Model
8904A
Operation
so
causes a glitch Any phase offsets have specific meaning when the frequencies of the four
channels are harmonically related.
If
the channels are at non-harmonically related frequencies,
the phase reset will cause all the channels to reset
to
zero phase
(or
the specified phase offset,
if any). From that point, the channels will each continue on accumulating phase at their own
rate.
If
phase continuous frequency switching is desired, special function
#1
can be activated.
This special function
will
disable the automatic phase reset
so
that frequency switches
will
be
phase continuous. Activating this special
will,
however, result in operation where the specified
phase relationships of the four channels
will
not be valid. To overcome this, the user can
perform a manual phase reset
to
realign the channels from the front panel by pressing the
blue shift key and the the
f3
softkey.
Amplitude switches in the Channel Configuration Mode are also not phase continuous. When
the amplitude
is
changed, the output will be blanked
for
approximately
13
ms. This is done
so
that amplitude glitches resulting from changing analog attenuators
will
not be allowed
to
reach the device under test. There are
two
methods of achieving glitch free amplitude
switches with the
HP
8904A. The first method
is
to
use special function
#4.
This special
disables the output blanking function. With special function
#
4 active, amplitude switches
within
a given analog attenuator range will be phase continuous (see Amplitude Resolution
for
details on these ranges). When an attenuator range
is
crossed, however, there
will
be a
jump up
or
down in level depending on which attenuator switches
first..
The second method
is
to
use the capabilities of Option
003.
Option
003
gives the
HP
8904A the ability
to
fast
hop
the frequency, amplitude, or phase
of
channel
A.
Any amplitude hops using this
fast
hopping
mode will glitch free. The HP 8904A fixes the analog attenuation range
to
accommodate the
highest amplitude loaded into the
HOP
RAM memory.
All
lower amplitude values are created
using the multiplier in the Digital Waveform Synthesis IC. The disadvantage of using this
method
is
that
for
large amplitude jumps
(>
6
dB),
fewer bits will be used which degrades the
accuracy and residual distortion of the waveform.
In the Tone Sequence and
DTMF
Sequence Modes, the HP 89044A will perform phase
continuous switches between frequencies if no
off
time is specified between tones.
If
some off
time has been specified, the HP 8904A will cutoff the sinewave at the set time (regardless of
where the switching point will be) which will cause an audible click in the waveform. This
is
caused b;y the sudden drop
to
zero volts which is composed of high frequency energy.
If
this
glitch is not desired, the on and off times specified must be modified
so
that the switching
will occur at a zero crossing.
3-43
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