From: liz@poppyrecords.invalid.invalid
Jan Panteltje wrote:
> >liz@poppyrecords.invalid.invalid (Liz Tuddenham)wrote:
> >>Bill Sloman wrote:
> >
> >> On 15/11/2025 4:46 am, Liz Tuddenham wrote:
> >
> >
> >[...]
> >> >
> >> > The Johnson noise of a 50-ohm resistor at 20 C with 5 Kc/s bandwidth is
> >> > -130 dBm. If we take the rail-to-rail output voltage of an op-amp to be
> >> > about +20 dBm, that gives 150 dB difference. Claims that a device is
> >> > capable of achieving this sort of figure in any practical situation is
> >> > stretching the limits of credibility.
> >> >
> >> > If the signal is lower than +20 dBm, you either have to start cooling
> >> > the device or reducing the measurement bandwidth, otherwise 150 dB S/N
> >> > ratio is impossible to achieve.
> >>
> >> Johnson noise is broadband. The harmonic of interest in evaluating a low
> >> distortion sine wave oscillator are rather narrow band.
> >
> >When you get down to signals as low as -150 dBm there will be Johnson
> >noise and intermodulation products contributed by the components within
> >the oscillator. Selecting individual harmonics with a narrow-band
> >filter will give an unrealistic number that doesn't represent the total
> >unwanted output.
> >
> >> You are comparing apples and pears as your "5 Kc.s bandwidth" makes
> >> clear. if you have been cribbing from a more modern source it would have
> >> been a 5kHz bandwidth.
> >
> >You have no idea what I did, so stop posting offensive nonsense.
> >
> >I have written a calculator based on a spreadsheet which gives the RMS
> >noise as a voltage and as dBm from user inputs of resistance, bandwidth
> >and temperature. I chose 5 Kc/s as a tolerably low figure for audio
> >work; it would be a suitable measurement bandwidth for a 1 Kc/s signal
> >if you wanted to include the third harmonic. I chose 50 ohms as it is
> >the lowest output impedance in common use - for audio work 600 ohms
> >might have been more appropriate but the figures would have been even
> >worse.
> >
> >I have always written "Kc/s" and "Mc/s" because I regard them as more
> >intuitive than having to remember who the units have been named after.
> >I presume you didn't notice this in all my previous posts.
> >
> >If someone wants to claim distortion products at or below the Johnson
> >noise, they need to justify those claims. Selecting a narrow-enough
> >bandwidth for the measurement could give any figure you like.
>
>
> You are all narrowing it down to dBm, and basically to audio only. DB
> (decibel) measures the relative difference between two power levels, while
> dBm (decibel milliwatt) indicates an absolute power level relative to 1
> milliwatt.
>
> If you have a 16 kV horizontal output transformer pulse as in an old BW
> tube TV as example then -160 DB is 16000 / 10^8 = 0.000160 volt, or ,16
> mV.
>
> It is all relative, [...]
I agree - but there was no indication that we were talking about signals
in the kilovolt region. I took the practical example of +20 dBm as
being the highest signal level normally encountered, which is 150dB
above the lowest Johnson noise normally encountered.
With exceptonal signal voltages and source resistances the 150 dB figure
can be bettered - but anyone making claims in that region needs to be
explicit about how the measurements were made, otherwise we could be
misled into assuming they were using the 'normal' criteria for such
measurements.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
