# Thread: 3 dB vs 6 dB bandwidth

Replies: 2 - Pages: 1 - Jun 6, 2012 6:41 AM Last Post By: Timbo Threads: [ | Next ]
 Posts: 5 Registered: 05/16/12
3 dB vs 6 dB bandwidth
Posted: Jun 5, 2012 3:19 PM
 This seems like a simple question but I can't seem to find or derive a calcuation. How do you convert 3 dB bandwidths to 6 dB bandwidths? (I doubt that multiplying by two is the correct answer!) Example: I have a requirement to use 10 kHz or greater 6 dB resolution bandwidth. I'm using a 4440a spectrum analyzer that is calibrated to 3 dB bandwidths. What bandwidth should I use and how did you calculate it. (I need the formula, not just a number.)
 Posts: 1,113 Registered: 07/24/09
Re: 3 dB vs 6 dB bandwidth
Posted: Jun 5, 2012 3:48 PM   in response to: Timbo
 Hi - The PSA has 160 RBW settings. They are referenced to 3 dB bandwidth, but you can easily figure out how to map a 6 dB bandwidth to our 3 dB bandwidths. The relationship is basically 6dB bandwidth = 3 dB bandwidth * sqrt(2). So, for instance if you need a 1 MHz -6 dB bandwidth using the PSA's 3 dB bandwidths, then the equivalent 3 dB bandwidth is 1 MHz / sqrt (2) = 707 kHz. The closest two bandwidths the PSA has are the 750 KHz and 680 kHz bandwidth. The 680 kHz 3 dB bandwidth equates to 962 kHz 6 dB bandwidth. This is only 3.8% off from being 1 MHz 6 dB bandwidth. Since our RBWs for PSA are spaced at 10% increments, whenever you chose an equivalent 6 dB bandwidth you'll be at most 5% off, which is really good. So, if for instance you need a 8.25 kHz -6dB bandwidth, then the equivalent 3 dB bandwidth to choose in the PSA is 5.6 kHz. If you need a 10 kHz or larger 6 dB bandwidth, then you would use a 7.5 kHz or larger 3 dB RBW. The PSA has some 6 dB bandwidths that you can choose from the front panel that are needed for electromagnetic compliance measurements, but all of those bandwidths are mapped behind the scene to the equivalent 3 dB RBW filter. Note - I am going to move this Forum thread to the Signal Analyzer section so more folks can learn from this post in the future! Hope this helped -
 Posts: 5 Registered: 05/16/12
Re: 3 dB vs 6 dB bandwidth
Posted: Jun 6, 2012 6:41 AM   in response to: tabbott
 Thanks! I read in app notes somewhere that the bandwidths were related by the square root of two but I wasn't sure how to do the calculation. I'd really like to derive that equation sometime... but at least I have the formula now.