Lou McFadin

W5DID

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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