35
votes
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Why do microwave ovens use magnetrons?
Magnetrons are cheap, reliable, pretty efficient (65% or so- and they tolerate high temperatures so heat sinking is easy) and made with mature technology. They are also reasonably tolerant of VSWR ...
- 331k
19
votes
Accepted
How do I calculate the cutoff frequency of a low pass rc circuit?
The specific formula applies only for a first order RC low pass filter. This is derived from its frequency response:
$$H(j\omega)=\frac{1}{1+j\omega RC}$$
The cutoff frequency is defined as the ...
- 1,179
16
votes
How do I calculate the cutoff frequency of a low pass rc circuit?
For a simple RC low pass filter, cut-off (3dB point) is defined as when the resistance is the same magnitude as the capacitive reactance: -
\$R = \dfrac{1}{2\pi f C}\$
It's a simple math trick to ...
- 384k
13
votes
Accepted
40 GHz spectrum analyzer with N-type coax input?
The R&S instruments are equipped with female test port adapters.
Edit: archived pdf
You can replace the N type with a more suitable type for higher frequencies.
- 331k
13
votes
Significance of cut-off frequency in a low-pass filter
H(w) =1/ (square root of 2) That sounds a bit confusing, we usually refer to the cutoff point as the -3 dB point.
That is the same though, -3 dB is half the power.
Let me explain: take your \$H(\...
- 78.8k
13
votes
Accepted
Cutoff frequency of RLC band pass and band stop filter
RLC Passband and Stopband
Your question doesn't explicitly take note of the fact that there are four distinct RLC filters that are either band-pass or band-stop. But I'd like to list them for those ...
- 70.2k
12
votes
Accepted
Deriving 2nd order passive low pass filter cutoff frequency
EDIT: Thanks to hryghr I see that the starting assumptions were incorrect. The transfer function magnitude can't be found that simply.
It is more than ten years since I considered my skills sharp on ...
- 1,131
12
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Why do microwave ovens use magnetrons?
The domestic microwave oven needs high power to cook the meal and high frequency to excite the water molecules. What is not needed is high stability because the water energy absorption spectrum is ...
- 12.4k
8
votes
Accepted
Cutoff frequency: exactly -3 db of power
The 3-dB cutoff is just one commonly used way to describe a filter. For some applications you might want to specify the 10-dB cutoff or 60-dB cutoff instead. It is convention that if someone says "...
- 121k
8
votes
Accepted
Do we always take the corner frequency of a filter at exactly -3dB?
It's not 3dB absolute, it's 3dB down from the peak, or some sort of nominal attenuation. So in your case, where the passband is -3dB, 3dB down is at -6dB.
Note that some filters (e.g. Chebychev) have ...
- 36.3k
7
votes
Deriving 2nd order passive low pass filter cutoff frequency
A lot of people confuse natural frequency with cut off frequency. The natural frequency is the frequency the system wants to oscillate at. The cut off frequency (or ...
- 71
7
votes
Accepted
Why do current transformers have a minimum frequency rating?
For a voltage transformer: -
If the applied voltage were very low in frequency i.e. 0.1 Hz, the primary current taken (if the voltage were 120 VAC or 230 VAC) would be several tens of amps and the ...
- 384k
7
votes
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Divider and lowpass combined
Not that I'm aware of, call it a filtered voltage divider :)
\$fc=\frac{(R_1+R_2)\sqrt{(10^{3/10}-1)}}{2\pi*R_1*R_2*C_1}\approx \frac{(R_1+R_2)}{2\pi*R_1*R_2*C_1} \$
\$R_1//R_2//C_1 \$
There are ...
- 728
6
votes
Do we always take the corner frequency of a filter at exactly -3dB?
It is not correct that for "all filters" the corner or cut-off frequency is defined by the "-3dB point" (magnitude 3 dB down with respect to the maximum).
This is only the case for
...
- 22.6k
5
votes
Why do current transformers have a minimum frequency rating?
Non-superconducting current transformers have a rolloff below a certain frequency, so the output for a given current simply goes down as the frequency decreases. It's not core saturation nor is it ...
- 331k
5
votes
Accepted
High pass filter - Cut-off frequency calculation doesn't match simulation
From the simulation, it looks like it fc=500Hz or somewhere around
there.
