Datron 4708 User Handbook Manual page 126

Harmonic
Distortion
Measurement
Appendix 6
to;
4708
User's
Handbook
Section
7
The
measurement of
True Harmonic
Distortion
is
not de-
tailed as part
of the vaification procedure.
However,
some
users
may
wish
to
check
this
feature
from time
to
time,
so
one
or
two
points
need
to
be
clarified.
The
very
low
noise content of
the
4708
output
is
included in
its
accuracy
specifications,
but not
in
its
TOTAL
HAR-
MONIC
DISTORTION
specifications
on
pages
6-5 and 6-
11.
The
latter
relate to true
harmonic
distortion only.
If the
4708
distortion
speciHcation
is to
be
verified,
the
measurement equipment must be
selected with care.
Some
distortion
met^
merely suppress
the
fundamental
and
measure
the
remainder
ov^
a
specific
bandwidth.
The
readings obtained include
wideband
noise
integrated
over
the
full
bandwidth,
introducing
measurement enors which
in-
crease the apparent
harmonic
distmtion (particularly at
low
fundamental
levels,
where
the
harmonic envelope rapidly
descends into the noise
floor).
Measurement
of
true
harmonic
distortion
only,
in
any
signal,
can be a
lalxMious process.
To
measure
the
value of each
single
frequency htumonic,
(as
would be
required to verify
the
4708
THD
specification),
any wideband
noise
must be
filtered
ouL Very
selective
bandpass notch
filtering
is
re-
quired;
this
is
usually
achieved by phase-locking
the
meas-
urement
circuitry to
the
signal,
as
performed
by
a selective
signal level meter.
A
modem
automatic spectrum analyser
is
more
satisfactory.
The
harmonic amplitudes can be displayed
on
a
screen
against
a
grid,
the noise levels
can
also
be
seen, oc a cursor
can
be used
to set
a bandpass
notch
filter
to the
harmonic
fre-
quency
required,
to
give a
direct digital readout.
THD
measurement
is
simplified
and speeded
up by
this
method.
It is
therefore
recommended
that either
a
spectmm
analyser
or
selective level
meter be employed.
Suitable instruments
are:
Hewlett-Packard
HP3585
Spectrum
Analyser,
or
HP3586
Selective
Level Meter.
7-A6-1