28
4430-V & VN BPSK/QPSK VME Demodulator & Optional Modulator User’s Manual
Doc: 4430_V,VN.fm, © 11 Jul 2005, 14:55
3.3.9
Calculation of the Input Low Pass Filter Frequency
To calculate the demodulator input low pass filter frequency, the demodulator
carrier frequency must be added to the symbol rate multiplied by two. The fol-
lowing equation applies.
1.
f
L
= Demod fc + (2 x Symbol Rate)
3.3.10 Setup Precautions
Certain values and selections within
Section 3.3
and
Section 3.4
are interrelated.
Settings that are likely to provide intermittent or unstable operation of the de-
vice should be avoided. Likewise, since there are several analog sections of the
board design, some of the ranges provided in the previous sections have bell
shaped operational characteristics. This does not point out deficiencies in the
design but rather the nature of analog designs and part tolerances.
➢
NCO Setup Sequence
There are two critical aspects of setting up either the dual demodulator NCO or
the modulator NCO: the sequence of setup steps, and repeating the setup steps.
The registers must be setup in the sequence shown in
Section 3.5
. This sets all
NCO registers in a programming state.
All NCO registers setup sequences should be executed twice. The NCOs used
in the hardware design of the 4430 require that they be written to twice to ensure
proper setup. This is a response to an anomaly from the NCO manufacturer.
➢
Data Filter Settings
Another concern is to make certain that the data filters are not set to a frequency
that is too close to the loop bandwidth (LBW) setting. In the case that the set-
tings are to close to the LBW, the loop filter will be impacted and the demodu-
lator may not lock up correctly. The data filter cutoff (f
D
) should be at least 20
times the LBW setting.
➢
Loop Bandwidth Setting
When selecting the loop bandwidth, selecting a value at the bottom of the LBW
range, increases the likelihood of inconsistent demodulator lock. Selection of a
reasonable loop bandwidth setting is critical for consistent operation. Unless the
f
L
=
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com