Your simulation looks more like 15 kHz (3 dB point) and the reason is because you haven't used enough resolution in your AC ...
- 384k
5
votes
High pass filter - Cut-off frequency calculation doesn't match simulation
Well, mathematically speaking we can write:
$$\mathcal{H}\left(\text{s}\right)=\frac{\text{sL}}{\text{sL}+\text{R}}\tag1$$
Using \$\text{s}=\text{j}\omega\$, we get:
$$\left|\underline{\mathcal{H}}\...
- 5,061
5
votes
Accepted
Active low pass filter with cut-off frequency ~4250 Hz lets a 20 kHz signal through
Back of the envelope calculation...
20k/4250 = about 4
It's a second order filter, so at a frequency 4x above its -3dB point it should attenuate the signal by 4 squared, or about 16.
Output signal is ...
- 59k
5
votes
How can I find the cutoff-frequency in real experiment?
It seems to be your task to figure out how to do this, and we don't solve homework for people. But your actual question here seems to be: I don't understand why I should find the method to determine ...
- 12.7k
5
votes
Accepted
Simulating an RLC bandpass filter with LTspice
The reason the "gain" is above zero is that it's not gain, it's voltage expressed in dBV and you're putting 5 VAC into the circuit. Change your source AC amplitude to 1 and try it.
- 4,140
4
votes
Why would an 8-order LPF switched capacitor have a less steep roll-off curve (MAX7405)?
For a lowpass with order n=8 you can expect a magnitude drop (far above the cut-off frequency) of 48dB/Octacve. I think, the presented curve does show such a slope - however, only approximately. Why ...
- 22.6k
4
votes
Accepted
Low Pass Filter With Hall Effect and ADC Help
The browser doesn't like that link to the datasheet, so I couldn't read it. Provide the link to just the PDF file, not a page with all kinds of fluff around it.
In any case, the reason for the low ...
- 306k
4
votes
Accepted
Region of Operation of MOSFET and current cutoff frequency
Disclaimer: the answer below relates to a mosfet transistor used as a voltage amplifier.
Summary
Transition frequency \$f_T\$ is a figure of merit (FOM) of a transistor reflecting both frequency ...
- 1,633
4
votes
Deriving 2nd order passive low pass filter cutoff frequency
Solving a simple 2nd-order circuit like this one requires a few lines obtained by inspecting the circuit. This is how Fast Analytical Circuits Techniques or FACTs described in "Linear Circuit Transfer ...
- 14.5k
4
votes
Deriving 2nd order passive low pass filter cutoff frequency
In my opinion this 'cut-off' frequency is not defined as the -3dB point. The real transfer function is: $$ H(s)=\frac{1}{s^2R_1R_2C_1C_2+s(R_1C_1+R_1C_2+R_2C_2)+1}$$
The 'common form' of a second ...
- 1,179
4
votes
Accepted
3-dB Frequency of second order transfer function
I show you how to obtain the 3dB cut-off frequency for the low pass filter \$G_1(s)\$. You can calculate the cut-off frequencies of the band pass filter \$G_2(s)\$ in a similar way, as long as you ...
- 3,603
4
votes
Finding Cutoff Frequency
For this type of a circuit the cutoff frequency is not at Vin/Vin = 0.707.
Because the R1 and R2 form a voltage divider, and "mid-band gain" (maximum of Vout) is equal to R2/(R1 + R2) = 0.9[V/V], ...
- 12.3k
4
votes
Derivation of cutoff frequency and phase shift for RC low pass filter
Cut-off frequency or 3-dB frequency is defined as the frequency of the input signal at which, the magnitude of the output signal reduces to \$1/\sqrt2\$ of the input, or the power reduces to half ( i....
- 10.4k
4
votes
What is the formula for cutoff frequency for LCL filter?
You should find a different teacher if he teaches you wrong words, which serve as a basis for future understanding. That said, the circuit is meant to be either doubly terminated -- one resistance as ...
- 17.7k
4
votes
What is the formula for cutoff frequency for LCL filter?
Building on the answer given by a concerned citizen, I have applied the fast analytical circuits techniques or FACTs as an alternative to the series of equations given in the text. By splitting ...
- 14.5k
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