Operation
Model
0904A
Although the automatic filter selection algorithm normally picks the optimum filter, there
are cases where you may want
to
manually pick one of the
two
filters.
For
example, you may
want to limit the bandwidth of the noise waveform. By using the gaussian filter, the noise
bandwidth can be lowered from 600
kHz
to
200 kHz.
Manual selection of filters is accomplished by pressing the
FILTER
key, and then entering the
output number you are using
(1
for standard instruments,
1
or
2
for Option 002 equipped
instruments). When the output number
is
entered, the filter that is presently active for that
output
is
displayed. You can then use the
fl
key
to
select the other filter and enable
or
disable
the automatic filter selection function
if
desired.
Phase Resolution
The nominal phase resolution for the
HP
8904A is 0.1 degrees. This
is
the smallest increment
which can be entered from the front panel
or
over
HP-IB.
The actual phase resolution of the
hardware
is
better than the 0.1 degree limit.
A
12 bit word
is
used in the Digital Waveform Synthesis IC
to
set the phase of each channel.
Since the phase range
is
0
to
359.9 degrees, each bit in the twelve bit phase word represents a
phase increment of 0.08789063 degrees. Since the resulting phase resolution from the digital
process
is
not an exact value, the instrument firmware limits the resolution
to
0.1
degrees.
The
HP
8904A attempts
to
set the phase
as
close
as
possible
to
the requested value using
the available phase resolution. This means that for any given phase offset there
will
be a
predictable error caused by the non-exact phase resolution of the hardware. In any case, the
maximum value of the error due to the resolution can be mathematically shown
to
be one
half of the hardware resolution: 0.04394531 degrees.
Although the phase resolution is 0.1 degrees, there are some cases where the phase resolution
can be degraded. Sinewaves and triangle waves always have 0.1 degrees of phase resolution
at all frequencies. However, the ramp and squarewave waveforms can have reduced phase
resolution
at
certain frequencies. These special frequencies are EXACT sub-multiples of the
digital-to-analog converter clock rate of
%
=
1.6777216
MHz.
These are the frequencies
where the squarewave has no jitter and the ramp waveform has greatly reduced jitter. (Refer
to
the
Jitter
information discussed earlier in this section). Example frequencies are: 26,214.4
Hz,
13,107.2
Hz,
6,553.6
Hz,
3,276.8
Hz,
1,638.4
Hz,
819.2 Hz, and 409.6
Hz.
The phase resolution
at these frequencies is limited because the nearly instantaneous transitions in these waveforms
can only occur
at
the clock rate.
For
example;
at
26,412.4
Hz,
one degree
of
phase equals 105.169 ns of time:
24
1
Time for
1
degree
=
126,412,4
Hzx360degrees)
=
105.1.69 ns
-
595.046 ns. This means
However, the DAC clock can change its output every
that for the phase to change
at
all,
the phase must be moved 5.66751 degrees. In other words,
the phase must be advanced enough
so
that the transition portion of the waveform can move
forward
or
backward
to
the next DAC clock. This effect reduction in phase resolution is
reduced by one-half for each successive exact frequency which
is
lower in frequency. This effect
of reducing the phase resolution does not effect square
or
ramp waves
at
other frequencies
because the jitter present averages out the switching point of the fast transitions and therefore
allows small changes in phase. This process is analogous
to
the use of dithering with noise in
digital-to-analog converters.
1
MHz
-
Phase Continuous Switching
In the Channel Configuration Mode, the
HP
8904A does not normally provide phase
continuous frequency
or
amplitude switching. In normal operation, the
HP
8904A performs
a “phase reset” whenever the frequency
or
destination of any of the four channels is
changed. This
is
done to assure that the proper phase relationships between channels will
be maintained. The phase reset brings all the channels into phase alignment, but in doing
3-42
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