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Modulation
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Figure 243 Test setup for constellation and eye diagrams
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Modulation
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Figure 244 (a) Constellation diagram; (b) eye diagram
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last and, as the received signal is degraded, the eye will begin to close and the BER will increase To view a constellation diagram, attach the I to the X input of the scope and the Q to the Y input (which will normally be channel 1 and channel 2, respectively), and turn on the X-Y mode and the display s persistence function Now attach the symbol clock to the external trigger (turn on EXT TRIGGER) The constellation should now be visible, along with faint lines joining the various points The lines are the actual symbol transitions between the constellation points, called the symbol trajectories To obtain just the constellation points (without these symbol transition lines), a BNC connector on the back plate of some oscilloscopes can be connected to the demodulator s symbol clock so that the oscilloscope s electron beam will be turned on only at the moment of sampling This will blank the transitions between constellation dots by triggering the beam only at the constellation point instants Common impairments, and how they look in a constellation diagram, are shown in Fig 245 To view an eye diagram, keep the same setup as above, but turn off the scope s X-Y function Set its HORIZONTAL TIMEBASE to obtain 3 symbol times per 10 divisions, and then view the eye diagram The eye should be open; in fact, the height of the eye can be considered the noise margin of the receiver s output, while the left and right corner of each eye indicate the amount of frequency jitter present The wider the eye, the less jitter, while the taller the eye, the less noise
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BER tests The bit error rate test measures the ratio of bad bits to all the trans-
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mitted bits, frequently over a complete range of input/output powers When
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Modulation
Two
Figure 245 (a) A perfect QAM-16 constellation diagram; (b) a CW signal interference tone effect on the
constellation diagram; (c) signal degradation caused by a poor SNR; (d) signal degradation caused by the digital radio s local oscillator instability; (e) slight overdriving of the transmitter s power amplifier; (f) multipath causing an uneven amplitude across the passband of the digital signal
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Modulation
Modulation
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 correction 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 communications 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 obstructions, 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 diagrams 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 246 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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