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Measuring 3dB BW

Question asked by on Nov 24, 2008
Latest reply on Nov 25, 2008 by odanzy
I searched but could not find the answer...

I'm trying to programmatically measure the 3dB BW of a filter using the
AgNA_ChannelMeasurementMarkerQueryBandwidth in CVI. However I'm missing something key

After I have a marker up on the screen I issue
AgNA_SetAttributeViReal64 (handle, "Channel1:Measurement1:Marker1", AGNA_ATTR_CHANNEL_MEASUREMENT_MARKER_BANDWIDTH_THRESHOLD, -3);

According to the help file, this "Turns on and sets markers 1 through 4 to calculate filter bandwidth. The value sets the point below the maximum bandwidth peak that establishes the bandwidth of a filter. For example, if you want to determine the filter bandwidth 3 db below the bandpass peak value, set value to -3."

I then issue
AgNA_ChannelMeasurementMarkerQueryBandwidth (handle, "Channel1:Measurement1:Marker1", &bw, &cf, &q, &loss);

and I get back non-valid data. I've tried turning on Mrkr 2-4 manually before setting the BW threshold but that doesn't help either. What am I missing?

Thanks in advance