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Hi
Ross,
That was good
timing of your inquiry about the Firebox noise! I just returned from a four-day
DXpedition (chasing foreign MW and tropical bands) to the Washington coast, and
encountered noise from my Firebox, too. This was my first serious use of my
relatively-new SDR-1000 and I had not noticed any noise at my home location
(fairly RF-quiet).
However, in a
DC-only DXpedition setting with zero noise other than atmospheric, any RFI is
immediately noticed. I tracked down a pulsing, broadband hissing noise to the
Firewire DC input, which is connected to my deep-cycle batteries via a
distribution system feeding all other DC gear. With all other equipment
turned off, I noticed no difference in the noise level when unplugging the I/O
cables and the Firewire cable. The only thing remaining was radiation from
the unit's case or power cable, or conduction via the power cable. I tried
temporarily grounding the Firebox's metal case, without
effect.
I happened to have
a Amidon FT-140-J torroid core (1.40" diameter) on hand, so I did a field
mod of cramming as many turns of the DC cable through the torroid as would fit.
The coaxial DC connector is attached less than an inch past the torroid.
Voila... noise
gone! The only noises remaining were very low-level spikes and buzzes here and
there, visible on the panadapter. I tracked these down to RFI emanating from the
Compaq X1000US laptop screen (noises change pitch and intensity when I touch
my hand to the surrounding LCD cover, aluminum in the X1000 series). I
don't know if the RFI is coming directly from the laptop (screen) casing,
or going out through the various parallel & USB cables. I've always
thought this was a pretty quiet laptop, but the SDR-1000 shows up any little
signal in a RF-quiet environment. In the future I'm going to
experiment with RFI clamp-on ferrites on every cable.
By the way, after
calibrating the SDR-1000's level with a XG-1, the background noise with Beverage
antenna attached was around -132 dBm at medium preamp setting, and -140 to
-144 dBm at the high setting. Very impressive! These readings were just after
local dawn at the coast, where noise levels can be eerily low in
mid-winters (WA State has the fewest T-storm days per year in the continental
USA). A Beverage antenna aimed out over the ocean at dawn knocks down
domestic mediumwave and tropical bands frequencies to the east,
leaving weak DX from Asia and the Pacific in the clear.
So...try a torroid
core to clamp down the noise from the Firebox. If your setup is like mine,
the RFI is coming from the power cable. The "J" series Amidon core is one I've often used over the years for
constructing impedance matching transformers, baluns, etc.
73,
Guy
Atkins
Puyallup,
WA
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- Re: [Flexradio] RF Noise from Presonus Firebox Guy Atkins
