On Mar 29, 2013, at 1:22 PM, Ed Muns wrote:
> Another alternative is to put a "ground loop
> eliminator" in one or both of the paths. There are many formats of ground
> loop eliminators available on eBay and I keep a supply handy for quick
> fixes. They are $2-6 each and the $20 Radio Shack version is also about $6
> on eBay without the R/S label.
When the "Ground Loop Isolator" came up on this reflector back in May 2012, I
went out to the local Radio Shack to pick one up to measure. This is what I
discovered (and sent to a couple of other folks):
>> I compared two cases...
>>
>> (1) My standard test cable with the Bourns LM-NP-1001-B1 transformer wired
>> in-line.
>>
>> Result: -89 to -91 dBc 3rd order IMD (like most software modems,
>> cocoaModem does not measure higher order IMD).
>>
>> (2) Radioshack Isolator (cable shortened and terminated with a 3.5mm Neutrik
>> phone jack)
>>
>> Result: -63 to -64 dBc 3rd order IMD.
The test generator was set up to put out about -3 dBu.
As of this posting, in unit quantities, the Bourns transformer is $2.25 a piece
at Digikey, and $1.78 at Mouser. You will need one for transmit, and one or two
for receive, depending on how many receivers you are using for decoding. The
Radio Shack isolator is listed at $19.99 at their web site.
Note that the Bourns is not good for wideband SDR (e.g., Softrock) if ou plan
to build a skimmer. At 10 kHz, they are down by 2.5 dB. The LP-PAN, for
example, uses the much more expensive and wider bandwidth Triad transformer.
Two of the Triad transformers, at $13.93 each at Mouser, for I and Q will run
you almost $30 -- but you get what you pay for when you need 200 kHz of
coverage :-).
I didn't measure the Radio Shack's frequency response, but with that kind of
IMD, you shouldn't use it as part of an SDR's audio chain anyway.
The RadioShack isolator is definitely good enough for AFSK transmission, since
most transmitters' IMD are in the region of -40 dB PEP or worse (0 dBc relative
to 1 tone is -3 dB PEP for a 2 tone signal; QST rig reviews use dB PEP). The
ground loop isolator will definitely not widen your AFSK transmit signal more
than what an even a good transmitter is already doing.
However, whether you want to use the Radio Shack isolator for receiving is up
to you own druthers. I am just posting the numbers so people can make up their
own minds.
If there is only a single signal coming through it, you are likely to be in
good shape since the SNR loss is too small to bother RTTY demodulators. But if
you rely on good software modems to also do the QRM filtering (and optimal
decoding) for you, it is 30 dB worse than even the dirt cheap Bourns
transformer.
73
Chen, W7AY
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