Frequency-related parameters are set using the FREQ / SPAN Menu. 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 measurement. The center frequency and span can be specified, the start and stop frequencies can be entered.
Entering Start and Stop Frequencies
The frequency settings are displayed along the bottom of the spectrum or spectrogram graph. These parameters can be accessed directly or via the FREQ SPAN menu.
1. Select FREQ SPAN on the main menu.
2. Select START FREQUENCY to open the start frequency parameter entry keypad.
3. Enter the desired start frequency. When entering a frequency with the keypad, available frequency units (GHz, MHz, kHz, and Hz) will be displayed along the left edge of the menu.
4. Select the appropriate frequency unit to terminate the entry or press ACCEPT to terminate the entry with the current frequency unit.
5. Select STOP FREQUENCY to open the stop frequency parameter entry keypad.
6. Enter the desired stop frequency. When entering a frequency with the keypad, available frequency units (GHz, MHz, kHz, and Hz) will be displayed along the left edge of the menu.
7. Select the appropriate frequency unit to terminate the entry or press ACCEPT to terminate the entry with the current frequency unit.
Note
To quickly move the start or stop frequency value up or down, press the + or – slider controls to increment the frequency by the set FREQUENCY STEP. You can also drag the frequency using the slider.
The center frequency will be set to exactly the middle of the start and stop frequencies. The current settings are shown along the bottom of the spectrum or spectrogram graph (refer to the user guide for view the relevant graphs).
Entering a Center Frequency
1. Select FREQ SPAN on the main menu.
2. Select CENTER FREQUENCY to open the center frequency parameter entry keypad.
3. Enter the desired center frequency. When entering a frequency with the keypad, available frequency units (GHz, MHz, kHz, and Hz) will be displayed along the left and right edge of the menu.
4. Select the appropriate frequency unit to terminate the entry or press ACCEPT to terminate the entry with the current frequency unit.
Note
To quickly move the center frequency value up or down, press the + or - slider controls to increment the center frequency by the set FREQUENCY STEP. You can also drag the center frequency using the slider or by dragging the trace directly.
The current settings are shown along the bottom of the spectrum or spectrogram graph.
Using Offset Frequency
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.
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, the offset value is displayed at the left of the screen in the status panel (see Status Panel). To remove a frequency offset, open the FREQ SPAN menu and set FREQUENCY OFFSET to 0 Hz. You can also access this parameter directly from the left side status panel.
Note
Offset frequency apply to start, stop, center, and marker frequencies.
Setting the Span
1. Select FREQ SPAN on the main menu.
2. Select SPAN to open the span frequency parameter entry keypad.
3. Enter the desired span frequency. When entering a frequency with the keypad, available frequency units (GHz, MHz, kHz, and Hz) will be displayed along the left edge of the menu.
4. Select the appropriate frequency unit to terminate the entry or press ACCEPT to terminate the entry with the current frequency unit.
5. To select full span, press the FULL SPAN. Selecting full span overrides any previously set start and stop frequencies.
Note
To quickly move the span value up or down, press the + or - slider controls to increment the span in a 1:2:5 sequence. You can also drag the span using the slider or by pinching the trace in or out.
Zero Span IF Output (Option 89)
Zero Span IF output effectively allows the spectrum analyzer to be used as a receiver front-end, converting the input signal at the spectrum analyzer RF In connector to a signal centered at the IF output frequency. You can then process the IF signal in a way that meets your needs. That may mean using an A-to-D converter or some other signal processing method.
Zero Span IF Output provides an IF signal of 325 MHz with FFT capture bandwidth ≤ 32 MHz and 300 MHz with FFT capture bandwidth = 100 MHz. The nominal RF output level is –4 dBm with a 10 MHz, –20 dBm input, and with 0 dB input attenuation and preamp off. The spectrum can be inverted in certain RF input bands. The IF output signal is present only when the spectrum analyzer span is set to ZERO SPAN and the IF OUTPUT is toggled on (see FREQ / SPAN Zero Span Menu). You can select the IF output bandwidth of 750 kHz, 7.5 MHz, 14 MHz, 30 MHz, or 100 MHz using the IF OUTPUT Menu (Option 89).
Note
The selectable bandwidth values may differ among instrument models and options.
Selected IF Filter Bandwidth
IF Output Frequency
100 MHz
300 MHz
30 MHz
325 MHz
14 MHz
325 MHz
7.5 MHz
325 MHz
750 kHz
325 MHz
The normal IF BW uses analog bandpass filters in the normal RBW chain. By changing the RBW, different filter bandwidths are selected. While the IF bandwidth is influenced by the selection of RBW filters, the digital RBW filters themselves are not used.
The spectrum analyzer has several mixer bands. Depending on the operating frequency, the local oscillator may be above or below the input frequency. When the local oscillator frequency is below the input frequency, an increase in the input frequency results in an increase in the IF output frequency. When the local oscillator is above the input frequency, an increase in the input frequency moves it closer to the local oscillator frequency and the IF output frequency consequently decreases. The following table shows the bands and indicates where the LO frequency is, in relation to the RF frequency.
