[eagle] Re: Preliminary CAN-Do EMI Findings and Suggestions to Mitigate in the 70 cm Receiver

Juan Rivera juan-rivera at sbcglobal.net
Fri Jun 15 08:26:44 PDT 2007


Chuck,

I actually think that the noisy power coming out of the CAN-Do module is
perfect for these tests.  The more I think about it the more delighted I am
that its there.  I have no other way to generate noisy power.  This is
great!  I'll attempt to capture this noise with my oscilloscope and post it
in my log if I can.  My scope is not feeling well these days so I may need
to dig up another one.

Even if the receiver eventually receives its power from another source, we
need to assume that it will have switching noise on it from other payloads.
That's the only reasonable assumption as far as I'm concerned.  The receiver
must be able to function in the presence of noisy power and the CAN-Do
module is a perfect noise source.

Based on the above I think I'll leave the CAN-Do module alone for now and
see if I can add some simple filtering to kill the spurs in the IF while
running on the switched CAN-Do power.  If it can do that then I think we
will be on the right track.

Every other payload designer should take heed and plan to operate with noise
on their DC inputs too.  This is probably a topic that needs some high-level
attention.  Are there any power specs that describe maximum expected noise
levels?  Perhaps a standard DC input filer should be designed and
incorporated in every power supply input.  Why leave this up to each
designer?  Come up with a standard filter using SMD components.  I would
think that this filter would also work to reduce switching noise from the
payloads and help prevent that noise from propagating back out to other
payloads on the power bus.  All this looks like an area that needs
attention.

I'm open to advice here.  All I was thinking of doing was adding about 10 uF
across pins 39/40 to ground, an RF choke in series, and then another 10 uF
cap and maybe a second smaller value to ground at the load end of that RF
choke.  I can cobble something like that together and stick it on the PCB in
dead bug style.  I can keep adding stages until I get rid of the noise.

73,

Juan
WA6HTP

-----Original Message-----
From: Chuck Green [mailto:greencl at mindspring.com] 
Sent: Friday, June 15, 2007 7:53 AM
To: juan-rivera at sbcglobal.net
Cc: eagle at amsat.org; David Smith; Dave Black (Work); Dave Black (Home);
Samsonoff at Mac. Com; Juan.Rivera (Work)
Subject: Re: [eagle] Preliminary CAN-Do EMI Findings and Suggestions to
Mitigate in the 70 cm Receiver

Juan,

I have a couple of suggestions.

Based on previous messages I presume you will not want to use the 
switched power from the CAN-Do! regardless of the noise issue. I also 
see that the switch transistor in the CAN-Do! is in the same area of the 
PCB as the CAN-Do! power supply.

1) Put a jumper between pins 2 and 3 of the switch transistor (TO-220). 
This will bypass the switch and any noise it is picking up from the 
power supply. This will keep the 2 amp fuse in your power source and the 
current measurement circuit.

2) You might also eliminate the 2 amp fuse from your power source by 
placing a jumper from pins 1,9 of the DA15P to pin 2 of the switch 
transistor after removing the transistor (or just cutting it's leads). 
This will preserve the current measurement of the power going to your 
module.

3) You might also bypass everything by putting a jumper from pins 1,9 of 
the DA15P to any unused user pins on that same connector (pins 
2,3,5,10,11,12) then picking up the power from the corresponding user 
pins on the 40 pin connector. If you do this, you should provide a 
current measurement circuit on your module and attach it's output to one 
of the analog input pins of the 40 pin connector.

The above may eliminate or reduce the noise in your power source while 
accomplishing other power source objectives.

Chuck

Juan Rivera wrote:
>
> All,
>
> I've completed a preliminary evaluation of the CAN-Do RFI problem. 
> Please see my log for details by clicking here 
>
<http://www.juanr.com/pages/hobbies/ham_radio/Eagle/CAN-Do_Noise_Troubleshoo
ting.htm>.
>
> Preliminary Findings...
>
> 1. The spurs seen in the receiver IF enter the receiver in two ways
>
> o As radiation from the CAN-Do inductor
>
> o Via CAN-Do module switched +14 VDC power on pins 39 and 40
>
> 2. I/O line filtering appears to be unnecessary
>
> 3. The switched power from the CAN-Do module pins 39 and 40 must be 
> filtered
>
> 4. The CAN-Do module will either have to be kept physically separated 
> from any sensitive components, redesigned, shielded or a combination 
> of the three.
>
> 5. Moving the Can-Do module 2-1/4" forward of its normal location on 
> the receiver PCB lowers the EMI level below the noise floor.
>
> Suggestions...
>
> . Create a full size PCB for the next generation receiver and shift 
> the RF components to the back of the board (away from the CAN-Do module.)
>
> . Add DC input filtering to the receiver
>
> 73,
>
> Juan
>
> WA6HTP
>
> ------------------------------------------------------------------------
>
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