[eagle] Re: Jim's comments

Jim Sanford wb4gcs at amsat.org
Wed Jul 18 17:33:50 PDT 2007


All:
I'm not going to say much on this, because I want the discussion to 
flow, and then for John to make recommendations that we can peer reivew 
re the redesign.

I will say this:
Juan makes an interesting proposal, which will solve much of the EMI 
issue for weak signal receivers.  I still think we need to reduce, as 
much as reasonably possible, the noise generated, radiated, and 
conducted by the CAN-Do! widget under all postulated uses in Eagle.  
There are many reasons why this is just a good idea.

73,
Jim
wb4gcs at amsat.org

Juan Rivera wrote:

> 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 à 
> http://datasheets.maxim-ic.com/en/ds/MAX1836-MAX1837.pdf
>
>  
>
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