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[Linrad] Re: SDR-IQ Birdies

At 03:13 PM 10/15/2009, you wrote:

>Hi Guy,
> > Pickup is through the antenna from something radiated inside the
> > shack. It appears that the SDR-IQ is the source of this interference,
> > which can be reduced somewhat by several clip-on ferrites on the USB
> > lead close to the SDR-IQ. The USB plug is too large to pass through
> > any of the ferrite rings I happen to have on hand, though I could
> > obviously buy some larger ones.
>I suggest you wrap your SDR-IQ carefully in aluminium foil and
>remove the outer plastic from the two cables entering the unit.
>They are both screened and you should make a good connection
>between each screen and the aluminium foil.
>This way you will eliminate any voltage difference between the
>two cables leaving the wrapped unit and you will also eliminate
>any voltage between different points on the foil. All currents
>will flow inside the enclosure and you should easily reach 60 dB
>attenuation. More aluminium will improve until the limit that
>is set by the leakage through ths screen of the USB cable.
>Make a small wire loop and connect to the 144 MHz input. It
>could be one turn with 20 mm diameter. When you place that loop
>against the various conductors in your setup you will easily see
>where currents of the interference frequencies flow. Once you know
>where the currents are it will be reasonably easy to do something
>about them (as long as they are inside your own house...)
>Leif / SM5BSZ
I enclosed my SDR-IQ inside a Hammond Diecast box, using a brass 
shield to mount DB-9 connector.  The USB jack isn't particularly 
shielded and the cable shield is only connected to the computer 
ground.  Computers often do not like their ground tied to radio 
equipment grounds, so beware!  I measured my noise floor at -134 dBm 
using a BNC 50-ohm termination on the antenna connector.  Very little 
evidence of birdies on the Spectravue display when terminated this 
way.  But I see a fair amount when an antenna is connected.  Perhaps 
use of clamp-on ferrites on the USB cable might be effective if there 
is leakage there.

73, Ed - KL7UW, WD2XSH/45
  BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
500-KHz/CW, 144-MHz EME, 1296-MHz EME
DUBUS Magazine USA Rep dubususa@xxxxxxxxxxx

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