Simple RF LC filter circuit diagram

This is a very interesting and practical radio frequency filter circuit. It is a simple three LC components. On the one hand, it attenuates a certain frequency, but at the same time, it also has a boosting and reinforcing effect on a nearby frequency to avoid excessive attenuation effect. In past product applications, low-power home wireless phones sold in the United States have a very close frequency of 4*9MHz because the receiving and transmitting antennas are shared. This circuit can be used to separate the two signals.

As shown in the figure, input from A and output from B:

Components L2 and C1 resonate in parallel, producing a notch frequency. Components L1 and L2 resonate in parallel with C1 to produce the pass frequency. That is, it blocks low frequencies and allows high frequencies to pass. (Because the inductance value of L1 and L2 in parallel becomes lower, the resonant frequency becomes higher.)

When the inductance value of L1 is much larger than that of L2, the two frequencies can be close to each other, which is convenient for application.

The blocking frequency point is: Freq-trap = 1 / 2π√ (L2 * C1)

The pass frequency point is: Freq-pass = 1 / 2π√ (L1// L2) * (C1)

ck="window.open(this.src)" alt="Click to see larger image" />

As shown in the figure, input from C and output from D:

Components L3 and C3 resonate in parallel, producing a notch frequency. Components C2, L3 and C3 resonate in parallel to produce the pass frequency. That is, it blocks high frequencies and allows low frequencies to pass. (Because the parallel capacitance value of C2 and C3 becomes higher, the resonant frequency becomes lower.)

When the capacitance value of C2 is much smaller than that of C3, the two frequencies can be close to each other, which is convenient for application.

The blocking frequency point is: Freq-trap = 1 / 2π√ (L3 * C3)

The pass frequency point is: Freq-pass = 1 / 2π√ (L3) * (C2 + C3)

Another common application is that point B or D is connected to ground, and point A or C is connected to the signal path, which can achieve exactly the same effect.