About SR770 program's PSD measurement mode

[GLSu]

Hi there,

 Since my FFT analyzer SR760 has some problems with its floppy driver, I decided to use the downloaded LavVIEW program to get PSD data instead of "store/recall" directly from the machine. However, when I press "start" and then choose "trace" in the program, the outcome always shows up the value in "spectrum" mode (in dBV), although I have set the measurement mode in "PSD" (in dBV/sqrt(Hz)) for both machine and program panel.

 Does anyone encounter this kind of problem before? I can just take those PSD data directly from the screen on the machine, but it becomes so tedious since there are 400 points. Solution to either the program or the conversion between PSD and spectrum is good.

 Thanks!!

 Larry

[TJR_741]

Larry and other SR770 users,

We ran into this same issue, and I wanted to share the results of our investigation.

1. The SR670/770 LabVIEW drivers available through NI use the SPEB command to transfer the data as a binary data stream. Despite what the SR770 documentation claims, it seems that this command only transfers the spectrum (in dBVpk or Vpk). It does not transfer either the PSD or the Vrms spectrum.

2. The SR770 has an alternative data transfer command SPEC. This command transfers exactly the trace that is displayed on the analyzer: spectrum Vpk, spectrum Vrms, PSD, etc. This data transfer is much slower, though, and the analyzer has to be paused when it is executed. If the analyzer is still collecting data, the trace is overwritten by new data before the transfer finishes.

3. The conversion from Vpk to Vrms is straight forward after retrieving the data. The conversion from Vpk to PSD is not documented, and we have not found a way to calculate it based on instrument settings. However, my colleague identified and implemented a work-around. The VI is programmed to read both the Trace and the Marker Y value. The Marker Y value matches the configuration and so can return the accurate PSD at one frequency point. A comparison between the returned spectrum and this marker value allows us to calculate the PSD because the PSD is Spectrum/Resolution Bandwidth and the marker allows us to determine the Resolution Bandwidth. If a log measurement is made, the difference between the marker and spectrum is used. If a linear measurement is made, the ratio is used.