Find the RF Interference Power and Impact on Receiver Sensitivity

In wireless system, RF interferences are found any where at any time, the in band RF interference can not be filter out and will cause noise rise to receiver system which will degrade the receiver sensitivity. To accurately test the system capability such as data rate or throughput, it is required and important to find the exact RF interference signal power level and find its impact on receiver sensitivity.
This article discusses the basic Method of Procedure on how to find the RF interference signal power level even if it is lower than the noise floor of the Vector Signal Transceiver (VST). We configure the Vector Signal Generator (VSG) to create a RF signal for emulating RF interference signal, and loop it back to Vector Signal Analyzer (VSA).  Refer to below for block diagram, consider PXIe-5841 as an example:

The procedure of measuring RF interference signal power level is summarized as below:

• Record the VSA noise floor with a 50Ω dummy load connected to RF input port

• Record the displayed total channel power include noise power and interference power

• Calculate the interference signal power with this formula CP_{interf_measured}  = 10 x Log (10^（CP_{SA_display}^{/ 10）} - 10^（CP_{SA_noise}^{/ 10）} )  

Where,
CP_{interf_measured} is the measured Interference Channel power
CP_{SA_display} is the displayed signal analyzer (SA) Channel power
CP_{SA_noise} is the signal analyzer (SA) Noise floor

The intended audiences are NI-VST users who have good knowledge of RF interference, and familiar with NI tools such as RFmx, RFSA, RFSG.

Measure RF Interference Signal Power

1. Record the VSA noise floor

To measure the RF interference signal power, we need to measure and record the VSA noise floor before input simulated RF interference signal from VSG to VSA. Best practice is to connect a dummy load at VSA input port. In this article, as an example, the measured VSA noise floor CP_{SA_noise} is -67.34dBm in 98.3MHz bandwidth. Refer to below screen shoot for the measure noise floor:

2. Generate the simulated RF interference signal from VSG with below configurations:

• Center frequency: 3.5GHz
• Bandwidth: 98.3MHz
• Power setting at VSG: -70dBm
• Jumper cable loss: 1dB estimated
• Power input to VSA: -71dBm

3. Read the displayed power of -65.81dBm in VSA, shown below:

4. Calculate the simulated RF interference power level (CP_{interf_measured}) with the formula discussed in overview section

5. Calculate more simulated RF interference power levels (CP_{interf_measured}) for other displayed power values (CP_{SA_display})

• Apply the formula shown in overview section and refer to table below for more values of calculated RF interference power
• Noticed the last row of the table, the displayed power level (CP _{SA_display}) is some 3 dB higher than either the measured interference power level (CP _{interf_measured}) or the SA noise floor (CP _{SA_noise}) because both are around -67.30dBm, two equal power RF signal add up increase the power by 3dB.

RF Interference Impacts On Receiver Sensitivity

1. RF interference impacts on receive sensitivity is quantified with receiver sensitivity degradation (RSD) is calculated with the formula NR = RSD = 10 x log (1+10^(ΔP/10))                                (2)