Re: VHF receiver sensitivity problem
- From: "PeteS" <ps@xxxxxxxxxxxxxxxxxxx>
- Date: 27 Feb 2006 02:40:27 -0800
Phil Allison wrote:
"Deefoo"
For a customer on a ship in a remote location I've build a COTS 160MHz
radio
modem into a waterproof diecast aluminium box that is grounded (the modem
originally comes in a plastic box). The antenna used to connect to a BNC
connector but I replaced that by a screw terminal because of space
restrictions.
** 1st bad move.
There is also an extra microcontroller board in the same box
** 2nd bad move.
with Ethernet connections on the same screw terminal as the VHF receiver
antenna.
** Huh ?
All cables (antenna, Ethernet & power) come in through cable
glands.
** Watertight - maybe.
RF tight is another matter.
The VHF modem contains a PIC (10MHz) and it doesn't look like any special
effort has been done to prevent interference from the PIC to reach the
input
stage except maybe that it is as far away as possible from the antenna
input. So I've mounted the extra microcontroller (22MHz) on the PIC side
of
the modem.
** All has to do with the actual frequencies being created by the uP.
None at all can be allowed on 160 MHz or else the receiver is compromised.
Now the customer has done comparative tests with the original setup (VHF
receiver in plastic box without extra microcontroller) and with the
"improved" setup and found that the receiver sensitivity has significantly
diminished for the "improved" setup.
** Many reasons are possible - but RFI from your handiwork is top of the
list.
Unfortunately I can't do any easy measurements and even if I could, I
probably do not have the right equipment (basically just a multimeter and
an
oscilloscope).
** Oh dear.
......... Phil
I assume this is a standard Marine band setup. Apart from Phil's
comments (which I concur with - obviously, if the current setup is
worse, it's something or things that was done in the 'improvement),
there are certain tests you can make to help isolate some things.
If it's the processor, you'll typically find it interferes fairly
narrow band. Is the relative sensititvity worse on some channels (or
channel groups) than others? if so, which channels? That would normally
point to specific interference from harmonics / switching.
Apart from that, the RFI holes you have added should be properly
shielded.
You'll need more than a scope and meter, though, to fully test what's
going on.
Cheers
PeteS
.
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