Center (Frequency)

When the center frequency is changed, the VSA changes the measurement start and stop frequencies, but does not change the span. For example, in the following picture, changing the center frequency causes the span to "slide" up or down the x axis.

$image\freqCenter_wmf.jpg$

When the center frequency is changed, the VSA automatically sets Time Data to Zoom. For information on other ways to set the Center Frequency, see Changing Frequency Settings.

Multi Channel Center Frequencies

The 89600 VSA supports independent center frequencies on multiple channels with certain hardware, including the following:

all supported oscilloscopes
dual box N4010
dual box MXA when using 2 RF Radio Frequency: A generic term for radio-based technologies, operating between the Low Frequency range (30k Hz) and the Extra High Frequency range (300 GHz). channels

In the Frequency tab of the MeasSetup dialog box, individual Measurement Channels may be selected using the buttons at the top of the dialog.

For the set of hardware listed above, the center frequency can be changed separately for each channel or for all channels by selecting the Apply to all channels checkbox in the Center Frequency dialog box (displayed when using the center frequency hotspot on a trace).

Display Limits (Trace Blanking)

A center frequency can be selected that ranges from the minimum start frequency to the maximum stop frequency. Because of this, it is possible to select a center frequency that causes a negative start frequency. It is also possible to specify a center frequency that causes the stop frequency to exceed the VSA's frequency range.

If a center frequency is specified that yields a start frequency below the VSA's frequency range, there will not be anything displayed to the left of the minimum start frequency. For example, if the frequency range of the VSA is 0 to 39 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second. and a center frequency is specified that yields a negative start frequency, there will not be anything displayed to the left of 0 Hz. This is called Trace Blanking.

Likewise, if a center frequency is specified that yields a stop frequency above the VSA's frequency range, there will not be anything displayed to the right of the maximum stop frequency. In the previous example, there will not be anything displayed to the right of 39 MHz.

$image\freqBlanking_wmf.jpg$

Center frequency limitations with 2-channel I+jQ(Ch1+j*Ch2) input mode

For details about setting the center frequency for I+jQ(Ch1+j*Ch2) mode, see I+jQ Span and Frequency.

Using "Show All Points"

Use the Show All Points parameter (Utilities > Display Preferences > Trace tab) to view the blanked trace data. The Show All Points parameter displays frequency data even if it extends beyond the VSA’s standard frequency and valid data range. The data may contain alias products, so use caution when selecting this parameter.