I have been doing some radio astronomy experiments with USRP using a
30 MHz dipole antenna (actually it is more of a riometer experiment).
I am running into various interference issues. E.g., at one point I
noticed that if my laptop power supply is too close to my USRP I get
switching power supply harmonics in my signal. At other times I have
been pretty sure that the inteference comes through my antenna.
Here is what I have been thinking to combat the interference problems:
Locate antenna at about 100-200 m away from any electronic devices
(except for the antenna preamp)
Locate USRP as far as possible from electronic devices (is 5m
really the maximum distance, or is there some other trick?)
Obviously filter and amplify signal properly before putting it in
Use well regulated and filtered power supplies for amplifiers and
USRP
Anything else that I should take into account? I don’t have that much
experience with minimizing interference, except for the things that I
have learned during the last couple of weeks while experimenting.
I have been doing some radio astronomy experiments with USRP using a
30 MHz dipole antenna (actually it is more of a riometer experiment).
I am running into various interference issues. E.g., at one point I
noticed that if my laptop power supply is too close to my USRP I get
switching power supply harmonics in my signal. At other times I have
been pretty sure that the inteference comes through my antenna.
I have found huge differences between laptop power supplies from the
manufacturer of the laptop (like Lenovo, HP, etc.), and cheap
replacement supplies made to look like the originals. Once I was
testing something on the USRP while the computer was attached to one of
these supplies and I saw a huge mess on the spectrum analyzer. I spent
an hour trying to figure out what was happening, only to find that it
was there even if there was no USRP hooked up at all – it was all
radiating from the power supply.
If you really need low noise, run the laptop off of its internal battery
only.
If you really need low noise, run the laptop off of its internal battery
only.
What also works is running everything (laptop and USRP) from external
batterypacks.
The USRP needs 6V, which can be provided by 5 standard NiMH batteries in
series (use large D-size cells or put several penlites in parallel)
Alternatively you can use non-switching powersupplies.
(Just use a LAB power-supply or build your own with a transformer,
rectifier, big capacitor and linear regulator)
More tips to get rid of RF spuriuos:
Put ferrite beads around any power or digital cable.
(mains power, DC power, USB, ethernet)
Put everything in fully closed metal cases (powersupply, laptop, USRP)
Make sure you don’t run into heat problems this way.
If you do run into heat problems, only use small round holes in the
metal casings.
1000 small holes is much better then 10 big holes from a RF point of
view.
What also helps for laptops is to attach coppertape to the inside or
outside of the full casing of the laptop.
(We used this to get flatscreen TV’s we developed CE complient)
Take care this tape doesn’t cause any shortcircuits.
A low-budget solution is to use aluminium foil.
Make sure you use high-quality coax cables for the RF-connections.
Make sure to use high-quality USB cables with good shielding.
Use a mains filter.
Run the USRP powersupply from a different mains group as the laptop.
Make sure your antenna is above a big metal plate (several wavelengths
big) which is very well grounded.
Make sure that your grounding is several meters into the ground well
into the groundwaterlevel.
Make sure that all digital equipment is below this metal plate
If you are really desperate:
Add extra 100 nF SMD capacitors between all powersuppply pins of every
chip and ground (both USRP and laptop)
Make a metal casing for the daughterboards.
Locate USRP as far as possible from electronic devices (is 5m
really the maximum distance, or is there some other trick?)
Juha,
You might try using a USB media converter. This would make it possible
to separate the USRP from the laptop by a much greater distance (cutting
down on radiated noise).