Modifications of the KIAE postamps for use with VTX preamps

     Since the VTX preamp output can be as high as 400 millivolts, the gain of the amplifier stage of the existing KIAE prototype postamp must be reduced so that its output doesn't saturate in order for the output pulse width information to be meaningfull. The "116" type amplifier used approaches saturation near 400 mv, so in fact very little gain is possible. Even one stage has gain of about four so the input was attenuated by a factor of three. The only advantages  this stage provides are the polarity inversion it provides for the comparator input and the non-inverting output which can be used as a signal monitor (  this would give a signal of half the amplitude of the signal at the comparator input into a 50 ohm termination )..
     Some tests were done on such a modified channel with the following results.

1. the actual gain for the VTX preamp signal at an attenuation of 10 db was 1.33.

2. At signal attenuation setting of 0 db some signal distortion/compression ( 30 % compression ) was observed indicating more input attenuation may be desired.

3. The signal monitor output was compared to the comparator input and found to match closely when the factor of two is taken into account.
4. Since the VTX preamp output risetime is specified as 5 nsec ( 10 - 90% ) one would expect the walk using this leading edge discriminator to be around 5 nsec.
      0 db - 10 db     1.0 nsec
      0 db - 20 db     2.4 nsec
      0 db - 30 db     4.5 nsec

5. Output pulse widths and leading edge jitter.
        0 db           44.8 nsec   ( see fig. 5 )                     0.3 nsec  ( see fig. 6 )
      10 db           36.4 nsec   ( see fig. 7 )                     0.4 nsec  ( see fig. 8 )
      20 db           29.1 nsec   ( see fig. 9 )                     0.7 nsec  ( see fig. 10 )
      30 db           20.0 nsec   ( see fig. 11 )                   1.1 nsec  ( see fig  12 )

6. For new design, the " 116 " IC should be replaced by an amplifier which will give a gain of around six without output saturation to give more accurate pulse widths and lower jitter.

7. The comparator circuit seems to work well.
 
 

                            Modifications of the KIAE postamps for use with VTX preamps

                                   EL2140 test
     Some tests using an Elantec EL2140 line driver instead of the " 116 " IC were done for comparison. In this application with single ended input the gain for each output polarity is unity which is lower than desired, but it's signal handling range is much higher. The only advantages  this stage provides are the polarity inversion it provides for the comparator input and the non-inverting output which can be used as a signal monitor (  this would give a signal of half the amplitude of the signal at the comparator input into a 50 ohm termination ).
     Some tests were done on such a modified channel with the following results:

1. the actual gain for the VTX preamp signal at an attenuation of 10 db was 0.85.       ( see fig. 13 )

2. At attenuation setting of 0 db no signal distortion/compression was observed .      ( see fig. 14 )

3. The signal monitor output was compared to the comparator input and found to match closely when the factor of two is taken into account.     ( see fig. 15 )

4. Since the VTX preamp output risetime is specified as 5 nsec ( 10 - 90% ) one would expect the walk using this leading edge discriminator to be around 5 nsec.      ( see fig. 16 )
      0 db - 10 db     1.1 nsec
      0 db - 20 db     2.8 nsec
      0 db - 30 db     6.0 nsec

5. Output pulse widths and leading edge jitter.
        0 db           35.5 nsec   ( see fig. 17 )                     0.3 nsec  ( see fig. 18 )
      10 db           29.5 nsec   ( see fig. 19 )                     0.5 nsec  ( see fig. 20 )
      20 db           22.3 nsec   ( see fig. 21 )                     0.9 nsec  ( see fig. 22 )
      30 db           11.9 nsec   ( see fig. 23 )                     2.6 nsec  ( see fig  24 )

6. This circuit is not as good as the " 116 " modification, except for lower distortion at the higher amplitudes, due to low gain. For new design, an extra stage of EL2140 or a different circuit with more gain would give improved results.