[hpsdr] Mercury Spurious Emissions

[Jeremy McDermond]

On Sep 10, 2011, at 4:54 PM, Joe Martin K5SO wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> 
> On Sep 9, 2011, at 5:52 PM, Jeremy McDermond wrote:
> 
>> It seems as if Mercury is radiating spurious signals into the antenna connector.  The symptom is showing up on 144.390 MHz.
> 
> Using my IC910 with a whip antenna a few feet from my HPSDR enclosure (Antec) I too can see a signal at about S3-S4 strength showing up at 144.390 (or image thereof) when my three Mercs are running.  However, the signal is NOT being radiated from the Mercury antenna connector.  In fact, when I directly connect the input of the IC-910 to the Mercury antenna connector the signal appearing at 144.390 MHz (or it's image, of course) actually goes away.  The emissions you are seeing are coming from the bus or the boards and not being output through the antenna connector, it seems.

I'm not seeing the same thing on my system.  If I disconnect the antenna connector from the system, I do see the S3-S4 signal, which isn't such a big deal.  If I connect the antenna to the system it becomes at least an S9 signal, if not more.  Without the antenna connector it isn't noticeable on the mobile that's like 20 ft away.  With the antenna connector on, it's S9 on that mobile.  Interestingly if I merely touch the shields of the BNC, I see this behavior.  This seems to indicate that it's coming through the ground side connection.

> That such VHF signals are present in the immediate vicinity of the HPSDR rig is not very surprising of course, no more so than similar type signals emanating from any bus-driven computer system, since the bus has relatively large amplitude square wave signals on it.   I was concerned that something was being radiated via the Mercury antenna connection, as was initially suggested...or at least I that's what I thought that's what the suggestion was.  Well, it isn't, in any case.

I'm not necessarily suggesting that this is a design issue or a general issue with Mercury in general.  I do know that the behavior of mine differs when I have the antenna ground connected.  I'm just trying to figure out whether I have a problem with my particular Mercury.  Of note is that my portable antenna is only around 10 feet away from the radio and there are no walls between the radio and the antenna.  So, proximity could be an issue, but it certainly seems for me that the antenna makes a difference.

I made a quick and dirty video showing the issue:

http://www.youtube.com/watch?v=BxJJFe9PjFc

> In this case it seems that standard, or perhaps elaborate, rf shielding techniques should be able to reduce the bus or board emissions to an acceptable level in the vicinity of the HPSDR rig, as has been already suggested.  Anyway, no wonder I didn't see much with the spectrum analyzer except for the 122.88MHz clock and it's harmonics when I connected directly to the BNC connector on Mercury earlier.  The problem you see isn't coming from there, clearly.
> 
> For your demos, in which folks may have handhelds right up next tot the HPSDR rig you may need to simply have a super shielded setup and not run with the covers off, but that hardly is conducive to showing people what the rig looks like.  At least I think you by now have a good idea what the issue actually is, right?
> 
> 73,  Joe K5SO

--
Jeremy McDermond (NH6Z)
Xenotropic Systems
mcdermj at xenotropic.com




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