Low Pass Filter design

Discussion in 'General Electronics Discussion' started by vin vino, Oct 13, 2014.

1. vin vino

5
1
Oct 13, 2014
Hi Friends,
I am designing a low pass RC filter circuit with the cut off freq (1khz). its working fine as per the logic but my problem is the out put voltage is mostly below 2.5V in all range.

Actually my need is if the in put freq is below cutoff that the out put voltage should above 2.5V and if it is above cutoff freq then the output voltage should below 2.5V.

2. Fish4FunSo long, and Thanks for all the Fish!

464
105
Aug 27, 2013
Hey vin vino, Welcome to EP!

Is this a strictly analog circuit? It would be best if you posted a schematic AND if the schematic does not clearly illustrate the purpose of the circuit, then you will likely need to give some details about the circuit's purpose. From the very few details you have provided, perhaps an OP-Amp configured as an inverting amplifier? or if it is a digital circuit, perhaps a simple logic inverter like a 74x04 or a schmitt trigger version like 74x14? But w/o more details these are just shots in the dark....

Fish

3. Harald KappModeratorModerator

11,281
2,583
Nov 17, 2011
Welcome to our forum.

Filters don't work that way. Although we use the term cut-off frequency, no filter really cuts off a signal at this frequency. Any filter has a gradual change from pass band to stop band (see e.g. here). How fast it transist from pass to stop depends on the so called filter order. The cut-offf frequency is by convention the frequency where the output of the filter is attenuated by 3dB.

A filter is usually not designed in terms of absolute voltages (or currents). Since the output at any frequency depends also on the amplitude of the input signal, a filter is described in terms of attenuation of the output signal relative to the input signal (Vout/Vin) measured logarithmically in decibels (dB).

Show us your design and complete specs of the filter (attenuation in the pass band, roll-off, etc.), your input signal and any other requirements for the output signal. We then may be able to help you further.

4. vin vino

5
1
Oct 13, 2014
HI Fish!
Here i sending the file info .. input may be either analog or digital but out put should be TTL with 3.3v.

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5. vin vino

5
1
Oct 13, 2014
Hi Harald,
My requirement is the input to the circuit will be an analog/TTL.. if the input frequency is goes above 1Khz out put should be ZERO.

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6. davennModerator

13,798
1,939
Sep 5, 2009
do you realise that putting a TTL ( square wave ) signal into a RC filter is going to severely distort the waveform ?

I think you need to tell us much more about what you are wanting to achieve, as your circuit stands at the moment, I don't think its going to have the desired effect

What is generating the input signal ?
why could it be digital or analog ?
component types are almost unreadable on your schematic ... what is the IC ?

Dave

7. KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
Is your input signal just a single frequency? In that case, a filter probably isn't the best option. One option is a frequency-to-voltage converter and a comparator with an adjustable threshold. Another option is a circuit using one or two monostables. Either of these can give a fairly accurate detection threshold.

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8. davennModerator

13,798
1,939
Sep 5, 2009
good thought Kris .... LM 2917 or similar

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9. vin vino

5
1
Oct 13, 2014
Ya Devenn & Kris...

input is from a hall sensor.. that input freq to the controller limit will not exceed (1KHz 0r 2.2Khz).
so i want to make a circuit which cuts off if the input freq is above the desired level.

10. KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
The LM2917 looks like a really good option. It has a frequency-to-voltage converter and an uncommitted op-amp/comparator driving a transistor, which can be used as a threshold detector.

There's a circuit to do exactly that in the data sheet for the LM2907/2917 at http://www.ti.com/lit/ds/symlink/lm2907-n.pdf. Look for the "Speed switch" circuit on page 9.

You can use either the 2907 or the 2917; the 2917 is smaller and has all the features you need, so I'll assume you will use that one.

You will need to drive the input (pin 1) from your Hall sensor, through a DC blocking capacitor (between the Hall sensor signal and pin 1) with a resistor of around 100k from pin 1 to 0V.

There's information in the data sheet to help you calculate the timing resistor and capacitors on pins 2 and 3, and the speed threshold is set by the voltage on pin 7; you can replace the two fixed resistors in the diagram with a potentiometer or preset potentiometer (trimpot) to make the threshold adjustable.

The only weakness of that circuit is that there is no hysteresis, which means that the output may witter around the switching point.

If you're using this circuit to disable the power to whatever is creating the rotation that activates the Hall sensor, and you want the rotation to stop once overspeed is detected, you will need a latch, which is activated when the LM2917 output goes active, to kill the power, and can only be deactivated manually. Without the latch, when the LM2917 detects overspeed and cuts the power, the rotation will slow down, so the LM2917 will detect that the speed is OK, and reconnect power. This will cause rapid cycling of the power, as the speed varies from below the overspeed limit to above the overspeed limit.

Any questions feel free to ask.

11. vin vino

5
1
Oct 13, 2014
Thank you Kris..

I will try with this ,
and let u know...

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