adixmaszParticipantadixmasz August 4, 2015 at 12:03 pm
I would like to use the Signal Hound USB-SA44B Spectrum Analyzer to measure noise.
I can get full sweep from this analyzer but I have a problem with the unit.
In my company, workers use dBm/Hz or nV/ Square root(Hz) units. Could you tell me how can I change unit from dBm to dBm/Hz?
I tried to use information from this site:
but your device isn’t an ordinary spectrum analyzer and I’m not sure about using this information.
I hope my questions are clear, could you help me?
I can offer you my software implementation of API 3.0.5. My LabVIEW Library has every function from API (version x32 and x64). I send you some print screen as attachment.
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AndrewModeratorAndrew August 4, 2015 at 12:25 pm
dBm/Hz = dBm – 10*log10(RBW)
The document you linked then shows you how to convert dBm/Hz to nV/Hz.
Our device input is 50 Ohm, which is used in the document.
Thank you for the LABVIEW example! We will take a look at it.
adixmaszParticipantadixmasz August 4, 2015 at 2:31 pm
Ok, I’ve noticed that before . When I changed unit this way I get different value depend of RBW. I used noise generator (constant noise value) so I should had the same value (unit dBm/Hz) for different RBW settings.
Do you think I should use noise bandwidth instead RBW? http://signalhound.com/support/forums/topic/noise-bandwidth-of-resolution-bandwidth-filter/
Justin CrooksModeratorJustin Crooks August 4, 2015 at 5:40 pm
The standard RBW works well for the BB60C, but not for the USB-SA44B. Because of the software-based image rejection, the default noise bandwidth is 2 x RBW, but then there is the software image reject algorithm. To get NBW close to 2 x RBW, you would need to either disable “spur reject” / image reject, or set video processing to power average and turn VBW down to 1/10 RBW.
To get noise BW close to RBW, you would need to filter out the image frequency. For wide sweeps, this is 21.4 MHz above or below the image frequency. For 200 kHz spans or less, this is 5.8 MHz above or 21.4 MHz above the measured signal. With image/spur reject on, filtering out either image will result in a good measurement.
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