Hi,

I am trying to measure output power of transceiver. The problem in hand is that the output of transceiver is differential and my spectrum analyzer is single ended. So what I have done is meaured single ended power while terminating the other end.

i.e.

1. Connected RFP to the spectrum analyzer, while RFN connected to 50 Ohms

2. Connected RFN to the spectrum analyzer, while RFP connected to 50 Ohms.

Now I have two sets of reading.

Please provide me details on how to combine the RFN and RFP to arrive at differential output power.

For e.g. RFN is 2.4 dBm and RFP is 3.4 dBm what is my differential (combined power)?

Should I add 3dB to RFN or RFP.

I am trying to measure output power of transceiver. The problem in hand is that the output of transceiver is differential and my spectrum analyzer is single ended. So what I have done is meaured single ended power while terminating the other end.

i.e.

1. Connected RFP to the spectrum analyzer, while RFN connected to 50 Ohms

2. Connected RFN to the spectrum analyzer, while RFP connected to 50 Ohms.

Now I have two sets of reading.

Please provide me details on how to combine the RFN and RFP to arrive at differential output power.

For e.g. RFN is 2.4 dBm and RFP is 3.4 dBm what is my differential (combined power)?

Should I add 3dB to RFN or RFP.

Since you have no phase measurement between the two channels (+ and -), you'll have to make the assumption that they are 180 degrees out of phase with one another (this may not be exactly true but there is no way to tell with a spectrum analyzer). This is a pretty fair assumption.

Then, if both measurements in dBm across 50 ohms were equal then, yes, you would just add 3 dB to get the differential power across 100 ohms. Since they are different, you must convert them from dBm to watts, then watts to voltage based on 50-ohm load, add the voltages(to get differential voltage), calculate power across a 100-ohm differential impedance, then convert to dBm.

RFN = 2.4 dBm = 1.738 mW -> -295 mV (over 50 ohms)

RFP = 3.4 dBm = 2.188 mW -> 331 mV (over 50 ohms)

Differential voltage (assuming signals are 180 deg appart) = 331 - (-295) = 626 mV

626 mV (over 100 ohm differential impedance) -> 3.919 mW = 5.93 dBm

You could follow a similar process to go from 50-ohm single-ended measurement to 200-ohm differential power if that were the desired application; only the last calculation would change. That was the easy part; allow me to complicate things Be sure that you present a very good 50-ohm load to both ports (i.e. a good termination on the unused port and relatively high attenuation on the spectrum analyzer for the measured port). Changes in SWR between measurements will affect accuracy so keeping it low to begin with is good practice. Remember that this calculation assumes they are 180 degrees apart and the phase difference of the two load relfection coefficients affect this. As an extreme, if the two ports were 0 degrees apart, you differential voltage swing would be 331 + (-295) = 36 mV -> -19 dBm across 100 ohms.

Another option would be to terminate each port with a load and use a differential probe with your spectrum analyzer. The U1818A/B has a good frequency range and can be powered off of the probe-power port on many of our spectrum analyzers. This way, you can connect the intended load to the output of your transmitter (ex: antenna) and measure it under the actual load conditions.