Page 94 - Complete Wireless Design

Modulation

Modulation 93

these powers are low, BER rate increases because of degraded SNR; when they
are high, amplifier compression is the major contributor to poor BER.
BER testing is an excellent way to check the signal quality of a digital radio
and its link. However, any in-line error correction and/or adaptive equalization
will make the BER appear far higher than it actually would be without these
two processes. Thus, your view of how close to a complete outage the digital
signal is may actually be hidden by error correction and equalization circuits.
This is why many receiver BER system tests are best done before the correc-
tion and equalization stages in order to obtain the raw BER. Nonetheless, it is
still quite useful to perform BER tests from one end of a complete communi-
cations system to the other—with correction and equalization engaged—to
confirm that the entire system meets BER specs over a set time period, and
the link is functioning as designed on a system level.
To prevent a digital wireless link from failing, it must be completely tested
to confirm that there is enough link budget to overcome any impairments
between the transmitter and receiver. Nevertheless, a generally poor BER in
a complete end-to-end system measurement will not indicate exactly where
the trouble lies. Finding the location of the weak stage in a communications
link is done by error distribution analysis. The fault could be caused by poor
antenna alignment, overdriven amplifiers, low signal output, path obstruc-
tions, multipath, cable losses, frequency drift, component malfunction, etc.
BER problems can be tracked down by observing the signal through a vector
network analyzer (VNA), by viewing the receiver’s constellation or eye dia-
grams with an oscilloscope, or by watching the signal in the frequency domain
on a spectrum analyzer.
To perform a BER test, a pseudo-random bit sequence is injected into the
transmitter’s baseband or IF input. Then, the signal to be BER tested can be
observed at the transmitter’s antenna output into a digital test receiver and
on to the bit error rate tester (BERT) or at the receiver’s antenna input into a
digital test receiver and on to the BERT, or after the receiver’s demodulator
directly into a BERT. In some high-accuracy, low-BER systems, performing a
complete BER test can take anywhere from a few minutes to several hours
because of the low amount of bit errors actually generated.
In the design phase, we should confirm that the BER will meet expectations,
since it may be found that in order to maintain a proper link budget (see Chap.
9, “Communication Systems Design”) we must increase transmit power,
strengthen the receiver and transmitter antenna gain, and lower receiver NF
to preserve the desired path length and quality of service at our preferred BER.

Two-tone test and measurement. The two-tone test to measure IMD has been a
vital part of analog radio for years, and is still important in digital radio for
preliminary testing.
To measure the two-tone third-order products at the output of a receiver—
at a set RF input level—hook up the test gear and receiver as shown in Fig.
2.46. Feed two signals, equal in amplitude and closely spaced in frequency,
from the two signal generators into the combiner. The combiner, which will

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