Subject: Re: Hi Chris !!
From: Christopher Orth (corth@budoe.bu.edu)
Date: Wed Jan 05 2000 - 10:14:42 EST
Ciao Massimo
I looked at the waveform plots, and I think it might be the
well-known fanout-WFD noise. For some reason the little donut might not
be doing its job anymore. It looks like the noise is at about 5kHz, and
based on my memory this is where the old problem was. (I am digging
through my notes to verify this now, but thought I'd email you this now
while it is still wednesday.) If this is true, the best thing is probably
changing the fanout module. But if you are unwilling to try this, or it
doesn't work, read on...
Do you have a circuit diagram for the WFD daughter card? I may
have a postscript somewhere.
The failure mode you describe is new - I have not heard of this
happening. But it sounds like it could be simply a poorly seated daughter
card (for example if the output of the daughter card is not making a good
connection then the ADC value will float to midrange).
I would check that.
The actualy pedestal is controlled by a resister (not a trim pot,
sorry), my notes show that this resister is R622. Together with R623
these two resisters form a voltage divider that provides a bias between
-2.5V and +2.5V for the final stage amplifier (U7 pin 5; pin 6 is the
other input and pin 7 is the output. pins 4 and 11 are +/- 6V supplies
respectively).
Another likely cause to the problem is the through-hole diode
at D600. This is used like a zener diode switch to only perform the final
stage amplification below some threshold. Check the solder connections of
this guy.
If this fails to find the problem:
There should (I hope) still be some daughter card spares. These
have been calibrated, though this was some time ago, and they were
calibrated in other WFD Boards. Still, they should be pretty close to
correct (no worse than any other card in MACRO say). The WFD NIM pulser
can be used to test. The NIM pulser produces pairs of almost square
pulses of alternating amplitude. One output (the top I believe) is the
signal; the other ouput is a sync output, synced to either the smaller or
larger amplitude pulse; this depends on the position of the switch. In
any case the sync is not used to test the WFD calibration. The values of
the signal amplitudes are selectable by the dial. If they aren't
labelled, they come in flavors of (yum!):
first setting 2.00 V, 8.00 V
1.00 V, 5.00 V
0.20 V, 0.80 V
0.10 V, 0.50 V
0.02 V, 0.08 V
last setting 0.01 V, 0.05 V
Usually we only check the first and second settings (1V-8V).
Any documentation that exists would be in the BOSTONPUB area or on the WEB
at http://budoe.bu.edu/~macro
Buona fortuna,
Chris
On Wed, 5 Jan 2000, ORSINI MASSIMO wrote:
> Ciao Chris,
>
> during last week we got some pedestal problem on SM1 channel 19 after having
> replaced the original WFD card which actually showed an oscillating pedestal
> on channel 18.....if you want,you can take a look at a couple of WFD plots on
> the Macro WEB page (hardware informations.wfd_plots.ped_var).
> Now the pedestal problem is a positive offset which appears sometime only and
> we would know how we can adjust it:
> Can we set it via software only ??
> Can we eventually adjust it by using some trimpot ?
> In case we'd fix it, where can we find the hardware procedure ?
> I suppose we should use the NIM WFD pulser but I did it only once a couple of
> years ago and now I don't remember it.
> Do you have any Web page about it ?
> Thank you.
>
> A presto,
>
> Massimo Orsini
>
> P.S.- Have a fantastic millenium without bugs !!!
>
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