From: jl@glen--canyon.com   
      
   On Sat, 15 Nov 2025 09:23:55 +0000, 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.   
      
   Stop making sense! Just stop it.   
      
   >   
   >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.   
      
   I never liked Hz. There's nothing wrong with megacycles.   
      
   Remember uuF?   
      
   >   
   >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.   
      
   That -150 dB oscillator sim is just using Spice as a video game,  to   
   waste time for people without much to do.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)