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 ...
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9 votes

Completely different circuit response with 1000kΩ than with 1MΩ resistance

In PSpice m and M are thousandths. You need to use Meg.
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  • 13.6k
8 votes

Why is superheterodyning better than direct conversion?

There are quite a few advantages. Taking a look at a typical superhet (up to the IF): The input signal at the RF input is small (as low as -122dBm in some narrow band voice systems I have worked on -...
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7 votes
Accepted

Detect an IR signal with a photodiode to use it with a microcontroller

Read the following data sheet extract from the TL074 and see why your circuit is ineffective: - So, if you fixed the supply rails the next problem will be that your output signal will be negative ...
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  • 379k
6 votes

Why is superheterodyning better than direct conversion?

Note that this answer is skewed toward analog radio reception. The rules are different for software-defined radios, and for digital services. The biggest drawback to direct conversion is sideband ...
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6 votes
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Is the resonance frequency actually at the center of the passband of a bandpass filter?

I remember learning in Circuit Theory that the resonance frequency of a series RLC circuit is the geometric mean (or geometric middle) of the half-power (cutoff) frequencies: I hope this helps.
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6 votes

How to properly design LC bandpass filters?

You have given enough parameters to design the required filter: source termination 50 ohms load termination 50 ohms bandwidth 200 kHz center frequency 1.81 Mhz Any number of "applets" can ...
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  • 18k
5 votes
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"Top coupling" in VHF bandpass filter

That filters has 4 sections, which if we number them 0 to 3 ... 0 is coupled to 1 by mixed coupling which is mostly inductive 1 is coupled to 2 by capacitive coupling through Cc 2 is coupled to 3 as ...
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  • 140k
5 votes
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Picking the right opamp

This specific application Presumably you want to power the op amps with the (single) 5 V DC supply, so your main requirements are: an op amp that can operate with a 5 V supply or less an op amp which ...
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5 votes
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Analysing a band pass sallen key filter

You can use modified nodal analysis to solve for all unknown node voltages and unknown currents. Once you get the node voltage, you can find the transfer function. For the analysis, I denote node and ...
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5 votes
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Digitally selectable filter

It would be better to have 560 ohms to ground, and use only one transistor, switching a value of 3k (should be 2.9k) to ground. That way most of the current flows in a real resistor, which minimises ...
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  • 140k
5 votes

Are these two circuits the same?

These two circuits can provide similar bandpass frequency response. However, the lower-frequency cutoff must be considerably lower than the high-frequency cutoff. This means that the frequency span ...
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  • 18k
5 votes

Why is superheterodyning better than direct conversion?

Yes, direct-conversion receivers exist, but they require special care, especially with certain kinds of modulation. For example, with SSB modulation, in order to reject the unwanted sideband, your ...
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  • 164k
5 votes

Why is superheterodyning better than direct conversion?

Direct conversion is conceptually simple, but requires considerable engineering to do it right. Besides Dave's & Tim's answers, there is a subtle pernicious potential problem with direct ...
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  • 18k
5 votes

Building a band-pass filter using an op-amp

The maximum output amplitude of an op-amp, above a certain frequency, decreases with frequency. From the LT1128 datasheet we have this graph What this graph tells us is that even though a particular ...
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4 votes

Discrete high order filter (passive?)

Well, not completely passive. But not what's usually meant when one uses the words 'active filter'. You can't be "a little pregnant". A filter also can't be "not quite passive, but also not active": ...
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4 votes

Filter for radio receivers

The LM358 and LM741 do not have enough bandwidth to do what you want. At 200kHz the LM358 can only manage about 10dB of gain. It also has poor slew rate and a high phase shift at this frequency, ...
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4 votes
Accepted

Sallen-key second order bandpass filter low frequency response

A square wave of 200 Hz contains odd harmonics all the way to infinity. The 25th harmonic of 200 Hz is 5 kHz hence you see the band pass filter extracting this harmonic and greatly amplifying it. ...
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4 votes

How could I design a circuit for a solar panel so that it only produces output from a red laser?

Firstly, "laser" tag doesn't (usually) use lasers - and for good reason: Lasers are dangerous and can blind people. Use with caution. Laser tag normally operates using a focused infra-red beam. ...
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4 votes

Designing a circuit from a transfer function

Your transfer function can be obtained in different ways and one of them could be by cascading two stages with a buffer in-between. Your original expression follows the form \$H(s)=H_0\frac{1+\frac{s}...
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4 votes

Completely different circuit response with 1000kΩ than with 1MΩ resistance

Not sure what kind of simulator that is, but a common pitfall is: 1M = 1 milli (Ohm, Volt, Ampere) and 1Meg = 1 Mega (Ohm, Volt, Ampere). Because they don't care about capitalization they can't make ...
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4 votes

Transfer function of a bandpass

These answers are obviously correct but they represent the brute-force approach. Nothing wrong here but considering all these lines, it is likely that mistakes or typos get in the way of the correct ...
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3 votes
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LCR circuit with squarewave input: Why does the output take a decreasing amplitude through each cycle?

Your LCR resonator has losses, so it is a decaying sinusoid wave at 5 MHz. Before it decays to zero, it is re-excited by the negative-going edge of your 1 MHz square wave. Then re-excited by the next ...
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3 votes

Designing a multi-stage BJT amplifier

Your design is a very basic and might work but since it has no feedback it is not so predictable in how it will behave in practice. The "proper" engineer's way of designing this is by use of feedback....
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3 votes
Accepted

Why is center frequency of a bandpass filter is given by the geometric average of the two cutoff frequencies?

Why is the center frequency of a band-pass filter is given by the geometric average of the two cutoff frequencies instead of arithmetic average? Because its the ratios that are relevant, not the ...
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3 votes
Accepted

Twin-T filter decay rate (drum circuit)

After some experimentation I finally solved the riddle: It was noise affecting the feedback loop. After shrinking the feedback resistors by a factor of ten and increasing the capacitors by the same ...
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3 votes

Impedance calculation

Perhaps the easiest way to determine this without using any math is to simulate it in a program such as LTSpice. If you aren't familiar with this tool, I strongly encourage you to look into how to use ...
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  • 325
3 votes

Ultrasonic transducer actual bandwidth for filtering

If total BW is defined as the 0.707 amplitude threshold ( -3dB) and on the lower half BW averages to -35kHz at 965 KHz, we might approximate it as a BW of 70kHz with a Q= 1000/70= 14 or inversely 1/Q=...
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3 votes
Accepted

Low capacitance value capacitors in LC bandpass filter

When you get unrealisable component values in your theoretical design, it's a sign that you need to change your filter topology. That second stage series resonant tuned circuit has an impedance that'...
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  • 140k
3 votes
Accepted

Audio Equalizer LTSpice Simulation

But it IS working, at least within the limitations of your opamp. The peaks in your response curves fall about where they should. In fact, if you draw the asymptotes to your response curves, they hit ...
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