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Right-click on V(out) at the top of the plot window and change it to V(out)/1u in the pane that opens. But that is never going to work properly, or as you expect, in practice, because those large capacitors resonate with internal and lead inductances, probably well below 1 MHz.

Hi LTspice Group,

?

I have simulated a filter which is a cascaded of two LC low-pass filters.
I get the Bode plot Magnitude in dB versus frequency and phase versus frequency.

?

However, I need to display the magnitude in dBuV versus frequency.
dB is just a value but dBuV is a value with reference 1uV.

I tried few things but no one worked.

Is there a way to show dBuV vs Frequency?
Has anyone tried it before?

?

LCLC filter is uploaded.

?

Comments are appreciated!

Ray

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1. If you are only concerned with the response well below 1MHz, there is not much point in sweeping up to 10MHz.

2. You can't change the axis labels to dB?V within LTspice, but you can add text to the plot pane: 0dB = 1?V. With Windows, you can get ? with ALT+0181 on the numerical keypad, with NumLock enabled.

3. To see the effects of even small stray inductances, put 100nH in series with each capacitor. 24 dB gain at about 10 kHz, but see below about input impedance.

4. The input impedance [1V/I(L1)] is very low, less than 1 ohm up to 300 kHz and a minimum of 2 milliohms at about 10 kHz.

Hi John,

Thanks for your response.

fc of the first LC is 11.25kHz and that of the second?one is 32.83kHz which are << 1MHz.
I intend to show dBuV?instead of dB on the vertical axis of the Bode plot if that is doable with LTspice.

Thanks,
Ray

On Sat, Apr 12, 2025 at 3:18?PM John Woodgate via <jmw=[email protected]> wrote:

Right-click on V(out) at the top of the plot window and change it to V(out)/1u in the pane that opens. But that is never going to work properly, or as you expect, in practice, because those large capacitors resonate with internal and lead inductances, probably well below 1 MHz.

On 2025-04-12 22:07, Ray. Koo via wrote:

Hi LTspice Group,

?

I have simulated a filter which is a cascaded of two LC low-pass filters.
I get the Bode plot Magnitude in dB versus frequency and phase versus frequency.

?

However, I need to display the magnitude in dBuV versus frequency.
dB is just a value but dBuV is a value with reference 1uV.

I tried few things but no one worked.

Is there a way to show dBuV vs Frequency?
Has anyone tried it before?

?

LCLC filter is uploaded.

?

Comments are appreciated!

Ray

--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only If something is true: * as far as we know - it's science *for certain - it's mathematics *unquestionably - it's religion

Virus-free.

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If that weird design works for you, OK. This group doesn't do circuit design, but if you want to have a sensible low-pass filter, you could well use one of the filter design apps at

Hi John,

Please see my answer to items 1, 3, and 4.
1. The intention?was twofold, (a) to verify attention rate at -40dB/Dec and (b) to check filter behavior at higher frequencies

2. I could get???into the Vertical?Axis range but ltspice did not display it. That was an interesting suggestion.

3. Caps have esl = 10pH & 20pH, respectively.?
I re-run the simulation with esl and noticed the effect of stray inductances.

4. Yes, you are?correct there.

Very Best,
Ray

On Sun, Apr 13, 2025 at 1:09?AM John Woodgate via <jmw=[email protected]> wrote:

1. If you are only concerned with the response well below 1MHz, there is not much point in sweeping up to 10MHz.

2. You can't change the axis labels to dB?V within LTspice, but you can add text to the plot pane: 0dB = 1?V. With Windows, you can get ? with ALT+0181 on the numerical keypad, with NumLock enabled.

3. To see the effects of even small stray inductances, put 100nH in series with each capacitor. 24 dB gain at about 10 kHz, but see below about input impedance.

4. The input impedance [1V/I(L1)] is very low, less than 1 ohm up to 300 kHz and a minimum of 2 milliohms at about 10 kHz.

On 2025-04-13 02:10, Ray. Koo via wrote:

Hi John,

Thanks for your response.

fc of the first LC is 11.25kHz and that of the second?one is 32.83kHz which are << 1MHz.
I intend to show dBuV?instead of dB on the vertical axis of the Bode plot if that is doable with LTspice.

