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Re: Making a Q-meter / References etc

 

On Tue, 30 Aug 2022 at 22:53, Rodger Bean <rodgerbean@...> wrote:

Further to the application of dB to inverting amplifiers.

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All of the calculators that I have checked this on, return an error message when the ratio (Vout/Vin) is negative.


I will show you a calculator that does not - see attached.

But consider a VNA connected to a wide-band amplifier with a gain of 10 dB. The output power is always 10 dB higher than the input. The VNA will show |S21| as 10 dB, irrespective of the phase shift through the amplifier. An inverted output is just a phase shift of 180 degrees. I would say the phase angle is irrelevant to the calculation of gain.

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When using a logarithmic ratio, a + prefix denotes a ratio greater than unity. And a ¨C prefix a ratio of less than unity. By convention the prefix can be omitted if gain, or loss, is implied. I.E. the 100W amplifier had a power gain of 30dB, or the attenuator had a loss of 3.5dB. In both cases the prefix is implied in the description.


True, and in both cases, the phase of the output is irrelevant in computing the gain (or loss) in dB.

However, I do think people should use their common sense, and would enter two positive numbers, despite the phase being shifted 180 degrees.

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Rodger Bean
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