VTX versus MEGA preamp comparison

1. The VTX preamp does shaping whereas the MEGA preamp does not.
2. The MEGA preamp output is differential whereas the VTX preamp output is single ended.
3. The VTX preamp has four times the gain of the MEGA preamp.

 

       The most usefull way to compare them would be to compare the outputs of a postamp/comparator after it is optimised for use with each preamp. Pulse width and rising edge jitter would be compared over a wide signal input range. Since the output pulse width can be used as an indicator of input pulse width, the postamp needs to operate in a linear region and not shape the pulse more than the VTX preamp already does.
        To accomplish this, it was decided to modify the MEGA postamp/comparator which was  available and also under consideration for use in the experiment
         Initial testing showed that the IC used as the postamp has very nonlinear response when operated at the lower gain required, even without the shaping R-C networks, which leads to reducing the input pulse to about half it's width. This behavior is documented in the data sheets for this part, the LM592. To solve this problem a pin-compatable IC, the uA733/LM733, was used instead.
         Again, comparing signal to noise ratios after the optimised postamp at the input to the comparator is a usefull indicator of how things are going. It is worth noting that the input to this comparator is differential and that the postamp has converted the single ended VTX signal to a differential one at this point.
 

 VTX        Comparator Input -  signal to pk-pk noise ratio (+ input)        MEGA

Attenuation	Amplitude (mV)	Noise (mV)	ratio	Amplitude	Noise	ratio
0 db	1,500	50	29.5 db	920	50	25.2 db
-10 db	560	25	27 db
-20 db	175	25	17 db	90	40	7 db
-30 db	56	26	6.4 db	32	42	-2.4 db

 
 

         This leads to the following comparator outputs.
 

VTX        Comparator Output -  Pulse Width and Rising Edge Jitter ( nsec. )        MEGA

Attenuation	Width	Jitter	Width	Jitter	comment
0 db	40	<1	~56	2	(14 falling edge)
-10 db	30.8	1
-20 db	22	1	24	4	(double pulsing)
-30 db	11	2	13	-	(double pulsing)

 

           Some of the double pulsing on the MEGA preamp setup is due to a rising tail on the comparator input indicating that the shaping is not ideal.

                                                                                 Conclusion

 
      From these tests it appears that the VTX preamp is a clear winner, mainly due to much lower noise and higher output which requires less postamplification. The only disadvantage would be if the single ended drive leads to more noise pickup over the cable lengths required. This problem could be overcome by adding a differential twisted pair driver IC, such as the Elantec EL2140, near the detector end and doubling the post amp gain to make up for loss in the cable terminations.
 
 
 

                                                 List of Figures

    fig.   1  VTX preamp out                       0 db             envelope mode
    fig.   2               "      "           "           - 10 db                        "
    fig.   3               "      "           "           - 30 db                        "
    fig.   4        MEGA   "          "                0 db                        "
    fig.   5             "         "          "           - 10 db                        "
    fig.   6             "         "          "           - 20 db                        "
    fig.   7             "         "          "           - 30 db                        "
    fig.   8         NE592   frequency response data sheet
    fig.   9         uA733          "                  "           "       "
    fig. 10        MEGA postamp  modifications
    fig. 11        VTX preamp to MEGA postamp          0 db    average mode
    fig. 12                                 "                                         0 db     envelope mode
    fig. 13                                 "                                      + 5 db     preamp overdrive
    fig. 14                                 "                                      - 10 db    average
    fig. 15                                 "                                      - 10 db    envelope
    fig. 16                                 "                                      - 20 db          "
    fig. 17                                 "                                      - 30 db          "
    fig. 18        MEGA preamp to MEGA postamp      0 db           "
    fig. 19                                 "                                      - 20 db           "
    fig. 20                                 "                                      - 30 db     average
    fig. 21                                 "                                      - 30 db     envelope