[time-nuts] TAPR TICC boxed (input protection)
kb8tq at n1k.org
Tue Apr 11 09:18:42 EDT 2017
If you go back in the thread, it started out as a “general purpose front end” design. One of the
suggested parameters on that design was a high impedance input capability in the 1mega ohm range.
Noise on a hi-z input is always an issue and input protection just makes it worse.
About the only thing we have not dug into is the question of just how robust this or that protection
approach is. A setup that will withstand being plugged into an European 250V wall outlet for 24 hours
would likely be a bit more parts intensive than something that withstands the occasional exposure
to +12V …. This all may seem a bit “nutty”. It’s worth noting that the HP 5335 is not at all happy
if you drive it with a 5V square wave and set the input attenuator to zero db (= you probably blow out
Input protection does matter and getting it right is not a trivial thing. There will always be compromise.
> On Apr 11, 2017, at 8:22 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> I have a really naive question: how can picoamp leakage parts be relevant in low impedance input pulse conditioning to an interval counter?
> Tim N3QE
>> On Apr 11, 2017, at 7:46 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
>>> On Apr 11, 2017, at 7:05 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
>>> David wrote:
>>>> I ended up qualifying 2N3904s based on manufacturer and lot and I
>>>> think we ended up using ones from Motorola. I wish detailed process
>>>> information like National had was available from every manufacturer.
>>> It is, if you ask the process engineers for it. (From the Big Boys, that is. These days it seems discrete devices are being fabbed by dozens of "garage" operations. I can't speak for them, and wouldn't think of buying product from them.)
>> If you dig into where your simple discrete part was made, you might be surprised. That’s even true of outfits you would
>> consider to be a “Big Guy” from days gone by. The real answer to selection today is to buy the automotive part.
>> That’s about the only thing anymore that locks down the sourcing, testing, and the process. If you buy a “normal” part,
>> it might have been made anywhere by just about any process. Yes, that’s scary and it raises a lot of questions. It is
>> a change that has happened over the last decade or two without a lot of publicity.
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