Design of LC filter 50, Design of LC filter

I can send you this book.

The differences between RC filters and LC filters are as follows:

1. The difference in volume:

Compared to LC filters, RC filters are easier to miniaturize or integrate, and the relative volume of LC is also relatively large.

2. The difference between wear and tear:

The RC filter is worn, and the LC filter can theoretically not be worn.

3. The difference between circuits:

RC is used for low-frequency circuits, and LC filters are generally used for high-frequency circuits.

4. Different characteristics:

The resistor in the RC filter consumes part of the DC voltage, and the R value cannot be very large, which is used in circuits with low currents and low requirements. Reinforced concrete is small in size and low in cost. The filtering effect is not as good as that of LC circuit, which is mainly due to small inductance resistance and small DC loss.

The AC inductance is high, and the filtering effect is good. The disadvantages are large size, heavy weight, and high cost. For demanding power supply circuits.

The differences between RC filters and LC filters are as follows:

1. The difference in volume:

Compared with LC filters, RC filters are easier to miniaturize or integrate, and LC is relatively larger.

2. The difference between wear and tear:

The RC filter has loss, and the LC filter can theoretically be loss-free.

3. The difference between circuits:

RC is used in low-frequency circuits, and LC filtering is generally used in high-frequency circuits.

4. Different characteristics:

The resistor in RC filtering consumes a part of the DC voltage, and R cannot be used very much, so it is used in circuits with low current requirements. RC is small in size and low in cost. The filtering effect is not as good as that of LC circuits; LC filtering is mainly due to the small resistance of the inductor and the small DC loss.

It has a large inductive response to alternating current and a good filtering effect. The disadvantages are large size, bulky and costly. It is used in demanding power supply circuits.

In the experimental LC filtering device, L=, C=15 F, the transition frequency of the slow plex letter W0=1/1 of the root number LC=4969rad S, then F0=790Hz, while the output fundamental frequency is 50Hz, and the switching frequency is 10K.

The characteristics of the LC filter, when the figure of merit is not particularly low, takes W0 as the transition frequency, and for the input signal whose angular frequency is much less than the transition frequency, the gain of the filter to its amplitude is 0dB, that is, it is not amplified or attenuated, and the phase shift after filtering is zero; For input signals with frequencies much greater than the transition frequency, the filter is attenuated at a rate of -40dB and the phase is shifted by 180 degrees (essentially inverted). Therefore, in order to obtain good filtering performance, it is generally necessary that the transition frequency of the filter is much greater than the output fundamental frequency, and at the same time much smaller than the switching frequency.

The components are the same, but the purpose is different.

LC oscillation is that when the foreign signal passes through LC, it will produce a phase displacement of 180 degrees, the oscillation is based on the principle that the output is output to the input plus a non-inverting feedback, when the amplifier is reversed, such as the collector output of the common emission circuit and the base input are reversed, and the addition of an LC between is positive feedback, an oscillation signal will be generated, and the oscillation frequency is the resonant frequency of LC.

LC filtering is used to make use of different impedances to different frequencies, and the circuit has a selective feature to open or short the unwanted frequencies and let the useful frequencies pass.

LC oscillation is usually active, LC filtering is usually passive, but it can also be combined with active components to improve the filtering effect.

The LC filter is to filter out the waves that resonate with this filter, so that these unwanted energy consumption is hail in this LC.

The resonant circuit is to be maintained, and energy is input every cycle, otherwise it will slowly stop vibrating.

The small signal resonant amplification circuit only amplifies the signal that can resonate with the LC resonator in the source, and other signals are considered useless and not amplified, and the LC in it is not a filter.

The color ring inductance can also be mutually induced, but it can be basically ignored. Their magnetic circuits are all confined to their own cores, and it is difficult to affect other inductors. The antenna inductance of the antenna receiver must generally be wound.

To determine according to the size and frequency of the output harmonics and the switching frequency and other parameters, the calculation is more complicated, and you need to find some professional books to read.

Two fixed filter models are examples:

For k-type filters, l=r (2 f)=;

c=1/(2πrf)=1/

Butterworth type l=2sin(2k-1 2n) *r (2 f)=c=2sin(2k-1 2n) (2 rf)= (where k,n=2) is different from the parameter calculation characteristics, the latter is better than the former in terms of cut-off frequency, the former is better than the latter in terms of delay characteristics, and the former is better than the latter in terms of impedance matching and reflection loss characteristics.

