MEGA postamp
Using the MEGA postamp requires the use of the pin compatable uA733 as the amplifier instead of the LM592. This IC type, of the same vintage as the LM592, has several drawbacks compared to more recent ICs available. Because of the large phase shift through these devices, they can't use feedback to improve linearity and reduce distortion. Their output impedance is also much higher which increases succeptability to crosstalk and reduces their ability to drive capacitive loads. They have fairly loose tolerances for specs such as gain, which would lead to variations between channels in output because the thresholds and output widths are adjusted with common controls. Nevertheless, the modified MEGA postamps are useable as long as these factors are kept in mind.
CAMAC prototype postamp
The prototype was tried without changes first and found to produce double pulsing near fullscale input. The preamp output also appeared distorted by ringing. These things indicated that the amplifier section of the postamp had far too much gain leading to saturation effects. After changing the input termination arrangement and reducing the gain such that full scale input produced full scale input to the comparator the output looked considerably better, allthough the gain was still a little too high.
modified prototype with VTX preamp
Comparator input
Comparator output
Attenuation Amplitude
Noise
ratio
Width
Jitter
0 db
340 mv 25 mv pk-pk
22.7 db
40 nsec
1 nsec
- 10 db
260
25
20.3 db
34.6
1
- 20 db
- 30 db
40
16
8 db
16.3
2.5
From this it can be seen that,with some further adjustments, this circuit could be used with the VTX preamp. Linearity and distortion would probably be similar to the MEGA postamp. This circuit appears to use a 116 type ECL line receiver IC which is specified mainly for its use in generating ECL logic levels from logic driven twisted pair cables.
Modern linear line driver/receiver test
The Elantec EL2140 Differential Twisted Pair Driver IC was used as a receiver since it has the differential outputs required to drive the following comparator. This unit has a fixed gain of two which gives an effective gain of one per output polarity when used with the single ended input from the VTX preamp. The EL2141 is an adjustable gain version but the bandwith is reduced too much for this application when higher gains are set. The waveforms at the comparator input follow the input very well and show no wrinkles when the comparator fires. This indicates that it might be worth trying some circuits using high speed opamps to make a receiver with more gain or finding a line driver IC that has higher bandwidth and adjustable gain. A circuit for a cross-coupled differential driver described in an Analog Devices application note and an article on broadband line drivers in Electronic Design magazine would be worth investigating. These circuits could be used as drivers near the detector for twisted pair cabling or receivers in the postamp for single ended cabling if differential drive for the following comparator is desired.
List of Figures
Fig. 1.
VTX to prototype preamp out
0 db envelope mode
Fig. 2.
"
" , comparator
in
0 db average "
Fig. 3.
"
"
"
- 17 db envelope
Fig. 4.
"
"
- 30 db "
Fig. 5.
VTX to modified prototype
0 db "
Fig. 6.
"
- 10 db "
Fig. 7.
"
- 30 db "
Fig. 8.
prototype circuit digaram and modification
Fig. 9.
VTX to MEGA with EL2140
input overdrive +
5 db average
Fig. 10.
"
0 db "
Fig. 11.
"
- 30 db "
Fig. 12.
"
preamp out
- 30 db envelope
Fig. 13.
"
comparator in
- 30 db "
Fig. 14.
Analog Devices app. note
Fig. 15.
"
Fig. 16.
"
Fig. 17
Electronic Design magazine broadband line driver excerpt
Data Sheets:
Outline
Data Sheet 1 High Speed
Op Amps
Data Sheet 2 Applications
of High Speed Op Amps
The above require the Adobe Acrobat Reader, Available free at http://www.adobe.com/prodindex/acrobat/readstep.html
