Happy New Year All,
If one takes as a starting point the arguments in the Internal
Note
2002/070 then it certainly looks like a full-scale
of 10.5pC is no
problem in itself. Even taking the "VPT
gain chosen by eta range" in
Table 2, and considering
the low luminosity case where the loss factors
are not
present, the full-scale in terms of energy is not pushed down to
the 1.25TeV "limit".
But the x12 to x6 gain switch is really rather low. Less than
200 GeV.
I suspect that this also is not a real problem,
and that we will, within
the next couple of months find
completely adequate means of reconstructing
range-changed pulses. This is almost true already.
On the other hand, is it really worth pushing down the
full-scale energy
and the 12 to 6 range-change energy so
hard just to improve the noise
performance by a few
percent? What about Chinese crystals with 30 or 40%
more
light? Why not relax in the luxury of a full-scale of ~20pC even if
it means 10% (say) more electronics noise?
All the best, Chris.
On Tue, 21 Dec 2004, Brown, RM (Bob) - PPD wrote:
> Thanks Mark,
>
> 976e was the figure that I got for the FET noise with C(Tot) = 130
pF. It is
> somewhat larger than the figure I found
in one of your talks, for the same
> feedback
components, presumably you had used a smaller value for
> C(Tot) and/or V(FET.
>
> For the dynamic range, my preference is for 6.8pF/5.6k, giving a
full scale
> of 10.5pC, but I would like to fly this
by Chris Seez before making any
> irreversible
decisions, since he did the initial studies on this topic.
> According to Chris's study, the important issue is the energy
equivalence of
> the charge at which gain switching
from the x12 to the x6 range occurs,
> rather than
maximum energy with x1 gain.
>
> (I plan to take a few days off after Christmas - back on
10th)
>
> Bob.
>
> -----Original
Message-----
> From: Mark Raymond [mailto:m.raymond@imperial.ac.uk]
> Sent: 21 December 2004 10:42
> To: Brown, RM (Bob) - PPD; claire.shepherd-themistocleous@rl.ac.uk;
Lodge,
> AB (Tony); Bell, Ken;
matthew.ryan@imperial.ac.uk
> Cc: Cockerill, DJA
(David)
> Subject: RE: protection resistor
noise
>
>
> Hi Bob,
>
>
Sorry, I was away yesterday. I quoted a values for V(FET) of 0.25
> nV/[root(Hz)] in the note I attached a few days
ago, and a value for C(FET)
> of 60 pF.
>
> The noise I measured for the
endcap feedback components (8p2//4k7 at an
> added
input capacitance of ~50 pF) was ~3300e. For 40 ns shaping the
> feedback resistor should contribute 2236e, and the FET noise should
be 976e
> (for C(TOT) = 130pF), making a total
prediction of 2439e. This is somewhat
> less than the
measurement, but there are some factors which are overlooked
> in the simple calculation.For example, the pulse shaping is assumed
to be
> perfect, and no contributions from the MGPA
first stage other than the
> input FET are included,
and no contributions from subsequent stages
>
> From experience, achieving 100% agreement
between predicted (or
> calculated) and measured
noise is difficult. Measurements can be easily
>
affected by interference in the environment. I am confident that the lab
> measurements are fairly accurate though, especially
since the barrel values
> have been pretty much
confirmed in the supermodule test beam system.
>
> I think the decision on endcap feedback components
should be based on
> dynamic range. The choice is
probably between 8p2//4k7 or 6p8//5k6. I am
> happy
with either. The noise difference will be small, and I believe both
> will meet the specification.
>
> I will get the VFE boards I have
modified soon after Christmas, so we
> should
probably decide by the end of the first working week in January (the
> 7th).
>
> Cheers, Mark.
>
>
>
> At 17:20
17/12/2004 +0000, Brown, RM (Bob) - PPD wrote:
> >Mark, thanks for this explanation.
>
>
> >I am slowly getting my head round the
various noise contributions. I
> >can more
or less reproduce the estimated figures that you give in
> >various talks etc, but I find some differences when I try and
calculate
> >the FET contribution.
> >
> >What value do you
assume for V(FET)?
> >
> >I assume that I have to include C(FET) in C(Total), what value
do you
> >take for this?
> >
> >Bob.
