How do i check Attenuation using LTspice

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ironasona

Joined Nov 12, 2020
53
How do i check Attenuation using LTspice .I had a look at the ''Basic circuit simulation with ltspice''but dont understand it a little bit , is the freq that the attention happens is at 10.09Hz at 2000Hz because that's where those 2 line meet ?
 

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crutschow

Joined Mar 14, 2008
27,697
is the freq that the attention happens is at 10.09Hz ?because that's where those 2 line meet ?
No.
The intersection of the amplitude response and the phase-shift have no particular significance.

Normally a filter is defined where the output of a low-pass filter (which this is) is -3db below the low flat part of the low frequency response.
Also of interest is how fast the filter rolls off above that frequency where the roll-off is a straight line (normally given in dB per octave or dB per decade of frequency change).

It's easier to do if you set the AC voltage at 1V, not 230V since the output dB value in LTspice is referenced to 1V (dB is always a reference value between two voltages)
That way the filter attenuation will be shown directly in dB.

Also the Vin and Vout node labels are floating.
The should be moved and the small dot attached to the desired wire node.
 

Thread Starter

ironasona

Joined Nov 12, 2020
53
It's easier to do if you set the AC voltage at 1V, not 230V since the output dB value in LTspice is referenced to 1V (dB is always a reference value between two voltages)
That way the filter attenuation will be shown directly in dB.
Sorry ,am kinda lost, so i've change the voltage source to 1V ,what part of the graph i need to look at ? to see the attenuation and at what frequency it happens. So would i just move the V(vout) to -3db? , so it would be -3db at 25 freq ?(sorry if u have to repeat your self but i just want to be 100% sure what am doing )
 

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ericgibbs

Joined Jan 29, 2010
13,586
hi irona,
I would suggest you stay focused on the original question that you are trying to solve for the circuit that you posted.

Don't get distracted by comments regarding Butterworth filters OPA's, or changing the component values etc.

They will come later in your studies.

In your original circuit simulation you used a 230Vac input source for the AC analysis.
Looking at your plot it shows a Gain of ~+48db at 1hZ.

Consider the circuit is an attenuator, so it cannot give any gain at the output for the input signal.

By using a AC test source voltage of 1Vac the plot will show 0db at 1hZ.
ie: Output = Input at 1hZ

As stated, post #11, the -3dB frequency is at 25.7hZ

Do you follow the explanation.?

E
 

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Thread Starter

ironasona

Joined Nov 12, 2020
53
hi irona,
With a 1Volt supply how do expect to get a 1Volt out.???
E
ye thats what i was thinking, i was asked to get an output voltage of 1V

Q''Adjust the amplitude of the signal source to obtain an output of about 1Vp-p (you will probably need a few mV input). ''

Or did i just misunderstood the question ?because my english isnt the best
 

ericgibbs

Joined Jan 29, 2010
13,586
hi
Your question:
Q''Adjust the amplitude of the signal source to obtain an output of about 1Vp-p (you will probably need a few mV input). '

It means adjusting the V15749 voltage source to get 1Vppk for a few millivolt's of V15749

Increase the supply V1 to say 5v.

What does the simulation show now.??

E
 

Thread Starter

ironasona

Joined Nov 12, 2020
53
hi
Your question:
Q''Adjust the amplitude of the signal source to obtain an output of about 1Vp-p (you will probably need a few mV input). '

It means adjusting the V15749 voltage source to get 1Vppk for a few millivolt's of V15749

Increase the supply V1 to say 5v.

What does the simulation show now.??

E
hi
Your question:
Q''Adjust the amplitude of the signal source to obtain an output of about 1Vp-p (you will probably need a few mV input). '

It means adjusting the V15749 voltage source to get 1Vppk for a few millivolt's of V15749

Increase the supply V1 to say 5v.

What does the simulation show now.??

E
when i increase V1 to 5V i get ,
 

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