" Even if a manufacture follows the device datasheets,
there's still a variable that comes into play with circuit
board layout and power supply decoupling. "
FOR A TEN-TEC EXAMPLE:
The Eagle has a similar noise problem when turning the Audio gain and RF
gain down.
The noise can barely be heard on the speaker but is loud and clear on "some"
headphones.
Dave, KW4M, found and fixed the problem ON HIS EAGLE.
I DO NOT RECOMMEND OTHERS TRY THIS . . . unless they are very experienced
with soldering on SMD gear.
A complete description of this mod can be found in Message #4477 in our
Ten-Tec Eagle group on groups.io.
---------------------------------------------------
Here is an excerpt from that message:
The processor noise is being picked up at the input of op amp U9 on the
DSP board. More specifically, signals SPUMISO and SPUMOSI are routed
very close to the input pins on this device. These traces are routed on
internal layers but are visible when holding the board up against a
bright light.
Eliminating the processor noise pickup into U9 will require a circuit
board layout change to move these traces. Perhaps TT will make this
change sometime on future radios. (Rick's Note: they never did!)
In the meantime, there is a fairly simple workaround. Since the Audio
Out stage U9 and the Line Out stage U11 are identical, these circuits
can be swapped. This change was made on my radio and the processor
noise cleared up. Actually, the noise is now being sent to the Line Out
port but the PC sound card will not be annoyed by it. [:)]
---------------------------------------------------
And here is a link to see a picture of that mod:
http://www.genebitsystems.com/david/LinkedPhotos/EagleDspFix.jpg
Perhaps Ten-Tec was aware of it when they first shipped, perhaps not.
But even if, it would have been expensive to redesign the circuit board
layout, and would have caused a long delay in the delivery time of the first
radios.
Since this noise does not normally impede reception in any way (under normal
condx), it is not a big issue - at least not for me.
Worst case, some users might need to buy new headphones...or sell the radio.
I will not attempt the mod on my own Eagle, out of fear of screwing
something else up.
My old eyes are poor and my hands are not as steady as they once were.
So in this case, it was not poor electrical design, but rather poor
mechanical design.
Cheers!
Rick, DJ0IP
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