?

I want to send a decent amount of current through it, so should I just go with a inductor that has high capacity, or is excess capacity going to negatively impact the performance of the circuit?

Is there in anyway I should account for the impedance of the device I will be coupling the tank circuit to, which might have an effect?

 

With air-core inductors you do not have to worry about core saturation due to high currents; size the wire based on current-carrying capacity. Here is a calculator for winding your own coil.

The self-resonant frequency should be a factor of five to ten times higher than 8Mhz.

Usually, if this is a tank circuit, then it would be configured as a Pi section low-pass filter which also matches the output impedance of the device to the load. The loaded Q of such a circuit is usually not more than 10, while the Q of the just the inductor by itself might be several hundred.

There are many on-line calculators that show how to configure an output matching network...

 

Hi!

I need to design and build a tank (LC) circuit. Reading around the internet, and basing it off of previous ones I have, it is fairly easy.

I however, really want to optimize the efficiency of the tank circuit for amplifying a specific frequency, specifically 8 MHz. I do want a fairly high Q (narrow frequency of amplification).

When shopping for inductors at digi-key, there are specs I do not quite understand, such as "frequency self resonant". Do I want to use an inductor that is self resonant at 8 MHz?

I want to send a decent amount of current through it, so should I just go with a inductor that has high capacity, or is excess capacity going to negatively impact the performance of the circuit?

Is there in anyway I should account for the impedance of the device I will be coupling the tank circuit to, which might have an effect?

The ARRL Handbook has tons of good information on tank circuit design. Here are a few hints, however.

1) You want to avoid any self-resonance point like the plague! These suck up huge amounts of energy.

2) The L/C ratio is HIGHLY dependent on both the source and load impedance. HOWEVER, you're usually better off erring on the side of high-C, Low-L when used as a parallel tank. This is because the LOAD resistance appears as a parallel resistance, thus lowering the LOADED Q.

3) There is a large difference between loaded and unloaded Q. The greater the ratio between unloaded and unloaded Q, the more efficient the circuit will be. Unloaded Q is determined by internal CIRCULATING resistance, while loaded Q is the "external" resistance.

Eric

 

http://home.freeuk.net/dunckx/wireless/maxpower2/maxpower2.html

More great info on the topic.

Eric