Why is frequency not measured in db in bode's plot?
Frequency is not a measure of signal amplitude or power hence, talking about &quot;this or that&quot; frequency in decibels makes no sense. If somebody says &quot;increase the frequency by 3 dB&quot;; it is meaningless because it isn't a signal or power quantity. If someone else says &quot;increase the signal by 3 dB&quot;, we know that means the power is doubled (or the signal is increased by \$\sqrt2\$). To apply that concept to frequency is pointless and meaningless.
There is nothing intrinsic to &quot;frequency&quot; that makes it relatable to power or root-power (as in voltage, current or other signal levels) hence, to use it so, makes no sense. After all, we use the base 10 logarithm of a power ratio multiplied by ten to convert it to decibels and, we use the log of a voltage ratio multiplied by twenty to produce an answer in dB. Which formula would make sense for frequency?
$$20\cdot \log_{10}(\text{voltage ratio}) = 10\cdot \log_{10}(\text{power ratio})$$
The number &quot;20&quot; is used because volts are squared to make power hence, the logarithm of a squared voltage ratio is multiplied by &quot;20&quot; to make it numerically equivalent decibel power. The base factor of &quot;10&quot; puts power into &quot;deci&quot; units.
Does adding a decibel frequency ratio to another decibel frequency ratio have any actual meaning that is useful? On the other hand, with gain stages you can make a lot of sense of adding the gains in decibels to produce an overall gain value in dB. Does that make sense for frequency; no.