[eagle] Re: CAN-Do-Too! ??????????
juan-rivera at sbcglobal.net
Mon Jul 9 20:21:11 PDT 2007
I concur completely with Bill,
I beg you, don't open that MIL spec! Abandon all hope ye who enter there!
There are actually two specs. One is how to test and the other is what the
government wants to see for EMI shielding on tanks, ships, aircraft, etc.
They're both impractical for our purposes.
What is does offer is a method of describing EMI so we can all discuss it.
They break it down into 4 categories':
I agree with Bill. Guessing about something as universal as the quality of
the primary power is not a good way to start. We need to characterize
"noise" from the perspective of the power distribution point. The amplitude
and spectral distribution of the conducted EMI coming out of there is a
critical item. The same goes for the switched power from the CAN-Do module.
Those two sources of noise will impact everything on the spacecraft.
That leaves radiated emissions and susceptibility to nail down. This is
where it impacts the module chassis directly. Can we get by with one
compartment with digital and analog circuitry together? This is generally
considered a very bad design practice. But we'll only be able to tell after
testing, and then it may only apply to the 70 cm Receiver so we have to be
careful. Some effort should be expended identifying other payloads that may
have EMI susceptibility issues - receivers, magnetometers, etc...
I think this is a good example of the phase, "measure twice and cut once".
If the requirements are unrealistic you'll find out very late in the
integration phase when there is no time to fix anything.
Lastly, we need to be able to generate a realistic noisy power source for
testing. It needs to be capable of being duplicated so we can have multiple
units for each team to use. Once we have a realistic noise source then we
can come up with a series of tests to determine if the unit meets the specs.
From: eagle-bounces at amsat.org [mailto:eagle-bounces at amsat.org] On Behalf Of
Sent: Monday, July 09, 2007 7:34 PM
To: Jim Sanford
Cc: eagle at amsat.org
Subject: [eagle] Re: CAN-Do-Too! ??????????
Jim Sanford wrote:
> 4. EMI Spec: You put a gun to my head, and I'm going to pull out the
> MIL-STD, which is probably overkill, but MAYBE NOT. I'd really prefer
> that one of you guiys current in the INDUSTRIAL world take on this
> task and come up with something good enough and reasonable. In
> general, I'd like to eliminate as much mass and extra "touch" labor in
> the assembly process (multiple shields) as possible. Bob Davis:
> Please weigh in here, if the possible milled modules simplify this
> issue, please enlighten us.
Jim, I think an EMI spec pulled out of a MIL-STD would be an over kill
effort and likely not representative of "our" real world. What we need
is not assumptions but data reflecting what the hardware does. The
problem is that we don't have the hardware - and that's my issue. The
power distribution circuits are what I call a "top level" subsystem
which feeds "everyone." Instead of working on the circuitry that will
run from it and trying to "imagine" what we can expect, let's focus on
breadboarding the key power distribution circuits and get some hard data
so we know what we're working with.
I'll again reiterate (as I have several times in the past) the same
thing for the satellite's 10 MHz reference. It's another top level
"subsystem" that will feed "all" LO's. Instead of guessing what we'll be
working with, let's get it defined too.
(I'm proposing we put that one to bed and use the time tested, well
defined, high stability, low noise HP (OK - Agilent) 10811 series of
reference standards used in most of their test instruments. If that
won't cut it - someone please tell me why!)
> 7. Requirements: Bob McGwier is correct, we really did start with a
> very top-level requirements document. It is not perfect (Bob has
> hated it from the word go), but can be found on EaglePedia under
> Functional Requirements. It is also in need of updating, after the
> October BoD decision. On my list to do. Like you, I have finite
> energy and time, but it is on my list. I think John did an EXCELLENT
> job of documenting the UHF Receiver requirements based on what he
> knew. The need for an EMI spec was not obvious, but is now, thanks to
> Juan's testing efforts and exceptional documentation. Lou:
> Functional requirements for power supplies? Bob Davis: Functional
> requirements for structure and thermal performance? etc. etc.....we
> have much to do, but I think worthwhile effort. By the way, I'm
> reading (in a few spare minutes here and there) an EXCELLENT book on
> requirements management. When I finish, look for a review of it on my
> project management page. I will also be providing suggestions on
> writing "good" requirements, based on that book and my recent
> experiences in the day job.
Yes, we have "top level requirements" for desired satellite functions,
operating modes and parameters and link budgets. But again, I think
we're sorely lacking in key "system" performance requirements based on
real data - - - which can only be obtained by building and testing
these key functions.
I'm not a power supply guy, but I'll offer to assist in building and
testing the circuits we need to characterize. Lou - point me in the
right direction - tell me what you need!
Regards...Bill - N6GHz
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