From: liz@poppyrecords.invalid.invalid   
      
   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.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
   --- SoupGate-Win32 v1.05   
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