From: bill.sloman@ieee.org   
      
   On 16/11/2025 6:04 am, 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.   
      
   Your explanation is an exact explanation of what you thought you did.   
   The fact that you fixed on a specific bandwidth is clear evidence that   
   you didn't actually understand the problem you were tackling. This may   
   be an offensive observation, but it's hard to point out how somebody has   
   managed to obfscate a problem without hurting their feeling.   
      
   > Please stop behaving like a troll, you are in danger of driving useful   
   > contributors away from this group.   
      
   We've always had plenty of trolls. The survivors are pretty   
   thick-skinned. The more notorious trolls have been prone to describing   
   people who disagreed with them as trolls. Cursitor Doom is perhaps the   
   most obvious example   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)