Finding the IF Center Frequency when Using the NI 5600/5601 in Downconverter Only Mode

Updated Apr 4, 2018

Reported In

Hardware

  • PXI-5600
  • PXIe-5601
  • PXI-5661
  • PXIe-5663
  • PXI-5652

Driver

  • NI-RFSA

Issue Details

Downconverter only mode for the NI 5600/5601 is a new feature that was introduced in NI-RFSA 2.1. When using the downconverter in this mode, we need to know the downconverted IF frequency. How can I find this IF frequency generated by the NI 5600/5601 downconverter?

Solution

The NI 5600 is specified to have an IF frequency range from 5 MHz to 25 MHz. The NI-RFSA driver will select a local oscillator (LO) frequency such that the downconverted signal is approximately centered around 15 MHz. Depending on the center frequency and frequency span of the RF signal, the tuning resolution of the LO will be limited to steps of 1 MHz and 5 MHz, which is the reason why the IF frequency will not always be centered at exactly 15 MHz.

However, unlike the NI 5600, the NI 5601 can downconvert the RF signal to an exact IF frequency by using the NI 5652 RF Signal Generator to source any specific LO frequency using direct digital synthesis (DDS). The downconverted IF frequency for the NI 5601 downconverter is set at either 53 MHz or 187.5 MHz depending on the input RF frequency as shown in the following table in the specifications sheet. 
 
The two attached VIs show how the IF frequency for the NI 5600/5601 downconverter can be programmatically calculated when used in downconverter the only mode. The RFSA Downconverter Center Frequency Adjustment.vi consists of two sessions: an NI-SCOPE session and an NI-RFSA session. The Get IF.vi is used as a subVI to obtain the IF frequency from the NI-RFSA session and specify the center frequency for the NI-SCOPE session. A portion of the block diagram for Get IF.vi is shown below. 

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