[eagle] Re: CAN-Do EMI - Let's Get Going on This!

[Louis McFadin]

Juan,
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  
module.

Lou McFadin
W5DID
w5did at mac.com


On Jun 29, 2007, at 10:25 PM, Juan Rivera wrote:

> Bdale,
>
> I'm sorry to pick on you, but you seem like a good guy to complain  
> to...
>
> 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  
> replaced
> 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  
> component
> 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  
> inductor
> 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  
> filtering.
> 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  
> CAN-Do
> 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  
> filtering
> 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
> intervals.
>
> If you sit back and think about the impact of a dozen noisy power  
> supplies
> all feeding EMI back to the common power source where they all mix  
> together
> 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  
> potential.
> 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  
> 375kHz
> 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  
> oscillator
> 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  
> circuitry,
> 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  
> next
> revision of the 70 cm Receiver, and also directly impacts the chassis
> design.  What do we need to do to get going on this?
>
> 73,
>
> Juan - WA6HTP
>
>
>
>
>
> _______________________________________________
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