Remote Spectrum Monitor™ MS27201A User Guide : Spectrum Monitor Measurements : Setting Amplitude Parameters
Setting Amplitude Parameters
The amplitude scale corresponds to the y-axis display. The MS2720xA supports both log units (such as dBm and dBV) and linear units of scale (such as volts or watts). Amplitude-related parameters are set using the AMPLITUDE Menu.
Setting Amplitude Units
This setting allows you to set the y-axis graticule units and applies to most of the spectrum monitor measurements, including markers, traces, and limit lines. To change the amplitude units:
2. Select the desired y-axis amplitude units from the following:
Logarithmic: dBm (default), dBW, dBV, dBmV, dBµV, or dBA
Linear: Volt (scalable to V, mV, µV, nV), Watt (scalable to W, mW, µW, nW, pW, fW),
A (scalable to A, mA, µA, nA)
When switching amplitude units, the following parameters are affected as described below:
Reference level is converted directly from the current unit into the new unit and scaled appropriately. For example, if the current units are dBm and ref level is set to –30 dBm, when changing the units to volts, the ref level is converted to 0.00707 V and scaled to 7.07 mV.
The reference level offset is always entered in dB. When linear units are selected, the dB offset is converted to an offset to match the selected units and applied to the measurement displays.
When changing from log units to log units, scaling (dB/div) remains unchanged. When changing from log units to linear units, scaling is not applicable in linear units.
When a linear unit is selected, the top graticule line is set to the value of the converted reference level and the bottom line of the graticule is set to zero. The remaining graticule lines are calculated by dividing the reference level into 10 equal parts. For example, if the ref level is 5 mv, the graticule lines will be labeled as follows (from top to bottom): 5.00, 4.50, 4.00, 3.50, 3.00, 2.50, 2.00, 1.50, 1.00, 0.00.
Trace in a “HOLD / VIEW” mode will be scaled to match the newly selected units.
Marker amplitudes will be scaled to match the newly selected units.
Limit line points will be directly converted and displayed in the newly selected amplitude units.
Trigger levels will be converted to the newly selected units.
Measurements and setups will be converted to the newly selected units.
Save and recall will retain the units configuration.
Power cycle will retain the current units configuration.
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 may overdrive and saturate the input circuit (refer to Indications of Excessive Signal Level). To set the current amplitude reference level:
1. To automatically set an optimum reference level, click AMPLITUDE > AUTO REF LEVEL.
2. To manually set the reference level, click AMPLITUDE > REF LEVEL, then enter the desired reference level.
Select AUTO ATTEN coupling of the attenuator setting and 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 monitor modes of operation and allows you to set the y-axis graticule scale for log-based units only.
2. Enter the desired number of units per division.
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 always in dB, even if linear amplitude units are selected. 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.
Attenuator Functions
The spectrum monitor includes a step attenuator at the RF input. This attenuator is used to reduce large signals to levels that make best use of the monitor’s dynamic range. By default, the auto attenuation automatically adjusts the attenuator as a function of the reference level. In the AMPLITUDE menu, the ATTEN LEVEL allows manual adjustment of the input attenuation. When auto attenuation is selected, both the reference level and the attenuation are increased. The following actions, listed in decreasing order of effectiveness, can facilitate the detection of low-level CW signals:
Decrease the reference level and attenuation. Refer to AMPLITUDE Menu.
Turn on the preamplifier.
Reduce RBW and or VBW (RBW/VBW = 10 is often optimal for this purpose). Refer to Setting Bandwidth Parameters.
Use trace averaging if VBW is already set to 1 Hz. Refer to Setting Trace Parameters
The preamplifier can be turned on and off by toggling PRE AMP via the status panel or the AMPLITUDE menu. Figure: 1. Preamplifier Off 2. Preamplifier On shows the noise floor with the preamplifier off (1) and on (2). Note that when the preamplifier is turned on, the noise floor drops significantly and a low-level signal is exposed. In order to use the preamplifier, the attenuation must be lower than 20 dB. If the preamplifier is turned on when the attenuation is greater than or equal to 20 dB, the attenuation will automatically drop to 10 dB. When AUTO ATTEN is toggled on, the REF LEVEL must be set to –40 dBm or lower to enable the preamplifier.
1. Preamplifier Off
2. Preamplifier On
Indications of Excessive Signal Level
The Remote Spectrum Monitor has built-in features to help prevent input overload. These include auto attenuation and reference level. The instrument will also indicate when a received signal is too high for the current setup by displaying an “ADC Overrange” notification in the title bar (Figure: ADC Overrange). Before proceeding with the measurements, adjust the reference level, the attenuation level, and disable the preamplifier if necessary. Adjusting the resolution bandwidth and frequency range may also help when measuring small signals that are near large signals.
ADC Overrange
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. Clicking 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 clicked.
The scale can be set from 1 dB per division to 15 dB per division. The default setting is 10 dB. Clicking the plus (+) or minus (–) control changes the value by 1. SCALE/DIV is not available when linear y-axis amplitude units are selected.
Selects the y-axis amplitude units. Available units are:
Logarithmic:dBm, dBW, or dBV, dBmV, dBµV, dBA
Linear: Volt (scalable to V, mV, uV, nV), Watt (scalable to W, mW, uW, nW, pW, fW), A (scalable to A, mA, uA, nA).
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 and will always be presented in dB units. Clicking 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 low-noise front-end preamplifier on or off. To ensure accurate measurement results, the largest signal into the instrument input when the preamplifier is turned on should be less than –40 dBm. The preamplifier cannot be turned on if auto attenuation is on and the reference level is above –40 dBm. Refer to Preamplifier.
Input attenuation can be either tied to the reference level (on) or manually selected (off). When input attenuation is tied to the reference level, attenuation is increased as higher reference levels are selected to make sure the instrument input circuits are not saturated by large signals that are likely to be present when high reference levels are required.
When auto attenuation is off, the attenuation value can be set manually to a resolution of 5 dB. Clicking the plus (+) or minus (–) control increments the value by 10.
Opens the AMPLITUDE FIELD STRENGTH Menu. Refer to Field Strength for a description of the measurement.