should be allowed to approach a steady value before a reading is taken. For the 1 Hz ENBW, this time is on the order of 15 to 30 seconds; for the 10 Hz ENBW, the output stabilizes much faster. The noise output will vary slightly since there will always be noise variations that are slow compared to the bandwidth. Any DC component in the output will not contribute to the noise. However, a large DC output will cause the noise computation to initially rise to a large value before approaching the final answer. As a result, the computation will take longer to settle.
If the OVLD indicator is blinking four times a second, then either the X or Y output is overloaded and the corresponding noise calculation should be ignored. If the OVLD LED is on continuously, then the input signal is overloading the ac amplifier or time constant filters. In this case, both noise outputs will be wrong.
To obtain a value for the noise density, the noise reading should be divided by the square root of the ENBW. Thus, when the ENBW is 1 Hz, the noise output is the noise density, and when the ENBW is 10 Hz, the noise density is the noise output divided by √ 10. For example, if the input noise is measured to be 7 nV with the ENBW set to 1 Hz, the noise density is 7 nV/√ Hz. Switching the ENBW to 10 Hz results in a faster measurement and a reading of 22 nV on the output. The noise density is 22 nV/√ 10 Hz or 7 nV/√ Hz. At frequencies » 10 Hz, the noise density should be independent of the ENBW.
Power
This is the instrument'sPOWER switch. When the power is turned off, the front panel settings are retained so that the instrument will return to the same settings when the power is next turned on. The SR530 always powers up in the LOCAL mode.
The D/A outputs X5 and X6 are not retained during power off. X5 always becomes the RATIO output at power on and X6 is always reset to zero.
When the power is turned on, the CHANNEL 1 OUTPUT DIGITAL DISPLAY will show the SERIAL NUMBER of the instrument and the CHANNEL 2 OUTPUT DIGITAL DISPLAY will show the firmware VERSION. The REFERENCE DIGITIAL DISPLAY shows the model number of
the instrument. All displays return to normal after 3 seconds.
Local and Remote
When the instrument is programmed via the computer interface to be in the REMOTE state WITHOUT
Defaults
If the LOCAL key is held down when the POWER is turned on, the instrument settings will be set to the defaults shown below instead of the settings in effect when the power was turned off.
Parameter | Setting |
BANDPASS | OUT |
LINE | OUT |
LINE X 2 | OUT |
SENSITIVITY | 500 mV |
DYN RES | LOW |
DISPLAYS | X Y |
EXPANDS | OFF |
OFFSETS | OFF (value=0) |
PRE TIME CONSTANT | 100 mS |
POST TIME CONSTANT 0.1 S | |
ENBW | 1 Hz |
REFERENCE MODE | f |
TRIGGER MODE | SYMMETRIC |
REFERENCE DISPLAY | FREQUENCY |
PHASE SHIFT | 0° |
Whenever default values are used at power up, the red ERR LED will turn on for about 3 seconds. If the ERR LED is on when the instrument is powered on without the LOCAL key down, then the instrument is ignoring the retained settings. This can be due to a low battery.
16