Thanks,
Ray

On Sat, Apr 12, 2025 at 3:18?PM John Woodgate via <jmw=[email protected]> wrote:

Right-click on V(out) at the top of the plot window and change it to V(out)/1u in the pane that opens. But that is never going to work properly, or as you expect, in practice, because those large capacitors resonate with internal and lead inductances, probably well below 1 MHz.

On 2025-04-12 22:07, Ray. Koo via wrote:

Hi LTspice Group,

?

I have simulated a filter which is a cascaded of two LC low-pass filters.
I get the Bode plot Magnitude in dB versus frequency and phase versus frequency.

?

However, I need to display the magnitude in dBuV versus frequency.
dB is just a value but dBuV is a value with reference 1uV.

I tried few things but no one worked.

Is there a way to show dBuV vs Frequency?
Has anyone tried it before?

?

LCLC filter is uploaded.

?

Comments are appreciated!

Ray

--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only If something is true: * as far as we know - it's science *for certain - it's mathematics *unquestionably - it's religion

Virus-free.

--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only If something is true: * as far as we know - it's science *for certain - it's mathematics *unquestionably - it's religion

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dBu? is decibels referred to 0.775 V, which is 1 mW in 600 ohms. dB?V, or better dB(?V), is decibels referred to 1?V.

?

dbu on the other hand is referenced to 1 uV.

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To be very precise, 0.775 V is approximately equivalent to 1mW in 600 ohms, which is sqrt(600x0.001) = 0.774596... That is the explanation of the voltage value.

John Woodgate:

>dBu? is decibels referred to 0.775 V, which is 1 mW in 600 ohms.
0 dBU is not 1mW in 600 ohms.

0 dBU is 0,775 V irrespective of impedance.

0 dBm is 1 mW in 600 ohms.

--------------------

Best regards,

Goran Finnberg
The Mastering Room AB
Goteborg
Sweden

E-mail: mastering@...

Learn from the mistakes of others, you can never live long enough to
make them all yourself.??? -?? John Luther

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(='.'=)
(")_(") Pyret, Ranglet, Aron, VovVov, Nero, Smurfen & Pussin:RIP

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This is very OT, but dBm is very widely applied to 50 ohm circuits in the RF field.

No, it's not. dBm applies only to 600 ohm circuits. If you want independant on load, you must use dBu (or dBV).

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Thank you, but that crazy filter is not my design.

Hello John,

The land of dB can be confusing at first.? When dB was invented, it was a way to keep track of power, as in watts.? Each +10dB was 10 times as much power.

Recall that power, P = V^2 / R.? Since power increases to the square of the voltage, 10 x voltage is +20dB.

The display defaults to reading 0dB as 1 volt.? For .001V, you go down -20 dB for each decade or -60dB.? Going to a microvolt, 1E-6 V, you get -120dB.? By the way, using the 1EX convention is easy as you don't have to recall any symbols.

As to what level you're using, it usually doesn't matter, because AC analysis assumes everything is linear.? So, you can inject 1 volt and pretend it's 1uV and that corresponds to 0dB.? The solver won't care :)

As to parasitic aspects, it is good to remember the DC resistance of the inductors, and better to find the resonance of the inductor and place an equivalent C across it.? Good fences build great neighbors, but including your parasitic values makes great models.

Most of the ceramic capacitors are on the order of 2nH, and it increases with the physical size of the capacitor.? I construct filters on the order of 10's of MHz to 1GHz, so the series inductors on caps become very important.? For 1MHz and under, you can probably ignore such tiny values.

? ? I wish you the best on your model and circuit,
? ? Mike

On Monday, April 14, 2025 at 06:22:02 AM CDT, John Woodgate <jmw@...> wrote:

This is very OT, but dBm is very widely applied to 50 ohm circuits in the RF field.

On 2025-04-14 12:10, Jerry Lee Marcel via groups.io wrote:

No, it's not. dBm applies only to 600 ohm circuits. If you want independant on load, you must use dBu (or dBV).

--
Best wishes John Woodgate RAYLEIGH Essex OOO-Own Opinions Only If something is true: * as far as we know - it's science *for certain - it's mathematics *unquestionably - it's religion

Virus-free.