How to Make an Accurate Frequency Measurement with the NI PXI-5620 Frequency Domain Digitizer

Primary Software: LabVIEW Development Systems>>LabVIEW Professional Development System
Primary Software Version: 8.5
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Modular Instruments>>RF Measurement Devices

Problem:
How do I make an accurate frequency measurement with the NI PXI-5620 frequency domain digitizer?

Solution:
When you want to make a highly accurate frequency measurement of a sine wave you use an instrument called a frequency counter. When using an NI PXI-5620 digitizer, you should use the Spectral Measurements Toolkit (SMT) Peak Search Example for niScope. This example will perform a zoom power spectrum upon an acquired data set, and perform an ensuing peak search measurement on the power spectrum to determine the X coordiante (frequency) of the tone. For more information on the patented peak search algorithm, reference the SMT User Guide linked below.

The following factors affect the overall accuracy in making a quality frequency measurement:

The quality of the 10 MHz reference signal.
The NI PXI-5620 uses an onboard 10 MHz reference. Locking the NI PXI-5620 to a high-stability oven-controlled crystal oscillator (OCXO) or Rubidium 10 MHz reference improves the accuracy of the frequency measurement.

The time duration of the data acquisition.
Frequency counters recognize the time duration of the data as gate time. When programming with the SMT, the data acquisition length is controlled by the resolution bandwidth (RBW) control. To get fine resolution in the frequency domain use a very low RBW setting, which consequently lengthens the duration of aquisition. For example, a 1 Hz RBW is very small, and to achieve this resolution you need approximately one second (1/1 Hz) of time domain data for the fast fourier transform (FFT).

Acquiring for a long time at a very fast rate normally results in the acquisition of a massive amount of samples. The digital downconverter (DDC) on the NI PXI-5620 decimates the acquired data for bandwidths ≤ 1.25 MHz to increase performance. This increase enables you to acquire for a long time at a lower sample rate, and reduces the amount of data that needs processing. The DDC allows you to do a zoom FFT and zoom power spectrum using a smaller RBW corresponding to a finer resolution on the X axis. The subsequent peak search will result in a more accurate frequency measurement.

The frequency offset between the signal source and the NI PXI-5620.
You will need to lock the sine wave source with the NI PXI-5620 (via the 10 MHz reference clock) in order to remove any frequency offset between the two devices. If the source and NI PXI-5620 are not locked together, they will recognize the 10 MHz differently. The following screenshots demonstrate the discrepancy between devices when they are not locked together.

I am generating a 1V pk-pk 32 kHz sine wave with the NI PXI-5421 into the NI PXI-5620. The NI PXI-5421 and NI PXI-5620 are not locked together. I use a 1 Hz RBW (to acquire a very fine resolution spectrum) and use a 10 kHz BW to utilize the DDC. I am also performing 100 averages.

I measure approximately 32.000136 kHz.

If now I export the NI PXI-5421 10 MHz reference and lock the NI PXI-5620 to this 10 MHz reference, both boards will have the same idea of what 10 MHz is and the frequency offset is removed.

I measure approximately 31.999995 kHz.

Attachments:

Report Date: 05/27/2005
Last Updated: 09/21/2007
Document ID: 3LQDLULE

Your Feedback!

Poor | Excellent

Yes

Document Quality?

Answered Your Question?

Please Contact NI for all product and support inquiries.