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If you disconnect the coax from the spectrum analyzer(SA)/receiver and the noise floor drops, then it is coming down the cable. If you are using a pre-amp the problem could be there.
If the noise floor does not drop, then the problem is in the SA/Receiver. Check to make sure the Attenuation is not set too high (-10db in normal). Also check to see if an offset has been set or correction factors have been entered incorrectly. Most Receivers have several different methods to input correction factors for antenna, cables, amps, etc. so they maybe entered incorrectly or twice (I have seen this happen). Remember, correction factors for antennas and cable lost is added to the result, corrections for external pre-amps are subtracted. Get that wrong and your get very strange numbers. A quick method to see if you are reading correctly is to input a known frequency and level, say from a signal generator. If you get correct values but still have a high noise floor, your SA/Receiver may have an internal noisy amp in the front-end stage which would require repair. You said that it was a 3 meter fully anechoic chamber. If you also use the chamber for Radiated Immunity test, always remember to disconnect the coax to your SA/Receiver. High power RF can couple through the coax and damage the front-end stage of the SA/Receiver. Also, when you are doing emissions testing, turn OFF the amplifier used for Immunity testing. We find that just the amp being turned on can raise the noise floor the receiver sees by 10-20db and it is very broadband. Hope this helps. Brian From: [email protected] [mailto:[email protected]]On Behalf Of ext Sent: 15 March, 2004 09:21 To: [email protected]; [email protected] Subject: RE: Noise level inside an fully anechoic chamber As there are high levels everywhere, please check the noise floor first. Remove antenna, or replace with 50 ohm terminator. Measure like you would have the antenna there. The result is the noise floor with the settings you use. Ari From: [email protected] [mailto:[email protected]]On Behalf Of ext [email protected] Sent: 15 March, 2004 17:10 To: [email protected]; [email protected] Cc: [email protected]; [email protected]; [email protected]; [email protected] Subject: RE: Noise level inside an fully anechoic chamber Wow, these are very high levels. There is certainly a problem. I would first suspect the cabling into the chamber- is is bonded at a bulkhead? Also, if the receiver/spectrum analyzer is in the chamber you will see lots of stuff. Dave Cuthbert Micron Technology From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Saturday, March 13, 2004 8:15 PM To: [email protected] Cc: Ken Javor; [email protected]; [email protected]; T.Sato Subject: Noise level inside an fully anechoic chamber Dear Experts, I am conducting pre-scans in a 3-meter fully anechoic chamber. It is noticed that even when no equipment is present inside the chamber the field strength is about 20 to 25 db micro volts per meter in the frequency range 30 to 350 MHz, about 27 to 32 db micro volts per meter in the frequency range 350 to 600 MHz and about 33 to 39 db micro volts per meter in the frequency range 600 to 1000 MHz. What is the level of ambient noise permissible in fully anechoic chambers? Which section of FCC deals with measurement site qualifications? Sincerely K.Balasubramanian Sr.Engineer, Hardware. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: with the single line:Title: Re:RE: Noise level inside an fully anechoic chamber
If you disconnect the coax from the spectrum analyzer(SA)/receiver and the noise
floor drops, then it is coming down the cable. If you are using a pre-amp the
problem could be there.
If the noise floor does not drop, then the problem is in the SA/Receiver. Check
to make sure the Attenuation is not set too high (-10db in normal). Also check
to see if an offset has been set or correction factors have been entered
incorrectly. Most Receivers have several different methods to input correction
factors for antenna, cables, amps, etc. so they maybe entered incorrectly or
twice (I have seen this happen). Remember, correction factors for antennas and
cable lost is added to the result, corrections for external pre-amps are
subtracted. Get that wrong and your get very strange numbers.
A quick method to see if you are reading correctly is to input a known frequency
and level, say from a signal generator. If you get correct values but still
have a high noise floor, your SA/Receiver may have an internal noisy amp in the
front-end stage which would require repair.
You said that it was a 3 meter fully anechoic chamber. If you also use the
chamber for Radiated Immunity test, always remember to disconnect the coax to
your SA/Receiver. High power RF can couple through the coax and damage the
front-end stage of the SA/Receiver. Also, when you are doing emissions testing,
turn OFF the amplifier used for Immunity testing. We find that just the amp
being turned on can raise the noise floor the receiver sees by 10-20db and it is
very broadband.
Hope this helps.
Brian
---------------------------------------------------------
From: [email protected] [mailto:[email protected]]On Behalf Of ext
Sent: 15 March, 2004 09:21
To: [email protected]; [email protected]
Subject: RE: Noise level inside an fully anechoic chamber
As there are high levels everywhere, please check the noise floor first.
Remove antenna, or replace with 50 ohm terminator. Measure like you would have the antenna there. The result is the noise floor with the settings you use.
Ari
-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of ext [email protected]
Sent: 15 March, 2004 17:10
To: [email protected]; [email protected]
Cc: [email protected]; [email protected]; [email protected]; [email protected]
Subject: RE: Noise level inside an fully anechoic chamber
Wow,
these are very high levels. There is certainly a problem. I would first suspect the cabling into the chamber- is is bonded at a bulkhead? Also, if the receiver/spectrum analyzer is in the chamber you will see lots of stuff.
Dave Cuthbert
Micron Technology
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of [email protected]
Sent: Saturday, March 13, 2004 8:15 PM
To: [email protected]
Cc: Ken Javor; [email protected]; [email protected]; T.Sato
Subject: Noise level inside an fully anechoic chamber
Dear Experts,
I am conducting pre-scans in a 3-meter fully anechoic chamber. It is noticed that even when no equipment is present inside the chamber the field strength is about 20 to 25 db micro volts per meter in the frequency range 30 to 350 MHz, about 27 to 32 db micro volts per meter in the frequency range 350 to 600 MHz and about 33 to 39 db micro volts per meter in the frequency range 600 to 1000 MHz. What is the level of ambient noise permissible in fully anechoic chambers? Which section of FCC deals with measurement site qualifications?
Sincerely
K.Balasubramanian
Sr.Engineer, Hardware.
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