10. Diode Bridge Part Two

Shows first measurements of the working bridge.


Bridge Input Grounded.



Bridge Output - 7A11 amplifier shows spikes, 7A11 works with a 100 to 150 MHz bandwidth and approximately 6pF probe capacitance for all ranges. In this measurement the sampling window signal has an 43 ohms series resistor in it's output, this slows the slope.





   
Reduced 7A11 bandwidth to 20 MHz (probe without alligator crimp)        Reduced 7A26 bandwidth to 20 MHz (measured with a coaxial cable)

The bridge output should not be loaded with too much capacitance - reduced measurement bandwidth decrease spike height - and decrease also measurement speed.

I'am disappointed, the spike will be too high after amplifying.

  • How many millivolts had your feedthrough spike on the bridge output (residue amplifier input) with an grounded bridge input?
  • Which bandwidth had the test equipment?



Measurements with the Residue Amplifier


Bridge Output Residue Amplifier.



  
Residue Amplifier xxxx not a good choice for high signal frequencies with Av=+40.
Peak 13mV  ( measured with 7A11 20MHz BW). 

Left edge of the sampling window starts with a slowly slewing slope - thermal effects?




 
Residue Amplifier xxxx - this faster CFA has more bandwidth, Peak 150mV.
Rf=1k3 is too high. ( measured with 7A11 20MHz BW)




 
Residue Amplifier xxxx - fast CFA, Peak 50mV. Rf=1k3 should be readjusted for Av=+40 gain, 2   (measured with 7A11 20MHz BW). Under the 10mV/DIV deflection good visible again the slowly rising slope - seems to be a problem of my settling time instrument.




Residue Amplifier xxxx - fast CFA, Peak 45mV.
Rf=1k3 should be readjusted for Av=+40 gain,
( measured with 7A11 20MHz BW)




Removing the short circuit on the bridge input, replaced with a BNC terminal - ready for applying an input signal.


Sine Wave on Bridge Input



1mVpp sine-wave applied on the bridge input. Scope triggers with the pulsgenerator, sine wave is free-running from a noisy source.  Camera opens the aperture for many milliseconds. Brighter scope CRT intensisty shows an 80mV peak, higher than in the former xxxx photo.



Using the xxxx Residue Amplifier


 
The high bandwidth xxxx Residue Amplifier has enough gain under high frequencies. Peak 150mV - too high for me.



Replacing the AC-Line Power Supply with three (12V and 6V) lead-acid battery results in - no improvement - this seem not to be the problem, the spike looks always the same.





Every residue amplifier shows that slow rising sloop. It seems to be from the settling time circuit. Approximately an unwanted 50µV error.




  
Applying the same sine wave signal on different residue amplifer. (Vertical 250µV/DIV)

The xxxx already limits the bandwidth, after switching the bridge OFF this amplifier can't follow fast. The xxxx does everything faster.

Note: in the xxxx measurement, the oscilloscope amplifier is already overdriven, the rising OFF spike reaches about 150mV - the oscilloscope is about three times overdriven.


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