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Re: oscillations



Hall D Electronics:

Hi All,

Just to answer Gerard's question:

> 2. We put on the new preamp and I asked Yves to connect as before. 
> Oscillations were observed. Here are the details of the connection: The 
> signal cable was ~3m, ERNI connector on one end, the other end broken out to 
> 100 (?) Ohm termination resistors on pair #1, 2-9, and 18-25. Pairs 10-17 
> were not terminated. (They are also open on the preamp board  end of the 
> cable, since no ASIC or termination resistor is there.) For one of the 
> channels (at a time, we looked at a few) the 100 Ohm resistor termination was 
> removed (Yves, is that right, or did we leave it?) and in it's place two

We left the 100 Ohm termination resistor in.

Cheers,
 	Yves





> RG-174 cables of probably unequal length, LEMO to clip leads, were connected. 
> Shield was tied to shield, one center conductor to one side of the signal 
> pair and the other center conductor to the other side of the signal pair. The 
> wire/clip lead lengths were on the order of 10-20 inches total, and nothing 
> was twisted, i.e., basically it was a circular loop of circumference 10-20 
> inches. These two RG-174 cables connected directly (through LEMO-BNC 
> adapters) to two channels of the scope, set to 50 Ohm termination.
>
> 3. I remarked that this may be a sub-optimal way of probing the signals. We 
> substituted my matching pad / transformer widget (see fig. 19 in 
> GlueX-doc-955-v1 - the center tap was _not_ grounded here) for the pair of 
> RG-174 cables with clip leads. No oscillations were observed. We moved on to 
> other stuff (attempts to tackle the EMI issue).
>
> I should also say, I have _never_ seen any evidence of instability of the new 
> preamp board under any reasonable operating conditions, so I don't think this 
> is something we need to focus much effort on.
>
> There is also the possibility that one or more of the LEMO connectors or 
> cables or adapters had a flaky ground connection. Certainly we found one or 
> two such things later, you'll not be surprised.
>
>> 
>> Yves - you may want to try turning off any fluorescent bulbs you have in
>> the lab for two reasons: the ballast transformers, if present, can be
>> very noisy; and the UV from the bulbs may cause extraneous pulses on the
>> straws through the gaps from the wrapping. I first observed this at BNL
>> (E850) where we had aluminized mylar straws. The UV from the bulbs
>> overhead, and which penetrated the straws through the gaps in the
>> wrapping, would knock electrons out of the aluminum surface inside the
>> straws and produce very noticeable pulses. 
>
> This is a very good point! At the moment I think the whole chamber is wrapped 
> in aluminum foil, that takes care of the light too. But care should be taken 
> not to have UV light contamination on the straws. (There is a "thick" 
> lexan(?) sheet over the top of the CDC prototype for mechanical protection, 
> maybe that was already enough to block the UV.)
>
> The surface work function on
>> your copperized Kapton straws is much higher so that this may not be a
>> problem for you. 
>
> FYI the metallization on the kapton straws is still aluminum, and I don't 
> think there's any plans to change that.
>
> Another interesting observation was that we could make
>> the inside of the straws glow blue (plasma) in the dark at higher
>> voltages, in the streamer regime. The blue color was perhaps because of
>> the Argon (I believe we used Ar-ethane 80/20).
>> 
>> Thanks for the feedback. Regards,
>> Fernando
>