From: jl@glen--canyon.com   
      
   On Sat, 15 Nov 2025 19:04:45 +0000, liz@poppyrecords.invalid.invalid   
   (Liz Tuddenham) wrote:   
      
   >Bill Sloman  wrote:   
   >   
   >> On 15/11/2025 8:23 pm, 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'm pretty confident that you don't have much of an  idea what you did   
   >> either.   
   >   
   >You will notice that I have moved your comment by one paragraph so that   
   >it now comes after my explanation of exactly what I did.  Your post   
   >appears to have been done purely to be offensive and obfscate the facts.   
   >   
   >Please stop behaving like a troll, you are in danger of driving useful   
   >contributors away from this group.   
      
   He's been doing that for years.   
      
      
   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)