Read magnitude and phase on a Bode plot

A ratio, not an absolute voltage

For a voltage transfer function, magnitude in decibels is $20\log_{10}|H|$, where H is output voltage divided by input voltage under stated loading. A voltage ratio of 2 is 6.0206 dB; a ratio of 0.1 is −20 dB. The factor 10 belongs to power ratios. Mixing the two conventions doubles or halves the plotted result.

An unloaded RC low-pass with output across C has $H(j\omega)=1/(1+j\omega RC)$. Its corner frequency is $f_c=1/(2\pi RC)$. Let R = 10 kΩ and C = 100 nF. Then fc = 159.154943 Hz. At fc, magnitude is 1/√2 = 0.707107, or −3.0103 dB, and phase is −45°. The corner is not the point where output suddenly disappears.

Three useful checkpoints

Well above the corner, this first-order response approaches a −20 dB per decade slope. That asymptote is a useful sketch, not the exact curve near fc. Phase approaches −90° at high frequency. Logarithmic frequency spacing makes both the low-frequency passband and high-frequency rolloff readable.

Loading must stay in the model

If a 10 kΩ load is placed across the capacitor, DC gain is 0.5 and the pole uses 5 kΩ times C. The absolute output at the pole is then 0.5/√2, not 0.707107. A “−3 dB point” normally means 3 dB below the passband gain, so state the reference.

Plan an AC sweep across at least two decades around the intended corner, then add exact checkpoint frequencies. A broad sweep shows shape; exact points support numerical acceptance. Compare magnitude and phase together, because an incorrect node selection can accidentally produce a plausible-looking magnitude at one frequency.