Spectrum Analyzer Measurements : Setting Amplitude Parameters
Setting Amplitude Parameters
Amplitude-related parameters are set using the AMPLITUDE Menu.
Setting Amplitude Reference Level
The amplitude reference level is typically an absolute reference level set at the top of the graticule for the power level being measured. Signal levels above this set value will be outside of the display range and might overdrive and saturate the input circuit. To set the current amplitude reference level:
1. To automatically set an optimum reference level, press AMPLITUDE > AUTO REF LEVEL.
2. To manually set the reference level, press AMPLITUDE > REF LEVEL, then enter the desired reference level in dBm.
Select AUTO REF LEVEL to help ensure that harmonics and spurs are not introduced into the measurements.
Setting Amplitude Range and Scale
This setting applies to most analyzer modes of instrument operation and allows you to set the y-axis graticule scale.
1. Press AMPLITUDE > SCALE/DIV and enter the desired number of units per division (dB/division).
2. Set the desired y-axis amplitude units. Currently, dBm is the only available selection.
Reference Level Offset for External Loss or External Gain
To obtain accurate measurements, you can compensate for any external attenuation or gain by using a reference level offset. The compensation factor is in dB. External attenuation can be created by using an external cable or an external high power attenuator. External gain is typically from an amplifier.
To adjust the reference or amplitude level for either gain or loss:
2. Enter a positive dB value to account for gain or enter a negative dB value to account for loss.
3. The new reference level offset value will be displayed on the instrument and the y-axis and trace amplitude is adjusted accordingly.
IF Gain
The IF Gain can be turned on and off by toggling IF GAIN via the status panel or the AMPLITUDE menu. Figure: 1. IF Gain Off 2. IF Gain On shows the noise floor with the IF Gain off (1) and on (2). Note that when the IF Gain is turned on, the noise floor drops by typically 15 to 20 dB. The Reference Level must be -10 dBm or lower for the IF Gain to function.
1. IF Gain Off
2. IF Gain On
The reference level is the top graticule line on the measurement display. If the reference level offset is not zero, the offset reference level is displayed at this location. Pressing the plus (+) or minus (–) control increments the value by 10. The plus/minus (+/-) button on the keypad toggles between positive and negative values.
Auto reference level automatically adjusts the reference level to place the highest signal amplitude at about two graticule lines from the top based on the position of the trace at the time the button is pressed.
The scale can be set from 1 dB per division to 15 dB per division. The default setting is 10 dB. Pressing the plus (+) or minus (–) control increments the value by 1.
Displays the y-axis amplitude units in dBm.
Reference level offset compensates for the presence of external input attenuation or gain. The offset is applied to all amplitude related parameters and to measurements such as the y-axis scale and marker measurements. The default offset value is 0 dB. Pressing the plus (+) or minus (–) control increments the value by 10. The plus/minus (+/-) button on the keypad toggles between positive and negative values. Refer to Reference Level Offset for External Loss or External Gain.
Turns the IF gain On or Off. Default is Off. The analyzer has an IF amplifier with approximately 20 dB of gain. When you can turn on the IF Gain without overloading the analyzer (reference levels of –10 dBm or less), it allows full use of the dynamic range of the analyzer. If the reference level is changed to be more than –10 dBm and IF gain was on, it will be turned off for any reference level over –10 dBm. Likewise, the IF gain cannot be turned on for a reference level over –10 dBm.
The MS276XA employs special image rejection algorithms to reduce or eliminate spurs and images in the receiver. Under normal operation, the analyzer will take two IF samples (a high side and a low side) and compare the two to reduce or eliminate unwanted spurs or images. This setting selects an image reject algorithm of Normal, High Only, or Low Only. Normal is the combination of both the high-side and the low-side IF samples, which provide the benefit of spur/image rejection. The High Only and Low Only settings offer no image rejection and only show the results from the single IF sample obtained (either high or low). High Only or Low Only settings results in faster overall sweep speed, but with no spur or image reduction at all. In Zero Span, the Low Only setting is automatically selected unless IMAGE REJECT is set to High Only.