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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's POWER 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 LOCK-OUT, the LOCAL key will return
the instrument to LOCAL front panel control. If
the instrument is in the REMOTE WITH LOCK-
OUT state, no front panel key will return the status
to LOCAL. In this case, a RETURN TO LOCAL
command must be sent over the computer
interface or the power must be turned off and back
on.
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.