Measure RF Devices Frequency Response on NI VST Without Tracking Generator

In RF/Wireless industry, it is often required to measure the frequency response of RF devices, such as filters, duplexers, cables, attenuators, pre-selector and so on. RF engineers are familiar to use two types of equipment to perform this measuring: VNA (Vector Network Analyzer) and SATG (Spectrum Analyzer with Tracking Generator). However, if both VNA and SATG are not available, NI VST and LabVIEW (if they are available) provide an alternative cost-effective and fast way of measurement, the method of procedure is discussed in this article.
As an example, PXIe-5841 is used in this article to show how to measure a duplexer's receiver path frequency response. The intend audiences are NI VST and LabVIEW users who have basic knowledge of RF devices specifications such as insertion loss, attenuation or rejection, pass band, stop band, frequency response, and so on. 
Please be note that the method discussed in this article is for functional measurement only as the max hold in RFSA is a worst-case scenario and will measure the minimum values of both pass band insertion loss and stop band attenuation/ rejection. If accurate measurement is required, please refer to NI PXI Vector Network Analyzer and follow the procedures in NI PXIe-S5090 Getting Started. 

Set Up RFSA and Measure the Jumper Cable Loss

1. Connect the jumper cable to PXIe-5841, shown below:

2. Launch NI-RFSA: Start >> National Instruments >> NI-RFSA Soft Front Panel
3. Set up the RFSA according to the measuring: the duplexer receiver path with passband 2.5GHz to 2.57GHz and stopband 2.62GHz to 2.69GHz, set the RFSA start frequency at 2.38GHz and stop frequency at 2.70GHz.
4. Set both RBW and VBW to Automatic, leave other settings such as reference level and range as default, or you can adjust them per your preference shown below one example.  
5. Select Trace/Detector >> Max Hold from RFSA control panel, let the RFSA runs. Shown below: 

Set Up RFSG and Test the Jumper Cable Loss

1. Find the example vi:  LabVIEW Help >> Help >> Find Examples >> Search >> RFSG Configuration List Frequency and Power Sweep 
2. Set up the vi accordingly: the Start Frequency, End Frequency, Start Power Level, End Power Level, Dwell in Each Steps, Number of Steps, shown below:

3.  Run the vi and record the cable loss 1.07dB displayed in RFSA, shown below:

Test Duplexer Frequency Response

1. Connect duplexer to VST with the 2 low loss RF jumper cables just tested, duplexer COM port to VST RF OUT, duplexer Receiver port to VST RF IN,  shown below:

2. Refer to the below picture for receiver port frequency (2500MHz to 2570MHz).

3. Set RF Power Lever to 1.07dBm to offset the jumper cable loss, click on RFSG Run button to start RF signal generating and measure the duplexer frequency response, shown below: 
4. Check duplexer receiver path frequency response from RFSA, you can play around with the RFSA setting to have your preferred view. One example is shown below:

Refer to below for duplexer specifications:

The test results match the duplexer specifications well. For details, refer to below table:

Description	Specification	Test Result	Remarks
Passband Loss	1.5dB	0.09dB	Passband center loss
		0.33dB	Passband low edge loss
		0.39dB	Passband high edge loss
Stopband Rejection	60dB	56.14dB	Refer Marker 4
		60.05dB	Refer Marker 5
		63.56dB	Refer Marker 6
		66.58dB	Refer Marker 7