Model
8904A
Operation
How
the
HP
8904A
Generates
a
Signal
Waveforms are generated
by
a numerical synthesis technique. The heart of the synthesizer is a single
digital waveform synthesis integrated circuit (DWSIC). For each channel, the DWSIC generates a
continuous stream of numbers which represent instantaneous levels of the waveform. These digital
values are then converted
to
an analog signal by
a
digital-to-analog converter or
DAC
(one converter
for each output port).
The analog signal
is
conditioned by conventional analog circuitry and routed
to
the output connector.
The conditioning circuits include a sample-and-hold
to
remove
DAC
switching noise, filters
to
remove
quantization noise (smooth the transitions between samples from the
DAC),
amplifiers
to
boost the
maximum output, and attenuators
to
increase the outputs dynamic range.
For more detailed information on how the
HP
8904A
creates signals, refer
to
the
HP
8904A
Multifunction Synthesizer
Service Manual.
Operating Considerations:
All
Instruments
Amplitude Resolution
The amplitude resolution of the
HP
8904A
is
provided by
a
combination of digital and analog controls
and
can be specified in smaller increments than what
is
displayed on the front panel of the instrument.
Because of this method of adjustment, the amplitude resolution varies between output voltage levels.
Refer
to
the
Operating Technical Reference: Amplitude Resolution
information included later in Section
3
if
you need more information regarding very fine amplitude adjustments.
Floating Output
The
HP
8904A
is designed
to
allow the user
to
have a floating ground
that
is independent of the
chassis ground of the instrument. This is why there
is
a
LOW
and
HIGH
connection for each output.
You
may
also prefer a ‘normal’ chassis referenced ground for
your
application, using the shield
of
the
HIGH
connector for the ground reference.
The
FLOAT
control allows you
to
select the type of signal reference you require. The instrument always
powers up in the ‘floating ground’ mode.
If
you have specified the correct output destination and
amplitude for a signal, and there appears
to
be no signal present on the output, check
to
see that you
have specified the correct ground reference for your application.
Waveform Jitter
Due to the digital conversion process used
to
create the different waveforms in the
HP
8904A,
a
partial
waveform can be present when using a ramp, square, or triangle waveform. This type of distortion is
referred
to
as
‘jitter’. This affect can be seen by displaying a
48
kHz
square wave on an oscilloscope.
The highbow transition period appears to have
a
second offset trace. For more information on what
causes this affect, refer to the
Operating Technical Reference: Waveform Jitter
information included
later in Section
3.
3-7
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