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 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
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
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
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
4) A power supply capable of synching to a master
oscillator should be
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?
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