dBhz to dBm Conversion

The dBm figure is the power related to a specific carrier bandwidth.(BW)

Normaly, for a carrier , this is the power in the 3 dB bandwidth.

If you use a spectrum analyser to measure the carrier power, you must make sure to use a resolution bandwidth which is greater than the carrier BW, but not so wide it takes in the power of any adjacent carrier.

With the digital spec. ana. it is possible to set up line markers to measure the total power between ther markes. (In the old days you had to use the calculator to figure this out)


A single forward bearer type has been chosen for use in the narrow beam. This bearer has a bandwidth of 189 kHz and uses 16 QAM modulation with a bit rate of 200-492kbps. (F80T4.5X)

If the C/N0 is 69.5 dBHz:  (Bear in mind that this figure is (negative dB)    -69,5 dBHz)

The power in the bearer is:  -69.5 + (10log189K) => -69,5 + 52,7  =  -16,7 dBm

Additional comments to the discussion dBm and dBHz. The term log means logarithm with base 10. The acronym dB stands for deciBel. By Bel means the logarithm of a quantity. By deciBel means 10 times Bel. dBm is a logarithmic expression for power measured in milliwatt. Example: P=1 Watt is 1000 mW, or  P=10*log(1000) dBm=30 dBm.

dBHz is a logaritmic expression for bandwidth. Example: If a communication system has a bandwidth of B=3.5 MHz (like UMTS) then this is 10*log(B)=10*log(3500000)=65.44 dBHz

A conversion between dBHz and dBm is not meaningful and I am not able to answer the question of conversion between dBHz and dBm

White Gaussian noise is often expressed as noise spectral densitydensity in watt/Hz. For example termal noise has a spectral density of N0=k*T W/Hz where k is the Bolzmanns constant k=1.38*10^-23 Ws/degK, and T is the temperature in deg. Kelvin. This could be expressed in a logaritmic scale in dBm/Hz. Thus at room temperature N0=1.38*10^-23 * 300 W/Hz or -174 dBm/Hz. If for instance the the bandwidth is 3.5 MHz, the noise level N0*B measured in dBm is -174dBm +65.44 dBHz=-108.5 dBm . In the latter example dBHz is used to find the nois power in a given bandwidth when the noise density is known.

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