John Fields wrote in message . ..
On 16 Jan 2004 06:40:38 -0800, (Svante)
wrote:
Harmonic distorsion is expressed as the ratio between the distorsion
components and the fundamental. What surprises me is that it is the
VOLTAGES that are compared (in the electrical case) not the POWERS. So
if we have a second harmonic 40 dB down, the second harmonic
distorsion is 1 %, not 0.01 %.
(In this case the voltage of the harmonic is 1% of the fundamental,
and its power is 0.01% of the fundamental)
What is the reason for this convention? I'd think that power would be
more logical.
---
It is _much_ easier to notch out the fundamental(s) and measure the
voltage of the remaining component(s) than it is to measure their power.
Then, knowing the voltage of the offending component(s) appearing across
the load and the impedance of the load at the various frequencies at
which distortion rears its ugly head, their contribution to the power
being dissipated in/by the load can be easily calculated.
Precisely. ALL of the THD meters in existance are essentially
voltmeters with filters. Going from the ancient and venerable
General Radio 1936 or Hewlett-Packard 330, through the HP 334,
through the Sound Technology 1700, Amber, Marconi and so on,
they are ALL voltmeters that have a narrow-band notch filter
in them.
And, if you describe the ratio of the original to its distortion
products in dB rather than in percentage, what do you care?
A THD of 1% means the amplitudes differ by a factor of 100:1,
their powers differ by 10000:1, but it's 40 dB in both cases.
That's the entire point: A RATIO between two signals is EXACTLY
the same whether you describe the RATIO of the voltage, the RATIO
of the power or the LOGS of those ratios (assuming the impedance
is the same in all cases, which is a prefectly reasonable assumption).
There is no more or less logic to doing it one way or the other,
they are exactly equivalent.