Model
8904A
Operation
3-9.
SPECIAL
FUNCTIONS
Introduction
The special functions alter the instruments operating ‘environment’ in order
to
change how the
instrument responds
to
changes in different instrument settings
or
operating conditions. These
functions perform simple operations, such
as
disabling the beeper, and more complex functions, such
as
disabling the automatic phase reset in the Digital Synthesis
IC.
NOTE
Special finctions can only be changed when the instrument is at the Main
Selection Level. Performing an instrument
PRESET
turns
off
all special
functions. Using
the
MAIN
key does not effect specials.
Enabling and Disabling Special Functions
From the Main Selection Level, press the
SPECIAL
key. Use the
NEXT
and
LAST
keys to scroll through
the list of specials. When the special you are changing
is
displayed, press
f2
and
ON
or
OFF
to
enable
or
disable that function. Exit the SPECIAL menu by pressing the
f4
key.
Special
0:
Last State Recalled
on
Power Up
Enabling this function tells the instrument
to
remember the instrument settings
that
are present when
the unit is turned off. When the instrument
is
turned back on, the same settings
are
automatically
selected. This effectively provides a dedicated store/recall register
to
eliminate the need
to
reenter the
same settings.
Special
1:
Disable Automatic Phase Reset
When operating ‘normally’ in the Channel Configuration mode, the HP
8904A
resets the relative phase
of each channel
to
zero(0) whenever the frequency
or
destination of any channel
is
changed. (However,
any phase offset
that
has been specified
is
maintained).
All
signals
with a phase setting of zero(0) are
reset
to
a zero-crossing point in there respective waveforms, regardless of the phase relationship that
existed before the change occurred.
Enabling this special function prevents the phase reset from occurring.
As
the frequency
or
destination
of one channel
is
changed, the phase of the other channel(s)
is
not disturbed. Even when you
do
change the frequency
of
a channel, the phase angle of that channel is not changed. This allows for
phase-continuous operation of the
HP
8904A.
This effect can be demonstrated by displaying
a
0.5
Hz
sinewave on an oscilloscope and then changing
the frequency in
0.1
Hz
increments. When this special
is
off,
the phase resets to zero(0) when the
frequency
is
changed when the special
is
on, the phase does not change as the frequency
is
changed.
Special
2:
Disable Beeper
The beeper normally beeps twice when the instrument
is
powered up, and then beeps once any time an
error occurs during operation. Enabling this special prevents the beeper from working in either case.
Special
3:
Reverse Modulating Waveforms
The modulating waveform
is
reversed
to
provide alternate modulating schemes, especially when using
the ramp. Normally the ramp waveform
starts
at
the negative peak and ramps
up
to
the positive peak.
This special function reverses the ramp when used
as
a
modulating source by starting the waveform at
the positive peak and ramping down.
Sine, triangle, and square modulating waveforms are
also
reversed, but the effect
is
not as noticeable
since they are symmetrical waveforms.
3-37
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