load the tuned circuit

[unrealdave]

Hi Guys

Can anyone give me a layman's explanation of 'load the tuned circuit' in the following -

This design offered some amplification of the signal and it didn't load the tuned circuit (to the right) nearly so much.

[ppppenguin]

Any resistance across a tuned circuit (or in series with the inductor) will cause it tune less sharply. Or lower the Q to put it another way. Such resistance across a tuned circuit would be said to load it.

[unrealdave]

Cheers matey - I will be getting these notes over to you soon.

[unrealdave]

One more quick question

A crystal set uses high impedance headphones. This increases the magnetomotive force but also match's the impedance of the crystal set. Again in layman's terms why is matching the impedance so important in these sets?

[pwdrive]

Power.

Lawrence.

[unrealdave]

OK
so are you saying if the impedance is not matched it wastes power?

[pwdrive]

It's all in here:

https://en.wikipedia.org/wiki/Impedance_matching

With a crystal set there's no amplifier so can't afford to lose anything.

Lawrence.

[ppppenguin]

You've got an inherent conflict with a crystal set. You want an impedance match to transfer maximum power from the aerial to the headphones. But this invetibaly loads the tuned circuit so you'll get bad selectivity.

Incidentally, don't try impedance matching for max power transfer on the mains or a car battery. You might not like the results

[reset]

OK
so are you saying if the impedance is not matched it wastes power?

If you have two resistors in series the same current will flow through them. The power is I^2 * R so you have the most power when I and R peak. Leave one resistor value fixed and adjust the other. Raising total resistance lowers I . Reducing total resistance allows the current to increase but the higher current drops more power across the fixed resistor. The fixed resistance is you circuit and the variable is the phones so you get maximum power across the phones when the match the circuit impedance.
Hope this makes since. You could use a simple circuit use a 10k fixed then vary the load 5k, 10k, and 15k using a 10 volt supply.

Also in a radio circuit you are power limited. In the example above the battery will supply power as need. I a radio as you load the circuit you kill the circuit Q and lose power.

73

[unrealdave]

Got it - cheers guys

reset - I just took a trip through Lucedale, Mississippi on google maps - looks great