3-33
Using the Analyzer
Measurement Techniques
The WDCA sweep rate is limited by the sweep rates available from the laser
source. The precision, normal and fast sweep modes use 5, 10 and 40 nm/s
sweep rates as a default. If increased sensitivity requires a slower sweep, the
instrument will use progressively lower sweep rates. For example, entering a
high sensitivity in the fast sweep mode can cause it to use the 5 nm/s sweep
rate used by precision sweep mode. As you adjust sensitivity, you might notice
that the sweep time has significant discontinuities that occur when the laser
sweep rate is changed. Part of good sensitivity selection is observing the dis-
continuity and staying above it.
Accuracy Unspecified
Only the 5, 10 and 40 nm/s rates have wavelength and amplitude specifica-
tions, the instrument turns on the
Acc Unspec
(Accuracy Unspecified) nota-
tion when sweep rates below 5 nm/s are used. The Acc Unspec notation also
appears when the instrument needs a slower sweep rate than is available from
the laser source, and hence is forced to sweep too fast.
If you want to go to higher sensitivity but avoid the Acc Unspec notation, con-
sider going to the Fast Sweep Mode. This mode has less stringent amplitude
and wavelength specifications than precision or normal, so it can use more fil-
tering without slowing the sweep down. In addition, because it starts at the 40
nm/s laser source sweep rate, it can reduce the sweep rate down to 5 nm/s
before the Acc Unspec notation appears.
For very long sweeps, it may not make sense to sweep twice in order to cover
multiple gain ranges. The WDCA allows the user to turn off automatic gain
ranging and instead use digital filtering to achieve the desired sensitivity. As
with any filter, halving the bandwidth slows the sweep down by half and only
improves sensitivity by 3 dB. To turn off the automatic gain ranging, first enter
the desired sensitivity using
Amplitude
>
Sensitivity
. Then go into the
amplitude setup panel by pressing
Amplitude
>
Amplitude Setup
and
selecting
Auto Ranging: Off
.
N o t e
Sensitivity [Auto]
implies
Auto Ranging Off
, and will change the value in
the amplitude setup panel. Similarly,
Sensitivity [Man]
implies
Auto
Ranging On
, and will change the value in the setup panel when first activated.
This procedure assumes that you have normalized your test setup and posi-
tioned the response on the display. For more information on these processes,
refer to “Normalizing the Measurement” on page 3-11 and refer to “Status
Reporting for Laser Wavelength Zeroing” on page 3-13.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