> >
> >-----Original
Message-----
> >From: Mark Raymond [mailto:m.raymond@imperial.ac.uk]
> >Sent: 15 December 2004 16:47
> >To: Brown, RM (Bob) - PPD;
claire.shepherd-themistocleous@rl.ac.uk;
> >Lodge,
AB (Tony); Bell, Ken; matthew.ryan@imperial.ac.uk
>
>Cc: Cockerill, DJA (David)
> >Subject: RE:
protection resistor noise
> >
> >
> >Hi Bob,
> >
> >Decreasing the feedback
capacitor increases the gain for both signal
>
>and series (FET) noise so the ENC contribution from the FET is
> >unchanged. If Cf is reduced then Rf has
to be increased to maintain
> >the 40 ns time
constant. Increasing Rf leads to a reduced parallel
>
>noise contribution from this component.
>
>
> >Of course the final noise is a combination
of all contributions, so
> >another way of looking
at it is that as the Rf noise reduces, other
>
>components become more significant, but they don't actually increase.
> >The bottom line is that the bigger you can make
Rf, the better, but
> >this does come at the cost
of reduced dynamic range.
> >
> >The table you saw yesterday shows hand calculations of the
benefit of
> >increasing Rf. You can find it again
at:
> >
>
> http://www.hep.ph.ic.ac.uk/~dmray/pptfiles/EEisues.ppt
> >
> >The measured noise
for the 8p2//4k7 combination is also shown. The
>
>measured values are a bit larger than I estimated. It is possible that
> >there was some interference contribution in the
environment, although I
> >took some care.
> >
> >Anyway, if we assume
the noise is 3300e for Cf//Rf = 8p2//4k7, it
>
>should reduce to 3175e if we go to 6p8//5k6.
>
>
> >I don't think we should go any lower, as
we will become more sensitive
> >to stray
capacitance (in parallel with Cf) on the VFE card, which might
> >lead to channel to channel pulse shape variations (because Cf
> >determines the pulse shaping differentiation
time constant).
> >
>
>So, the noise benefit is rather small, but there is no point having
> >more dynamic range than you really need.
> >
> >Hope this
helps.
> >
>
>Cheers, Mark.
> >
>
>
> >At 15:19 15/12/2004 +0000, Brown, RM
(Bob) - PPD wrote:
> > >Mark, thanks for
this.
> > >
> >
>My current thinking is to reduce the dynamic range by decreasing C(f)
> > >to 6.8pF, giving full scale = 10.5 pC.
This is because the crystal
> > >and VPT will
'age' reducing the charge/MeV with time. At the start
> > >we can reduce the VPT bias to avoid saturation, then
increase the
> > >bias later to compensate for
aging.
> > >
> >
>However, I now realise that although decreasing the MGPA full scale
> > >decreases the R(f) noise, this improvement
is offset by an increase
> > >in the FET noise,
which starts to become important when C(f) becomes
>
> >small.
> > >
> > >Have I understood correctly? What is your estimate
for the total
> > >noise with
> > >C(f) = 6.8 pF and R(f) = 5.6k?
>
> >
> > >Bob.
> > >
> > >-----Original
Message-----
> > >From: Mark Raymond [mailto:m.raymond@imperial.ac.uk]
> > >Sent: 15 December 2004 13:20
> > >To: claire.shepherd-themistocleous@rl.ac.uk; Lodge, AB
(Tony); Bell,
> > >Ken;
matthew.ryan@imperial.ac.uk; Brown, RM (Bob) - PPD
>
> >Subject: protection resistor noise
> >
>
> > >
> >
>Hi,
> > >
> >
>I gave some consideration to the noise contribution of a possible
> > >protection resistor and am attaching a
note. I was too pessimistic
> > >about its
noise contribution as you can see, but it depends on where
> > >you put it. If it can go on the HV card, close to the VPT
then its
> > >contribution will be small. If it
goes on the motherboard and sees
> > >the
significant capacitance of the umbilical cable, then its
> > >contribution will be much larger.
> > >
> > >I assume this
accounts for Claire's measurements.
> >
>
> > >Cheers, Mark.
> > >
> >
>------------------------------------------------------------------
> > > Mark Raymond
> > > Blackett Laboratory
> > > Imperial
College phone: +20 7594 7799
> > > London SW7
2AZ fax: +20 7823 8830
> >
>------------------------------------------------------------------
> >
>
>------------------------------------------------------------------
> > Mark Raymond
>
> Blackett Laboratory
>
> Imperial College phone: +20
7594 7799
> > London SW7
2AZ fax: +20 7823 8830
>
>------------------------------------------------------------------
>
>
------------------------------------------------------------------
> Mark Raymond
> Blackett Laboratory
> Imperial College
phone: +20 7594 7799
> London SW7
2AZ fax: +20 7823 8830
>
------------------------------------------------------------------
>