[eagle] Re: Jim's comments

Juan Rivera juan-rivera at sbcglobal.net
Tue Jul 17 21:05:48 PDT 2007


Jim and gang,

 

I have a bit of new information for you regarding the CAN-Do step-down
converter:

 

The Maxim 1836 step-down converter is designed to deliver output currents of
up to 125 milliamps, but in this circuit it is only supplying 11 mills.  The
spec sheet shows that the efficiency and the switching frequency both drop
off badly at that low output current.  It's pretty clear that this IC is not
the ideal choice for this very small load.  I can think of several
alternatives:

 

1)       Replace the noisy switching downconverter with a linear regulator
and remove all traces of generated EMI

2)      Find another more suitable step-down converter and design a new
circuit and a new PCB

3)      At least for the 70 cm Receiver, disconnect the CAN-Do step-down
converter and power the CAN-Do module from the +5 volt receiver supply. It
shouldn't even notice another 11 mills.

 

Let's assume for the moment that we end up with a two-compartment enclosure.
What if we just disable the CAN-Do module's step-down converter and create a
small PCB that would attach to the 40-pin header and be the home for the
power supplies for whatever was in the other side of the enclosure - in this
case the receiver analog circuitry.  The CAN-Do module would get its power
from that little board and so would the receiver.  In the case of this
receiver we could put all the power supplies on a PCB about the size of a
large postage stamp.

 

Don, KD6IRE amplifies on my idea and suggests that the digital compartment
be placed off to the side - just wide enough to allow the existing CAN-Do
module to fit.  The main compartment would then be an 'L' shape, but with
access to the front panel.  All RF and IF I/O from the rear compartment
would come up the leg on the "L", with SMA connectors all located on one
side of the CAN-Do D connector.

 

Again speaking only from the perspective of the 70 cm receiver, I think this
would require the least amount of redesign and result in a pretty clean
passband.  The benefits would be:

 

1)       No major modifications to the CAN-Do module required (same PCB and
same connector)

2)      No connectors out the side or the top creating wiring headaches

3)      No more 5 kHz radiated or conducted CAN-Do EMI inside the enclosure
or propagated back up the DC power lines (5 kHz spurs completely gone)

4)      Moves the 157 kHz receiver switching power supply to the separate
compartment in the front on that little daughter board and gets rid of that
spur in the passband

5)      Minor modifications to the enclosure - just another internal sheet
metal piece

6)      The Receiver PCB form factor could remain the same or it could
change to the "L" shape, depending on what works best.

 

This still leaves the issue of flex unresolved and this suggestion might not
work for every project but it seems like it might be worth trying.

 

73,

 

Juan

 

P.S.  Maxim 1836 link -->
http://datasheets.maxim-ic.com/en/ds/MAX1836-MAX1837.pdf

 

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