GENERAL MEASUREMENT: 
  
  
   PHYSICS -->| 
              |->SAMPLE -->| 
  unwanted -->|            |->ELECTRONICS ->| 
  physics        unwanted->|                |->DATA--> CAKE
                 signals        unwanted--->|
                                data



  
  Physics that leads to cake {data}  =  signal

  Unwanted  {physics+signals+data}   =  noise + interference

                                          
                              signal      
Figure of merit:        ------------------
                          noise+interference 
                                             (frequency)




----------------------------------------------------------------------
           NOISE & INTERFERENCE IN A MEASUREMENT 1      
           ---------------------------------------


                      NOISE:

  Looks like  :       random-like "continuous" spectrum
  Origin      :       nature-made
  Lowest level:       set by physics and electronics


                      INTERFERENCE:

  Looks like  :       specific spectral components
  Origin      :       man-made
  Lowest level:       zero



  GOOD SETUP :      interference <  noise-floor 

----------------------------------------------------------------------
           NOISE & INTERFERENCE IN A MEASUREMENT 2
           -------------------------------------


  1- CHECK FOR INTERFERENCE: Use analog scope,
                             trigger mode = normal
                             (no trigger = no light)
                             turn trigger level knob
                             see a repetitive signal ?
                             (vary timebase)
                             --> Interference


  2- CHECK FOR NOISE:    a  calculate total RMS noise
                            from sample + electronics
                            for a given bandwidth
                            
                         b  use a filter to limit the
                            bandwidth, connect to scope
                            estimate top-top value,
                            RMS-value = (peak-peak)/6

                         c  If b < 2*a then setup = ok

----------------------------------------------------------------------
      SIGNAL BANDWIDTH versus MEASUREMENT BANDWIDTH     
      ---------------------------------------------

  Example:  Ibias -> Rsample -> wires -> Voltage amplifier

  some values:   Cwires : 300 pF
                 Rsample: 100 kohm
                 Rnoise : 50  nV/sqrtHz

  Signal bandwidth      : dc..5 kHz

  However: 
  As the output signal drops >5kHz also the noise level drops
  The usefull MEASUREMENT bandwidth can also be larger.

  example extending the signal bandwidth:
  Using a dedicated amplifier design with a
  differentiating response from 5kHz up to
  5MHz:  (ampnoise = 0.5 nV/sqrtHz)

  Signal bandwidth      : dc..5MHz

         noise <500kHz  : 50   nV/sqrtHz
         noise  1MHz    : 100  nV/sqrtHz
         noise  5MHz    : 500  nV/sqrtHz


----------------------------------------------------------------------
                    CONCLUSIONS
                  --------------

  -Only the ratio between the signal and the combined
   noise and interference in the used bandwidth should
   be considered, not the absolute levels.

  -Noise should be discriminated from interference
   to be able to improve and qualify the measurement.

  -Noise is an instrumentation and sample problem.
   Interference is a construction and environment
   problem.

  -A wellbuild measurement setup is limited by noise

  -Signal bandwidth is an overrated parameter as it
   has no direct relation to the measurement bandwidth.