and from

HP3581_Four

SENSITIVITY and AMPLITUDE REF LEVEL switch settings. The INPUT SENSITIVITY switch controls the input attenuation and gain of the input circuits to establish the maximum input level as outlined in Paragraph 3-37. The AMPLITUDE REF LEVEL switch controls an IF attenua- tor which determines the gain of the IF Amplifier and the meter sensitivity. As the AMPLITUDE REF LEVEL switch is rotated in a clockwise direction, the IF attenuation decreases, the IF gain increases and the full-scale sensitivity increases. 3-58. By observing the INPUT SENSITIVITY and AMPLI- TUDE REF LEVEL controls, it can be noted that the full scale (blue) markings on the INPUT SENSITIVITY switch dial are indicated by a white window that is mechanically linked to the AMPLITUDE REF LEVEL switch. Changing the position of either switch changes the full-scale sensiti- vity in a 30V, 10 V, 3V, IV sequence. Changing the INPUT SENSITIVITY switch setting changes the maximum input level and the full-scale sensitivity. Changing the AMPLITUDE REF LEVEL switch setting changes the full-scale sensitivity but does not effect the maximum input level. For example, if the INPUT SENSITIVITY switch is set for a maximum input of 1 V rms, and the AMPLITUDE REF LEVEL switch is set to the XO.l position, the full-scale sensitivity is 0.1 V rms but the maximum input level is still 1 V rms. Signals between 0.1 V rms and 1 V rms will overdrive the meter but will not damage the instrument or hinder its ability to measure signals within the 0 V to 0.1 V range.

NOTE

When using the linear (Voltage) scales, be sure the Calibration switch is set to the dBV/LIN (3581A) or dBm 900^/LIN (3581C) position.

3-59. Relative Measurements (Linear Scale). For relative measurements using the linear scale, the INPUT SENSITI- VITY and amplitude VERNIER controls are adjusted so that the reference signal produces full-scale deflection on the 0 to 1 meter scale. Whenever possible, this should be done with the AMPLITUDE REF LEVEL switch set to the XI position. Signals below the reference level are then measured in "percent of full scale" where the reference is 100% and each major division of the 0 to 1 meter scale is 10%. Signals below 30% of full scale can be measured on an expanded scale by changing the AMPLITUDE REF LEVEL setting. With the AMPLITUDE REF LEVEL switch set to the first unmarked position between XI and XO.l, the meter range is from 0 to 32% and the meter can be read using the 0 to 3 scale. A reading of "3" corresponds to 30%. With the AMPLITUDE REF LEVEL switch set to the XO.l position, the meter range is from 0 to 10% and the meter can be read using the 0 to 1 scale. As the AMPLITUDE REF LEVEL switch is rotated further clock- wise, the ranging sequence is 3%, 1%, 0.3% and 0.1%. The 0 to 1 meter scale can be read for the 100%, 10%, 1% and 0.1% ranges and the 0 to 3 scale can be read for the 30%, 3% and 0.3% ranges.

3-60. If, when measuring low-level signals, a full-scale reference cannot be obtained with the AMPLITUDE REF LEVEL switch in the XI position, a lower setting can be used. The setting on which the full-scale reference is obtained becomes the 100% range and the lower settings become the 30%, 10%, 1%, etc. ranges.

3-61. Alternative Method. Another method for deter- mining the relative amplitude of two signals is to first measure the absolute voltage levels and then calculate their relative amplitude using the following formula:

A= (V2/V1) * 100
Where:

A = relative amplitude in percent
VI = reference level in rms volts
V2 = signal level in rms volts

3-62. Using the Amplitude Ref. Level Control. Whenever possible, the AMPLITUDE REF LEVEL switch should be left in the NORMAL (XI) position and the INPUT SENSITIVITY switch should be used to set the full-scale sensitivity. This is because the Amplitude Calibration Procedure (Paragraph 3-173) is performed with the AMPLI- TUDE REF LEVEL switch in the NORMAL position and any error introduced by the IF Attenuator is adjusted out. When the AMPLITUDE REF LEVEL setting is changed from the NORMAL position, the accuracy of the IF Attenuator must be considered. This means that a possible worst-case error of ± 3% of full scale must be added to the amplitude accuracy specification. Amplitude accuracy is discussed in Paragraph 3-72. 3-63. The INPUT SENSITIVITY and AMPLITUDE REF LEVEL controls provide a total of 18 voltage ranges (30 V to O.ljuV). The top 11 ranges can be selected using the INPUT SENSITIVITY switch with the AMPLITUDE REF LEVEL switch set to the NORMAL position. The seven bottom ranges can be selected using the AMPLITUDE REF LEVEL control with the INPUT SENSITIVITY switch set to the 0.3 mV (full CW) position.
3-64- The AMPLITUDE REF LEVEL switch can also be used for expanded scale measurements where the input level is 0.3 mV rms or greater and the components to be measured are less than 30% of full scale. In this case, the AMPLITUDE REF LEVEL switch should initially be set to the XI position and the INPUT SENSITIVITY switch set to the lowest range that does not produce an OVERLOAD indication. The AMPLITUDE REF LEVEL control can then be adjusted so that the low level signals of interest can be measured.

3-65. Using the Log Scales.

3-66. The 3581 A log scales can be used for absolute measurements in dBV (1 V rms = 0 dBV) or dBm/600 ohms

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