Since you don't define what kind of filter.

Optional: K-type filter then l=r (2 f)=;

c=1/(2πrf)=1/

Butterworth type l=2sin(2k-1 2n) *r (2 f)=

c=2sin(2k-1 2n) (2 rf)= (where k,n=2) can also choose m deduction type, so it will not be calculated.

The latter is better than the former in terms of cut-off frequency, the former is better than the latter in terms of delay characteristics, and the former is better than the latter in terms of impedance matching, in terms of reflection loss characteristics.

In general, without knowing how to choose, I personally recommend going with the Butterworth type.

<> the above figure is the basic parameters of the commonly used classical algorithm, Butterworth type filter circuit, the cut-off frequency is 1 2 Hz(, the characteristic impedance is 1, first to determine how many orders are needed, such as the second order, first normalize, and then transform the cut-off frequency, m=200 l(new)=l(old) m, c(new)=c(old) m, and then transform the characteristic impedance k=50 1, l(new)=l(old)*k, c(new)=c(old) k, The calculated value is the value of the final LC filter to be designed.

Optional: K-type filter then l=r (2 f)=;

c=1/(2πrf)=1/

Butterworth type l=2sin(2k-1 2n) *r (2 f)=

c = 2 sin(2k-1 2n) 2 rf) = (where k, n=2).

This form is the standard shape filter, although it can be filtered with one less capacitor, but it seems that it is not convenient to match or some reason needs to add a capacitor.

The -3db of the peak is indeed the case where the peak is attenuated to one-half of the original root number, and it is also the case where the power is attenuated by half (the power is proportional to the square of the peak). You can calculate 10*log(1) and 10*log(, which correspond to 0db of the original power and half of the power attenuation, respectively, and then convert to the sub-bass value, which is about -3db.

C2 is also used to filter out high-frequency signals, mainly to increase the attenuation effect of the low-pass filter on high-frequency signals.

<> capacitors and resistors are connected in parallel.

Equivalent: <>

According to the principle of partial pressure:

So the transfer function:

RC filter and LC filter Theoretically, as long as the parameters are reasonable, they can form the filter you need parameters! However, from the engineering point of view, RC is generally used for low-frequency filtering, and LC is used for high-frequency filtering, because of the functional limitations of the device in the specific actual circuit! For example, if you need to use LC type to construct a low-frequency filter, then you will get a large inductance value according to theoretical calculations, which means that you need an inductor with a large volume and weight to form your circuit in the design circuit, on the other hand, an inductor with too much volume and weight will naturally have a large winding resistance, resulting in excessive signal loss and low figure of merit!

Conversely, in high-frequency applications, both the inductor and capacitance are small and can yield a high figure of merit. In the same way, if RC is used for higher frequencies, it means that there is a small RC time constant, which requires a small resistance and a large capacitance to meet the requirements, and often the actual circuit may not be able to buy a resistor with a small enough capacitance, and a capacitance with a large capacitance also means that there is a large distributed inductance and resistance. Moreover, the RC filter is more sensitive to the accuracy requirements of the device, in the low-frequency filtering, due to the relatively large time constant, the impact may be relatively small, but the slight deviation at high frequency will lead to a large error!

Then there is the fact that the actual RC filter circuit has large parasitic parameters at high frequencies, which leads to the deterioration of characteristics!

As for the situation that you said that a capacitor is directly connected to the ground, it should also be discussed in different situations, if your circuit is directly connected to a capacitor from the signal source, there is no load, in this case theoretically (without considering the parasitic parameters of the actual capacitance), the AC component capacitor of different frequencies will present different impedances, the higher the frequency, the smaller the capacitor to its impedance, that is, the capacitor to the signal with higher frequency bypass to the ground (the impedance of the capacitor is inversely proportional to the frequency of the signal). You can also think of this as a filter, but the difference between it and RC is that the frequency response curve of this filter cannot form a filter with a certain passband flatness.

But most of the time the filter must be connected to the load, even if you only connect one capacitor directly to ground, the load of your back-end is equivalent to the front capacitor to form an RC filter, or has a certain time constant!

Is this explained clearly?

The LC filter circuit has good stability, low sensitivity, high cost and large size; RC filter circuits are low cost and small in size, but due to the high frequency characteristics of capacitors, they cannot be used in too high frequency circuits. All of the above is for passive filters.