Thanks
Jim,
I spent some time this evening
trying to dig into the CAN-Do noise. I was able to reduce it by 3
or 4 dB by temporarily shielding the module with aluminum tape. Before
getting too excited we need to answer the question – is this noise going to be
a problem? I think as a general suggestion we should move the RF to the
far end of the enclosure. We have the space. I think it would be
best to just extend the traces and not get involved trying to decouple the
leads with ferrite beads and a cable – or perhaps build in some filtering into
the PCB and implement with SMD components. Extend the input and output
traces so we can stick with the three PCB mount SMA connectors. It might be nice
to know what steps have been taken in the past to reduce CAN-Do noise.
Stephen?
I have another idea that would
involve a milled enclosure with multiple compartments. I can see a
modular structure where the sections are assembled into a chassis and pulled
together with threaded rods. The individual sections would be kept in
alignment with pins. That would take care of all the RFI and mechanical
flexing issues. If you’re willing to consider this idea I’ll follow up
with details. Unless the existing chassis is made rigid and plumb it is
a bit concern of mine.
Moving to microphonics… I did
a quick check of available information on the Internet and found this: “Barium
titanate, which is the base ceramic material for most dielectric systems, will
exhibit microphonic effects…” That article went on to suggest tantalum
as a non-microphonic choice. Since it should be pretty quiet out there
in orbit microphonics shouldn’t be a problem. I did find out where the
bulk of is coming from however – C27 and C28. You’ll find them on
schematic page 4 between U4 and U5. Almost every capacitor on the board
appears to exhibit some microphonics but those two seem to be the
worst.
73,
Juan
From: Jim Sanford
[mailto:[email protected]]
Sent: Monday, June 11, 2007 6:21
PM
To: Juan
Rivera
Cc: Bdale Garbee; Bill
Ress; [email protected]
Subject: Re: [eagle] Re: Jim's Phase
Noise Number!!
Juan:
I have seen, in my professional life, a
similar situation where an inductor was radiating noise with unacceptable
impact. Replacing the inductor with a shielded version was more
effective than any overall shield, tho those did work.
Good luck, and
thanks again for all your hard work.
73,
Jim
[email protected]
Juan Rivera
wrote:
Please refresh me on the function of the watchdog
timer. In the state I was running, while observing the 3-second jumps,
the receiver was shut down but the CAN-Do module and dongle were both powered
up (pins 39 and 40 not connected to the receiver.) I thought the module
only reset if it detected no heart beat from the dongle.
I think this evening I will fire up the receiver to
recreate the noise I've been seeing and then start playing with some brass
sheet in front of the inductor as a shield. I tried probing all the pins
with a 10x scope probe connected to the SDR-IQ but I didn't see
anything. I'll repeat that test with a 1x probe. Based on last
night I'm currently thinking that the noise is radiated from that inductor
rather than conducted via the 40-pin connector. There are receiver
components very close to that inductor.
On 6/11/07, Bdale Garbee <[email protected]> wrote:
On Sun, 2007-06-10 at 20:04
-0700, Juan Rivera wrote:
> To my surprise the module jumps briefly to
about 57 kHz at regular
> intervals of about 3 seconds.
Yes,
that's the watchdog timer firing and causing a software
reset. I
have no doubt that the reset cycle causes a short-term
variation in the
current consumption profile on the board
itself. If the frequency is
power consumption dependent, note
that it's likely to move around a bit
during normal operation... might want
at some point to look at
conditions with the CAN-Do! running at a "normal"
20ms update rate.
> My next plan is to probe all 40 pins by using a
10x scope probe. I'll
> start another log called CAN-Do Noise
and start posting there as I
> start on this task.
As Lyle
pointed out in an off-list email, the power supply on the
CAN-Do! widget
was designed to be efficient, not necessarily to
be
quiet. Switchers always have noise issues... we've flown a
lot of them,
and this isn't the first time some filtering or shielding has
been
required to keep everything happy. You're doing the right
things to
characterize what's going on, and we'll all help if/as
needed.
We (the CAN-Do folk) are following your progress with great
interest.
Augmenting the CAN-Do! documentation with some summary of your
findings
and mitigation approach to aid future users is something we'll
look
forward to
doing.
Bdale