4
\$\begingroup\$

enter image description here

Above is a screen-shot from an LTSpice simulation. It is a freq. response of an LC tank circuit with a 5V sinusoidal AC source.

When I plot it, the y-axis is in dB.

When I look at the voltage it is 13.979dB which corresponds to 5V from the formula 20*log(V1/1).(1 is the reference voltage?)

But the current is plotted in negative dB. Why is that so? What does -dB mean here for the current? is that something to do with reference current in 20*log(I1/Iref)?

\$\endgroup\$
4
\$\begingroup\$

What does -dB mean here for the current?

0 dBI is exactly 1 amp (not to be confused with dBi which is the gain of an antenna compared to the benchmark (i)sotropic antenna)

A current of 100 mA is -20 dBI etc..

So when you have decibels for current as the Y axis then it is implicit that they the Y axis is dB relative to 1 amp (or dBI).

One further thing to consider is that the graph may be plotting peak current and peak current is \$\sqrt2\$ times bigger (for a sinewave). Read the LTSpice help advice to know exactly.

I've just checked on google about usage of the term "dBI" as representing current and nothing comes up. You can find dBuA for dB relative to 1 uA but nothing seems to mention dBI - I'm interested if anyone can provide some evidence to substantiate the usage of the term "dBI".

Sure, dBV are used and this is quite common and one website I visited compared decibels for current and voltage by calling them dB(volts) and dB(amps) - maybe "dBI" just isn't used so as not to confuse folk with dBi?

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Whether the output is peak voltage or rms depends on whether, when you configured the source, you intended its 5 A amplitude to be peak or rms. \$\endgroup\$ – The Photon Jan 22 '16 at 15:48
7
\$\begingroup\$

The decibel scale is a relative one, i.e. you can't really express a value \$x\$ in decibels, rather you always take something like \$20 \log(\frac{x}{x_0})\$ for some fixed \$x_0\$. For the usual scales we use, there is a well defined reference \$x_0\$ such as for \$dBm\$ is logarithm of power divided by one milliwatt, or dBSPL is the logarithm of sound pressure divided by 20 microPascals.

LTSpice doesn't know which reference to use, so it apparently always refers to one unit of the corresponding SI unit. So in LTSpice dB for voltage is \$20 \log (V/1 \mathrm{volt})\$ (which by the way gives exactly the value you report for 5 volts). Correspondingly, the current is referenced to \$1\$ ampere, so the dB values are \$20 \log(I/1 \mathrm{A})\$. These tend to be negative, since you don't usually have one ampere currents running through LC circuits when fed with 5 volts.

So yes, it has to do with \$I_{ref}\$, and \$I_{ref} = 1\mathrm{A}\$ in LTSpice. It cannot be changed as far as I know.

This is actually quite clumsy, since one is often interested in transfer functions, which should be referenced to the amplitude or power of the input signal. The only workaround I've found is to always plot a formula where I explicitly divide by the input amplitude, instead of plotting directly voltages in the simulation

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.