VHF receiver sensitivity problem

"Deefoo"

** 1st bad move.

** 2nd bad move.

** Huh ?

** Watertight - maybe.

RF tight is another matter.

** 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.

** Many reasons are possible - but RFI from your handiwork is top of the list. ** 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

Changing the antenna connector to a "screw terminal" is unclear, but it's not recommended. This will probably cause a mis-match. Depending upon the severity of the mis-match, and coupling of RFI into the antenna input, the sensitivity will be reduced. assume your antenna is mounted elsewhere, and comes in via coaxial feed (50 ohms?). I'm not sure what you mean by "space constraints". so perhaps you can provide a picture. If the antenna connector is too large, try replacing the connector with an SMA type on both the radio housing and the antenna feed cable, using proper RF cabling techniques is essential. Check out the Radio Amateur's Handbook for a guide to antenna connections.

Frank Raffaeli

modem

screw terminal _block_, not the same pins, obviously.

Well, since the original modem came in an unshielded plastic box RF tight should not really be an issue, now should it?

special

compromised.

significantly

the

and

The setup is at about 800 km from where I am...

Maybe placing a shield between the modem pcb and the microcontroller?

--DF

Hello DF,

If space is an issue use SMA or something, but definitely not a screw terminal. Not at 160MHz, and most definitely not via the same terminal block.

It isn't too surprising. Most likely crosstalk from the uC and modem straight into the antenna line.

Sorry to say but it appears that this might need a complete redesign. First of all a coax connector. Decoupling of the supplies. Then look at the materials. Dissimilar metals such as an aluminum box, steel screws and so on can pose a substantial problem in maritime equipment, especially on sea-going vessel because of the salt water. The usual scenario is that noise gets worse over a few months until something finally quits working altogether.

Regards, Joerg

modem

BNC

receiver

special

side

compromised.

(VHF

significantly

the

and

Could it be that the grounded aluminium box makes matters worse by confining any internally generated RFI to the box?

--DF

radio

This wouldn't be a NASA AIS receiver, would it?

This is really bad practice.

input

of

The PIC on the board doesn't interfere because the board design is preventing interference. How is your design with the extra PIC doing? Proper layout?

an

board?

The combination of all. But all this is hard to say as we don't see diagrams, PCB layouts, pictures. Making such a design work requires carefull EMC proof design. Not to mention the required approvals (compliance with IEC/EN standards required by the IMO on commercial vessels).

Meindert

There are no easy answers to your immediate problem.

A few suggestions though. Ship board equipment is not usually size critical so this may do the trick.

I assume this modification was intended to protect the modem from the various forces conspiring to corrode it to a crusty non-functioning lump. Can I suggest putting a completely unmodified and originally cased modem, centrally positioned in a LARGER diecast box, and then silicone sealing all cables where they penetrate the metal enclosure. Five centimetre spacing all round the modem as a minimum, ten if you can manage it. Silica gel packs to mop up any moisture. Polystyrene foam to maintain spacing from the walls of the enclosure.

If the receiver is still desensitised inside the box, then it is almost certainly the pic micro switching transients. A fallback then could be to ditch the diecast and do the job with a half dozen TOUGH poly bags sealed one after the other (ugh!), not forgetting the silica gel packs.

This will certainly not be pretty or compact, but it may just dig you out of the hole the project is in.

To start with, compare the receiver input sensitivity with the transients measured from the PIC.

Rene

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The major problem may very well be the aluminum box if you tied this to circuit common in several places, such as corner metal standoffs, creating circulating currents where there were none before. The antenna screw terminals at 160MHz should cause negligible loss as long as the launch from the board remains short. But now that you have mentioned the compact size of the packaging, the aluminum box can introduce a significant amount shunt capacitance at RF where none existed previously and this will add to input attenuation as well as possibly seriously de-tune any input frequency and IF selection circuits. My money is on de-tuning of the frequency selective circuits as the major cause of the problem. Somewhere in there is possible an AGC or field strength indicator which can be used to possibly adjust the tuning for maximum signal level- with a VOM.

board?

The only contact between the circuit and the box is through two varistors and two capacitors on the power lines. Power is isolated by a DCDC converter and the output is over Ethernet. Does your shunt capacitance hypothesis still hold in this case?

Thanks,

--DF

RF Rule #1. If it works OK as purchased and doesn't work after modifications, do the modifications one by one until it doesn't work. Then find out what the problem was with the last mod.

Jim's Rule #2. I don't care if you don't want to top post, but if you want to interpost or bottom post, learn how to snip.

Jim

What exactly did you do with the original antenna configuration? This would be the first thing to look at.

Secondly, the aluminum box introduces a ground plane that itself is grounded. If the modem had been purposely built with an internal ground plane, your aluminum box will distort/kill both transmission and reception. So first thing you could do is de-ground the box. Don't let anyone touch the box while testing. Put a towel or something between the case and the mount and then test. If you don't see significant improvement, and you didn't do anything strange with screw terminal to cause impedance mismatch, you're probably going to have to go back to that plastic box.

-Le Chaud Lapin-

Yes- the equivalent circuit is as shown below. There may also be an issue of proximity effect reducing the selectivity of the tuned tanks: View in a fixed-width font such as Courier.

. . . . . . RF Node . | . ckt gnd v ckt gnd . ckt brd --------------- - ---------------- .

varistors

converter

Oh OK, I see. I am planning tests next friday. Thanks to you and the other repliers I will first concentrate on the antenna connection and the box. It is easy to take everything out of the box so that will be test number one.

Thanks,

--DF

It

For those who care:

Some tests have been carried out and dramatic improvement of the receiver sensitivity was observed when connecting the antenna properly (BNC, not on screw terminal) as close as possible to the original antenna input. The metal enclosure did not seem to have any influence at all, with or without it the reception quality stayed the same.

Thanks a lot everyone for your suggestions!

--DF