[eagle] Re: CAN-Do EMI - Let's Get Going on This!
w5did at amsat.org
Fri Jun 29 19:36:07 PDT 2007
Why not think out of the box, Put the Can Do module outside the box,
perhaps on top of the module.
I think that is a more likely solution than re building the Can-do
w5did at mac.com
On Jun 29, 2007, at 10:25 PM, Juan Rivera wrote:
> I'm sorry to pick on you, but you seem like a good guy to complain
> I seem to be having a problem stimulating a discussion. After
> saying that I
> though the CAN-Do power supply needed to be completely scrapped and
> with one running at around 1 MHz I expected to be buried in email,
> but I've
> only received one message referring to this so far, and it wasn't from
> anyone working on CAN-Do.
> The CAN-Do module is unique in that it is going to be an integral
> INSIDE of every payload, so any deficiencies it may have are going
> to have a
> large impact.
> Let me restate my finding so far:
> There are 4 categories of EMI and the CAN-Do module / 70 cm Receiver
> combination is experiencing all four. They break down into
> radiated and
> conducted emissions and susceptibility, and they are generally
> referred to
> by a two-letter designation - RE, RS, CE, and CS.
> RE and RS go together - the CAN-Do module's switching power supply
> radiates the 5 kHz switching noise out the back directly towards the
> receiver (RE.) The Receiver's VCO's are both very sensitive to EMI
> and are
> impacted by the CAN-Do module if they are within 4-1/2 inches of the
> inductor (RS.) I've had to move the CAN-Do module off of the
> receiver PCB
> and interconnect it with a ribbon cable to deal with this problem.
> The good
> news is that I am fairly confident that It can be fixed by going to a
> two-compartment chassis, with a bulkhead separating the CAN-Do
> module from
> the analog Receiver to provide shielding. The bad news is that I
> think this
> means we need a milled module chassis.
> CE and CS also go together, and this is the real problem I see
> since you
> can't fix conducted EMI with a shielded enclosure. It requires
> The CAN-Do module is trashing the DC input from the power source
> and also
> feeding noisy power to the Receiver. The outgoing noise is the bigger
> concern because it will add to the CS problems for everything
> connected to
> the power source. In the other direction, the switched power from the
> CAN-Do module shows up in the IF output as 5 kHz spurs. Moving the
> module physically away from the Receiver only dealt with the RE/RS
> issue. I
> had to bypass the CAN-Do module and run clean power directly from
> the lab
> bench supply to deal with the CE/CS problem. This means that there
> is no
> current monitoring and no power control.
> The 5 kHz switching frequency is bad for two reasons - it makes
> this noise a much larger problem than it needs to be, and the
> impact is more
> severe since it is putting spurs all over the passband of the IF at
> 5 kHz
> If you sit back and think about the impact of a dozen noisy power
> all feeding EMI back to the common power source where they all mix
> and make their way back to each payload, it starts to look nasty.
> All these
> supplies will be drifting around and beating with each other to
> produce sum
> and difference noise on the power bus. 5 kHz noise is hard enough
> to get
> rid of but what if there are difference components at a few hundred
> Hz? How
> can you design a filter when you won't know what to expect until
> you hook
> everything up and turn it on? And by then you're out of time.
> I'm not making this stuff up. People I know have run into this exact
> problem before and the result was very bad.
> A while ago Howard Long made a suggestion that I think has great
> Here's what he had to say:
> ...in the original SDX PSU design I had in San Francisco last year
> is an
> SMPS using the LM2672 device. These can be fitted with an AC
> coupled SYNC
> signal to override the internal default SMPS frequency. I selected
> for my unit (6MHz divided by 16) to ensure its harmonics were
> outside the
> 10.7MHz IF passband. If the external SYNC fails the internal SMPS
> takes over.
> My Suggestions:
> 1) Revise the CAN-Do module to move the switching frequency up as
> far as
> possible to move spurs out of the passband of sensitive analog
> and to ease the burden on EMI filtering.
> 2) The power and grounds must be filtered in both directions to
> minimize CE
> back to the power source and to the payload.
> 3) The switching inductor should be a shielded to reduce RE inside the
> module chassis.
> 4) A power supply capable of synching to a master oscillator should be
> strongly considered.
> This topic needs to be elevated to the top of the queue. The EMI
> environment surrounding the CAN-Do module impacts the design of the
> revision of the 70 cm Receiver, and also directly impacts the chassis
> design. What do we need to do to get going on this?
> Juan - WA6HTP
> Via the Eagle mailing list courtesy of AMSAT-NA
> Eagle at amsat.org
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