or for relative measurements in dB. The 3581C log scales  can be used for absolute measurements in dBm/900 ohms or dBm/600 ohms or for relative measurements in dB. For absolute measurements, the unit of measure (dBV,  dBm/600 n, dBm/900 ^) is determined by the position of  the front panel Calibration switch.
3-67. When the Log 90 dB scale is selected, the dynamic  range of the meter scale is 90 dB although the specified  dynamic range of the instrument is 80 dB. The region  between - 80 dB and - 90 dB is useful for detecting the  presence of low-level signals but should not be used for  measurements.

3-68. With a dynamic range of 80 dB, only eleven full-scale  ranges are needed to cover the full measurement range of  the instrument. These eleven ranges are selected using the  INPUT SENSITIVITY switch. For absolute measurements,  the full-scale sensitivity ranges from + 30 dBV/dBm to - 70 dBV/dBm.
3-69. The maximum input level for the log scales is  determined by the INPUT SENSITIVITY and amplitude VERNIER settings. For absolute measurements, the full- scale sensitivity is indicated by the white window on the  INPUT SENSITIVITY switch dial. The log scales, however,  do not require an IF attenuator and the AMPLITUDE REF  LEVEL switch cannot be used to extend the measurement range. When the Log 90 dB scale is selected and the AMPLITUDE REF LEVEL switch is rotated away from the 0 dB (NORMAL) position, the meter reading is offset in steps of 10 dB. Each time the meter reading is offset, the full-scale reference becomes 10 dB lower as indicated by the white window. At the same time, however, the dynamic range decreases by 10 dB. With the AMPLITUDE REF LEVEL switch set to the - 70 dB position, the full-scale reference is 70 dB below its original value but the dynamic range is only about 10 dB.
3-70. The ability to offset the meter reading on the Log 90 dB scale is useful for some measurement applications. In most cases, however, all measurements can be made with the AMPLITUDE REF LEVEL switch in the NORMAL position. Any time the AMPLITUDE REF LEVEL setting is changed from the NORMAL position, the dynamic range decreases and a possible worst-case error of ± 1 dB must be added to the amplitude accuracy specification. 3-71. When the Log 10 dB scale is selected, the meter sensitivity is increased to 1 dB per division over a 10 dB range. The Log 10 dB scale corresponds to the top 10 dB of the Log 90 dB scale. Thus, by offsetting the meter reading using the AMPLITUDE REF LEVEL control, any 10 dB portion of the 80 dB range can be selected. For the Log 10 dB scale, the black (dB) markings on the AMPLITUDEREF LEVEL switch dial indicate the full-scale level with respect to the 0 dB reference. For example, if the switch is set to the - 10 dB position, the full-scale level is - 10 dB and the meter scale ranges from -10 dB to - 20 dB. Similarly, with the switch in the - 60 dB position, the full-scale level is - 60 dB and the meter scale ranges from - 60 dB to - 70 dB. t-l?

3-74. The Amplitude Accuracy specification is broken  down so that portions of the specification that do not  apply to a particular measurement can be eliminated. All applicable portions of the specification must be added together to obtain the overall accuracy specification. It should be noted that the overall accuracy specification reflects the absolute worst-case error that could possibly be encountered. Typically, all parameters are well within their specified tolerances and the probability of having a worst- case condition is very slight. As more parameters are added to the specification, the magnitude of the possible worst- case error increases but the probability of having a worst-case condition greatly decreases.

3-75. The Frequency Response, Amplitude Display and Input Attenuator specifications must always be taken into account when calculating the overall accuracy specification. Excluding the Switching Between Bandwidths and Amplitude Ref. Level specifications, the worst-case error is ± 2.7 dB in the Log mode or ± 9% of reading in the Linear mode.

3-76. The Switching Between Bandwidths specification can be disregarded as long as the Amplitude Calibration Procedure is performed on the BANDWIDTH setting that is used for measurements. If the BANDWIDTH setting is changed, the Switching Between Bandwidths specification must be added to the overall accuracy specification. Similarly, the Amplitude Ref. Level specification can be disregarded as long as the AMPLITUDE REF LEVEL control is in the NORMAL position. If the AMPLITUDE REF LEVEL setting is changed, the Amplitude Ref Level specification must also be added to the overall accuracy specification.

3-77. Internal Cal. Signal. 3-78. With the INPUT SENSITIVITY switch set to the CAL position, the high INPUT, terminal on the front panel