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Re: scope traces for new preamp



Hi Simon, Mitch and Gerard,

I just have a few comments and clarifications on the pictures that I and 
Simon took on Friday, which Simon posted on the link. Note that the 
preamp does not have any external pull ups - just the internal ASIC 
resistors to +1.25V.

1. Pictures 0-22 and a23 are anode pictures with 18 m of cable 
terminated into 100 Ohm and measured with two scope probes in 
differential mode (math ch1-ch2). The scales are 200 mV and 40 ns. The 
signals look nice and clean and have a rise time of the order of 16 to 
20 ns. Saturation is seldom observed.

2. Pictures 23-32 and c32 are for cathodes, as in 1 above. The scales 
are 100 ns and 40 ns. The signals also look nice and clean. The lower 
amplitudes will require a gain of 4 - 5 X higher to bring their 
amplitudes up to values comparable to the anodes. This is what we would 
expect based on the FDC geometry. The rise times are similar to those of 
the anodes (16 - 20 ns). Saturation is never observed.

3. Pictures 32-42 and c5s43 are for cathodes with 18 m of cable and fed 
into a shaper board with 5X gain. The scales are 500 mV and 40 ns. The 
pulses look reasonably good with high amplitude but with a slight 
reflection (maybe not 100 Ohm termination). Some saturation is observed. 
These pictures should be compared with those in 2 above.

4. Pictures 43-50 and a1s51 are for anodes with 18 m cable and fed into 
shaper with 1X gain. The scales are 500 mV and 40 ns. There is double 
pulsing, as a result of perhaps non-optimized shaping on the shaper 
board. Saturation is very seldom observed.

5. Pictures 51-54 and c1s55 are for cathodes with 18 m of cable and fed 
into a shaper with a 1X gain. The scales are 200 mV and 40 ns. There is 
double pulsing, as a result of perhaps non-optimized shaping as in 4 
above. Saturation is very seldom observed. Comparing to 3 above, and 
disregarding the shaping problem, clearly there is too much gain with 
the 5X shaper and preamp combination. Note that the preamp has a much 
higher gain than simulated (3.7 mV/fC vs. 2 mV/fC) but that may be ok if 
the shaper gain is lowered.

6. Pictures 55- 62 and a5s63 are for anodes with 18 m of cable and fed 
into a shaper with 5X gain. The scales are 500 mV and 40 ns. Saturation 
occurs very often but there is no double pulsing. The shaping constant 
seems ok.

Tests 5 and 6 were performed as checks, only. The results show that the 
gain (preamp and shaper combination) need to be set lower. The shaping 
constants on the shaper board with the 1X gain (for anodes) need to be 
checked. It may be opportune to revise the shaping constants for both 
boards, though.

Overall, I think we are in good shape.

Gerard, send me a schematic, layout and tell me the values for the 
components we need to change and we'll re-test.

Regards,
Fernando



Simon Taylor wrote:
> Hello, everyone.
>
> I've posted a set of scope traces for an anode wire and a cathode strip
> from the FDC prototype in the FDC electronic logbook, accessible via
>
> http://www.jlab.org/htbin/PHYSICS/halld/enote112.pl?nb=notebook&action=view&page=352
>
> Simon
>