# Measuring Power per Hertz of a RF signal

I am testing my communication application to see it's performance over different frequencies. So I need to measure power at different frequencies of operation. I have been told that my limitation for power to be transmitted is -50dBm/Hz. So I need to see to it that I don't exceed this rating. So i sent a signal at 500Khz bandwidth and checked the output power from the transmitter. I got the value to be around -60dBm. I then changed the bandwidth to 1Mhz to see that maybe the power value will double. But then the only change in power was from -60dBm to -57dBm. I then changed bandwidth to different values to see a correlation. I didn't find any. So how do we go about measuring power per Hz. Seeing the spectrum analyser values if I try to divide power value on spectrum analyser by bandwidth present i think it will be Inaccurate. I would like to know what I am doing wrong and how I can meet the rate of -50dBm/Hz. Any other approach that would help?

• 60dBm would be 1000W. Are you sure about those numbers?
– user16324
Commented Jul 17, 2013 at 20:14
• @BrianDrummond Sorry it was -60dBm to -57dBm. I edited it in the question. Commented Jul 18, 2013 at 4:36
• Perhaps I'm confused, but it seems like -60 dBm to -57 dBm is exactly the doubling of power that was expected from doubling the BW. Commented Jul 17, 2014 at 12:51

While "-50dBm/Hz" can be interpreted as "with 1 Hz RBW, no point exceeds -50dBm", it is generally expected that your signal power is somewhat evenly distributed over the entire bandwidth. Thus, it means that your transmit power maximum is this value times the transmission bandwidth, i.e. for 1 MHz bandwidth, you get +10dBm.

However, that assumes ideal distribution of power, which you hardly ever get, so that is a purely theoretical value. What is practically possible depends to a large extent on the scrambling algorithm you use.

The regulations should also specify how compliance is tested -- typically with an RBW of a few kHz, VBW > RBW, peak detector and a fairly long sweep time.

Measurements with an RBW of 1 Hz are not really useful in compliance tests, because they take ages and see only little of the actual signal power.

change video and resolution bandwidth of the spectrum analyzer to lowest setting. Further use slower sweep time then you should get the best dBm/Hz.

Best dBm/Hz is measured by using a narrow IF filter (RBW), narrow post detection filter (VBW) and highest measurement integration time (Slowest sweep time).

here is a link of agilent app note that explains RBW, VBW and Sweep time of a spectrum analyzer

http://cp.literature.agilent.com/litweb/pdf/5965-7920E.pdf