Rfi
- Drive motors:
- The amplifiers are the largest source of rfi
(from the pulsing)
- amplifiers inside shielded rack, inside
shielded container.
- filters on cables that go out to motors. good
shields.
- Drive encoders.
- 614Khz comb from
azimuth encoders (.pdf)
- put filter inside encoder enclosure (doesn't
need the sharp clock transitions).
- Warning: you want weather proof encoders so
there is not corrosion problem
- The encoder weather proofing uses rubber
gaskets on some of the connector plugs (connectors that are not used).
This causes rfi to leak out since you no longer have good metal to
metal contact on the connectors plugs.
- fiber modulators
- Modulators inside doubly shielded box.
- Ethernet fiber modulators have caused problems.
They are not well shielded. We try to put them in shielded boxes.
- pc,computers:
- 1/distance^2 really helps so try to keep
electronic devices off the feed platform
- shielded computer room (Faraday cage) for
backends and computers.
- Need to plan ahead for bringing cables into
and out of room. Cutting holes in the Faraday cage is not very smart
(but we still do it).
- Transparent conductive film on all windows cuts
down rfi transmission from inside buildings.
- Need windows since people want to look at the
beautiful telescope. This assumes there is enough metal (rebar)
in the concrete to stop transmission through the solid walls.
- pc clocks.
- some pc's chirp (frequency modulate) their
clocks over many Mhz to meet the emission requirements.
Instead of a large narrow interference you get a smaller wide
band problem.
- Electronics in the feed room:
- put all electronics in a shielded cabinet. Make
sure that cables that leave the cabinet are well grounded.
- power meters, multi meters, digital switching
devices.
- air conditioners:
- air conditioners to cool the feed room cause
rfi. We had to replace the digital thermostat with an analog version.
more info:
rfi info:
List of
birdies seen at AO.
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