What Are The Commonly Used In RF Unit Conversion Formula?

Updated May 12, 2022

Reported In

Hardware

  • PXI Vector Signal Transceiver
  • PXI Vector Signal Generator
  • PXI Vector Signal Analyzer

Driver

  • NI-RFSG
  • NI-RFSA

Issue Details

What are the commonly used in RF unit conversion formula?

Solution

These are the commonly used in RF unit conversion formula:
  • Vrms = Vpk / sqrt(2)
  • Powerwatts = Vrms2 / Zohms
  • PowerdBW = 10 * log(Powerwatts)
  • PowerdBm = PowerdBW + 30
  • PowerdBmV = PowerdBm + 10 * log(100 * Zohms)
  • PowerdBm = PSDdBm/Hz + 10 * log(BWHz)
Where, Zohms represents the input impedance of the device, BWHz represents the bandwidth range of user interest, and PSDdBm/Hz represents the power spectral density.

The following below shown the example of scenario using RF unit conversion formula: 
1. Conversion from Volts to dBm
Single-tone signal generated by vector signal transceiver (VST) has a frequency offset of 500 kHz from the carrier, IQ signal generated at a rate of 120 MS/s and output power is about -20 dBm. The signal is fed back to the VST device via the RF IN port. The I/Q level measurement results are shown in Figure 1 below, and the power measurement results are shown in Figure 2.
Figure 1
Figure 1. I/Q Level Measurement Results

Figure 2.jpg
Figure 2. Power Measurement Results
 
Based on the I/Q level measurement results, the signal power can be calculated by the following formula:
 
     I = Vpk * cos(theta(t)) = 0.03 * cos(theta(t))
     Vpk = 0.03 V
     Vrms = Vpk / sqrt(2) = 0.03 / sqrt(2) = 0.021213 V
     Power = Vrms2 / Zohms  = (0.021213)2 / 50 = 9 uW
     PowerdBm = 10 * log(Power) + 30 = 10 * log(9u) +30 = -20.45 dBm

From the above conversion equation, we can find out that the time domain measurement results of signal power are basically consistent with the frequency domain calculation results.

2. Conversion from dBm/Hz to dBm
Spectrum measurements captured using the NI VST are shown in Figure 3. The power spectral density at marker 1 is -84.50 dBm/Hz.
 
Figure 3.PNG
Figure 3. Spectrum measurements captured using NI VST
 
At a resolution bandwidth of 200 kHz, the signal power can be calculated as follows:
PdBm = PSDdBm/Hz + 10 * log(BWHz) = -84.50 + 10 * log(2 * 105) = -31.4 dBm