IF Inversion Bands
RF Band
Start Frequency (MHz)
Stop Frequency (MHz)
Spectrum Inversion
1
0
5350
Y
2
5350
7200
Y
3
7200
9200
Y
4
9200
11100
N
5
11100
13000
N
6
13000
16500
N
7
16500
20400
Y
8
20400
26950
Y
9
26950
33810
Y
10
33810
39900
Y
11
39900
47000
N
12
47000
54000
N
“Inverted” means that the IF is spectrally inverted from the input (as the input frequency goes higher, the IF goes lower). “Not Inverted” means that the IF is not spectrally inverted (as the input frequency goes higher, the IF goes higher).
You need to take frequency inversion into account when processing the IF signal. Assuming that the IF has been processed to yield I and Q data, inversion is easily done by swapping I and Q.
A residual frequency offset of the IF may exist compared to the RF due to the resolution of the first and second local oscillators. This offset is usually on the order of several kHz, but may be up to 10 kHz or so. To determine the residual offset, you need a second spectrum analyzer or a frequency counter.
1. Attach a signal source (or antenna) to the spectrum analyzer and set the center frequency to the center of the signal being received.
2. Set Zero Span from the FREQ/SPAN menu, and then press IF OUTPUT and enable the IF output.
3. Attach a second spectrum analyzer to the IF Out port and set the center frequency to 300 (or 325) MHz.
4. Set the span of the second spectrum analyzer to 100 kHz with the resolution needed to be able to measure an offset that may be 25 kHz or less.
5. Measure the frequency of the IF signal to see how far the signal is offset from 300 (or 325) MHz.
FREQ / SPAN Menu
FREQ / SPAN Menu
-
CENTER FREQUENCY
Sets the center frequency of the sweep range. The current span setting will remain constant or will be adjusted to accommodate the start and stop frequency range of the instrument. The center frequency can also be dragged on the display when gestures are not toggled off.
SPAN
Sets the sweep frequency range. The current center frequency will remain constant and the start and stop frequencies will be adjusted to accommodate the new range. Selecting the plus (+) or minus (–) control increments the span value in a 1:2:5 sequence. The span can also be changed by pinching the trace in or out when gestures are not toggled off.
FULL SPAN
Selecting this button sets the span to cover the entire tunable spectrum of the instrument.
LAST SPAN
Selecting this button returns the span to the previously set span value.
Sets the start frequency of the sweep range. The center frequency and span will be adjusted to accommodate the new start and current stop frequencies. Selecting the plus (+) or minus (–) control moves the start frequency in steps defined by the FREQUENCY STEP value.
STOP FREQUENCY
Sets the stop frequency of the sweep range. The center frequency and span will be adjusted to accommodate the current start and new stop frequencies. Selecting the plus (+) or minus (–) control moves the start frequency in steps defined by the FREQUENCY STEP value (set lower in this menu).
FREQUENCY STEP
Sets the frequency step value used for the plus (+) or minus (–) control.
FREQUENCY OFFSET
The frequency offset value accounts for frequency conversions outside of the analyzer. The offset frequency value is added to the start, stop, center, fixed marker, and normal marker frequencies. Selecting the plus (+) or minus (–) control moves the offset frequency in steps defined by the FREQUENCY STEP value.
Toggles the touchscreen drag feature on or off. When toggled off, the center frequency will not change when dragging the spectrum display. This can be useful when dragging markers.
PINCH
Toggles the touchscreen pinch feature on or off. When toggled off, the span frequency will not change when pinching the spectrum display. This can be useful when dragging markers.
Note that the Drag and Pinch features can also be toggled on/off by touching the locks under the display.
FREQ / SPAN Zero Span Menu
FREQ / SPAN Menu
-
CENTER FREQUENCY
Sets the center frequency of the sweep range. The current span setting will remain constant or will be adjusted to accommodate the start and stop frequency range of the instrument. The center frequency can also be dragged on the display when gestures are not toggled off.
SPAN
Sets the sweep frequency range. The current center frequency will remain constant and the start and stop frequencies will be adjusted to accommodate the new range. Selecting the plus (+) or minus (–) control increments the span value in a 1:2:5 sequence. The span can also be changed by pinching the trace in or out when gestures are not toggled off.
FULL SPAN
Selecting this button sets the span to cover the entire tunable spectrum of the instrument.
LAST SPAN
Selecting this button returns the span to the previously set span value.
Sets the frequency step value used for the plus (+) or minus (–) control.
FREQUENCY OFFSET
The frequency offset value accounts for frequency conversions outside of the analyzer. The offset frequency value is added to the start, stop, center, fixed marker, and normal marker frequencies. Selecting the plus (+) or minus (–) control moves the offset frequency in steps defined by the FREQUENCY STEP value.
