As the circuit here, http://www.newcircuits.com/circuit.php?id=rfr002

Why a tank circuit is used to turn the frequency? Isn't a tank circuit used to block a certain frequency?

 

It depends on how it is used. Just as an RC filter can be either a high-pass or a low-pass, depending on how it is hooked up. In general, a tank circuit makes the circuit do something different at the tuned frequency than it does at frequencies above or below it. Depending on how it's hooked up, it might pass signals at the frequency or it bigh block signals at that frequency.

 

I can understand what you mean, but I still don't understand how it can be done in a tank circuit example

 

Think of putting the tank circuit in a box and know you have two terminals. The tank circuit will easily let low frequency signals pass through it (the inductor looks like a short) and it will easily let high frequency signals pass through it (the capacitor looks like a short), but it will not let signals at or very near the resonant frequency through it.

So now consider what happens if you put this box in series with your signal. The frequencies that the circuit is tuned for are blocked and all the others get through. Now consider what happens if you put this box between the wire carrying your signal and ground (or, more generally, you take your signal measurement across the terminals of the box). The signals you don't want will be shunted to ground (or shorted out) but the signal near the tuned frequency will not be and will be seen by whatever the follow-on circuit is.

One way to picture it is to imagine that the box has two resistors in it, one of frequencies very close to the resonant frequency and one for frequencies well away from it. The first is a very large resistor and the second is a very small resistor.

 

Hi WBahn,

thanks for taking the time to explain it to me, now I can understand it.

Now comes another question, I only saw a few RF circuit, but it looks like all of circuit I saw, they only use a tank circuit to turn the frequency, I believe there must be a good reason.

I mean, why not use a inductor and cap in series to filter in the needed frequency, instead of shorting out the unwanted frequency?

those two methods look the same for me, at least in theory anyway.

please point out what have I missed here, thanks.

 

With respect to your original schematic for the FM transmitter the tank [parallel resonant] circuit presents a high impedance load to the collector at resonant condition - not a low impedance. This is a classic circuit topology which evolved from the typical RF amplifier used in vacuum tube amplifiers. The tank inductance passes DC current required to establish DC operating conditions in the amplifier. This important feature makes the parallel configuration a topology of choice.

Series resonant topologies are also commonly used - such as in inverters used in power supplies and induction heating systems.

The parallel resonant circuit has a long history going back to the spark-gap transmitter.

 

Hi t_n_k

Thank your very much for taking the time to explain it to me, that's very helpful.

I am afraid I still don't understand this circuit, I mean I can understand the tank circuit part, due to WBahn's and your explanation.

I think now my question lead to how the typical RF amplifier works, which you mentioned here:

This is a classic circuit topology which evolved from the typical RF amplifier used in vacuum tube amplifiers.

but I think that's another topic and I don't think it's an easy, so I will do some googling before I post another question here.