IBM 7220, Instruction Manual

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3-14
Chapter 3, TECHNICAL DESCRIPTION
3.10 Noise Measurements
The noise measurement facility is available only in the baseband mode (i.e. at
reference frequencies less than 60 kHz) and uses the output processor to perform a
noise computation on the Y output where it is assumed that the waveform is
Gaussian with zero mean. The zero mean is usually obtained by using the reference
phase control or the Auto-Phase function with a comparatively long time constant
(say 1 s) and the time constant is then reduced (to say 10 ms) for the noise
measurement.
The user is strongly advised to use an oscilloscope attached to the rear panel SIG
MON (signal monitor) output when making noise measurements as this is the best
way of ensuring that one is measuring a random process rather than line pick-up.
The indicated value of the noise (in V/
√
Hz or A/
√
Hz) is the square root of the mean
spectral density over the bandwidth defined by the setting of the output filter time
constant.
3.11 Power-up Defaults
All instrument settings are retained when the unit is switched off. When the
instrument is switched on again the settings are restored but with the following
exceptions:-
a) The signal channel reverts to AC coupling.
b) The GPIB mask byte is set to zero.
c) The REMOTE parameter is set to zero (front panel control enabled).
d) The curve buffer is cleared.
e) Any sweep that was in progress at switch-off is terminated.
f)
Synchronous time constants are enabled.
g) Display backlights are turned on
3.12 Auto Functions
3.12.01 Introduction
The auto functions are groups of control operations which can be executed by means
of a single command or two key-presses. The auto functions allow easier, faster
operation in most applications, however, direct manual operation or special purpose
control programs may give better results in certain circumstances. During application
of several of the auto functions, decisions are made on the basis of output readings
made at a particular moment. Where this is the case, it is important for the output
time constant set by the user to be long enough to reduce the output noise to a
sufficiently low level so that valid decisions can be made and sufficient time is
allowed for the output to settle.
The following sections contain brief descriptions of the auto functions.