Spectrum Analyzer Measurement Guide : Spectrum Analyzer : Making Spectrum Analyzer Measurements
 
Making Spectrum Analyzer Measurements
Required Equipment
Optionally, an antenna that is appropriate for the frequency range to be measured
Required Setup
Place the instrument in Spectrum Analyzer mode. Refer to Selecting a Measurement Mode.
Connect the input signal or antenna to the RF In test port.
Setting Frequency
The tuning frequency range can be entered in several different ways depending upon what makes the most sense, either for the user or for the application. The center frequency and span can be specified, the start and stop frequencies can be entered, or a signal standard and channel number can be selected from the built‑in list.
Offset Frequency
In addition, a user‑defined frequency offset can be entered to adjust the frequency that is displayed on the instrument from the actual swept frequency. For example, if the DUT is an antenna system receiving signals in the 10 GHz range and offsetting the signals to the 1 GHz range, you can set a frequency offset in the spectrum analyzer in order to display the actual received antenna frequency in the sweep window. For a measurement example, see Figure: 200 MHz Frequency Offset Example.
Both positive and negative offset values are allowed. Negative offsets can be useful for seeing differences from expected values. Enter a negative offset of the expected value, and the received antenna frequency should display in the 0 Hz range.
When enabled, Offset is displayed at the bottom of the screen (Figure: 200 MHz Frequency Offset Example), and the Center Freq, Start Freq, and Stop Freq submenu keys indicate that a frequency offset has been turned on by adding the word Offset before Center, Start, and Stop. For menu examples, see Figure: SPA Frequency Menu (1 of 2) and Figure: SPA Freq 1/2 with Offset Function Menu.
To remove a frequency offset, set the Freq Offset to 0 Hz.
Setting Bandwidth Parameters
Both resolution bandwidth (RBW) and video bandwidth (VBW) can be coupled to the frequency span automatically or manually set. When set to Auto RBW, RBW adjusts automatically in proportion to the frequency span. The default ratio of the span width to the resolution bandwidth is 100:1, and can be changed as follows:
1. Press the BW main menu key.
2. Press the Span/RBW submenu key. The current Span/RBW ratio is shown as part of the submenu key label. Change the value using the keypad, the directional arrows, or the rotary knob and then press Enter.
When auto‑coupling between the span and RBW is selected (the Auto RBW submenu key is toggled to “On”), this is indicated on the left side of the display with the RBW label and underneath it one to three digits followed by the frequency units; this represents the resolution bandwidth value. If manual RBW is selected (the Auto RBW submenu key is toggled “Off”), the label and value turn red and a ‘#’ symbol is shown in front of the RBW label. Adjust resolution bandwidth independently of the span. If an unavailable resolution bandwidth is entered, then the instrument selects the next higher resolution bandwidth. If a value greater than the widest RBW is entered, then the widest RBW will be selected.
VBW can be set two ways – manually or by auto coupling. Auto coupling of the VBW links the video bandwidth to the resolution bandwidth, so that VBW varies in proportion to RBW. Auto coupling is indicated on the left side of the display with the VBW label and underneath it one to three digits and the frequency units; this represents the video bandwidth value. If manual VBW coupling is selected, the label and value turn red and the “#” symbol is shown in front of VBW on the left side of the display. Adjust video bandwidth independently of the RBW. If a non‑existent video bandwidth is entered, then the instrument will select the next higher video bandwidth. If a value greater than the widest VBW is entered, then the widest VBW will be selected.
The ratio of the resolution bandwidth to the video bandwidth can be changed by pressing the BW main menu key, the RBW/VBW submenu key, and then using the keypad, the directional arrows, or the rotary knob to set the ratio. By default, the RBW/VBW ratio is set to 3. The current value of the ratio is shown as part of the submenu key label.
1. Press the BW main menu key.
2. Press the RBW/VBW submenu key. The current RBW/VBW ratio is shown as part of the submenu key label. Enter the desired value.
The RBW range varies with instrument features. Refer to the RBW submenu key description in BW (Bandwidth) Menu, and check your Technical Data Sheet for the RBW range of your instrument.