Tektronix AWG 2021 manuale d’uso

Signals and Measurements for W ireless Communications T esting[...]

2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Analog Carriers and Modulation 1 Basic Sine Wave Amplitude Modulation (AM) . . . . . . . . . . . . . . .3 2 AM with Adjacent Carriers . . . . . . . . . . . . . . . . . . . . . . . . . .5 3 Multi-T one T esting . . . . . . . . . . . . . . . . . . . . . . . . . . . . [...]

3 One of the most challenging tasks in designing wireless communications products is the develop- ment of a rational approach to characterizing and testing components, assemblies, and sub-systems. Baseband modulation and RF signal characteristics are becoming increasingly complex as standards and common sense force more efficient use of the finite [...]

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5 The best introduction to the AWG is to parallel the procedure of generating a carrier with a conventional signal generator. With a signal generator, one simply enters the carrier frequency and the output ampli- tude, such as 1000 kHz at 0 dBm. With an AWG, one creates a sequence of points to represent the waveform: A sin ω c t where A is the pea[...]

6 A record length must be selected that has an adequate number of points to reconstruct the desired waveform. The waveform period is 1 ms and there are 1000 carrier cycles in this period. A record length of 20,000 points would allocate 20 points per cycle, which adequately over- samples the ideal waveform. Any sampling system must sample at least t[...]

7 A simple addition to the AM signal demonstrates the flexibility of equation-based waveform descriptions. A common task in evaluating receiver performance is to evaluate the effect of adja- cent carriers. For the basic AM signal, one can easily add modu- lated carriers 10 kHz above and below the original signal (Figure 4). One simply adds two copi[...]

8 Frequency (kHz -90 -80 -70 -60 -50 -40 -30 -20 -10 0 980 985 990 995 1000 1005 1010 1015 1020 Figure 6. Spectrum analyzer plot of the 3 carriers. There are 3 kHz AM on the adjacent carriers and 1 kHz AM on the original carrier. Note the low level of close-in spurious components. Magnitude (dBm)[...]

9 Multi-T one T esting 3 The logical extension of adjacent carrier testing is multi-tone test- ing. In addition to simulating multiple carriers in a multi- channel system, multi-tones can quickly test filter response when a scalar or network analyzer is not available, or they can iden- tify intermodulation products resulting from saturation or non-[...]

10 The 11 tone equation was then modified so that the last 5 tones (71 through 75 MHz) are inverted. The two different multi-tone results are shown in Figure 9. The scope shows that the rms levels of the two signals are identical, but the peak-to- peak values are different. All eleven tones in the original signal added in-phase at t=0. This was not[...]

11 Frequency modulation introduces control of the phase argument, Φ , in the basic carrier equation: A sin ( ω c t + Φ ). FM is implemented by varying Φ in direct proportion to the integral of the modulating signal. Thus, for a modulating signal m(t), the FM signal can be written: A sin ( ω c t + k ∫ m(x) dx ) where k sets the peak frequency[...]

12 While basic single-tone FM is a built-in function of virtually all conventional signal generators, dual- tone FM modulation clearly contrasts the flexibility of the AWG approach. Dual-tone modulation tests can be used to measure intermodulation prod- ucts in a noise reduction compandor (compressor- expander) in FM receivers such as cordless phon[...]

13 Figure 13. The TDS 744A shows the intermodu- lation performance with expanders disabled and enabled. There is no distortion with the expander disabled. The first order intermodulation products are about 35 dB below the fundamental tones with expanders enabled. Figure 13 shows the demodu- lated output from an FM receiver with the expander disable[...]

14 A final example of conventional analog modulation combines most of the above techniques to simulate the stereo modulation used in broadcast FM. The modulating signal consists of three components, 1) the composite audio which is the sum of the left and right (L+R) channels, 2) the stereo pilot signal which is a 19 kHz tone, and 3) the difference [...]

15 The resulting 455 kHz signal is mixed up to the broadcast band and inserted into a stereo receiver. The stereo indicator is turned on, and the resulting left and right output signals are captured on the TDS 744A scope (Figure 15). The upper two traces are the right channel (1000 Hz) signal and spectrum. The lower two traces are from the left cha[...]

16 Although the removal of noise is a common design goal, a noise source can be an extremely useful test stimulus or signal impairment. The AWG 2041 provides a built-in noise func- tion, but its characteristics are quite different than traditional sources such as noise diodes. An AWG 1 noise waveform is actu- ally a calculated series of pseudo-rand[...]

17 the AWG’s 10 MHz low-pass filter (middle trace). The TDS 744A FFT spectra for the two signals are overlaid below the time domain waveforms. The salient characteristic of the unfiltered noise spectrum is that it rolls off with a (sin x)/x func- tion with the first null at the 32.768 MHz clock frequency and subsequent nulls at multiples of the c[...]

18 The AWG’s graphical waveform editor provides a variety of mathematical operators for exist- ing waveforms. Waveforms can be combined with other waveforms, or a waveform can be squared, scaled, differenti- ated, integrated, etc. Combining the Noise with the Carrier The signal and noise waveforms are summed using the AWG’s waveform editor (Fig[...]

19 Digital Phase Modulation — PSK 8 The modulating signals in the foregoing examples have been sinusoidal or continuous wave- forms. A simple step to digital modulation is made with a slight variation to sinusoidal modula- tion. Figure 21 shows one cycle of a sinewave that has been quantized into steps between –0.5 and +0.5. The equation defini[...]

20 The record length of 1024 points and a waveform period of 1 µs requires a sampling rate of 1.024 GHz. The resulting carrier frequency is 50 MHz. Since each level represents one of eight states or symbols, 3-bits of data can be transmitted per symbol. Of course, no data per se is associated with this particular modulating pattern since a sinusoi[...]

21 Baseband Digital Patterns 9 Before continuing with exam- ples of digital modulation, it is important to establish a method of creating arbitrary test data patterns. Figure 24 shows direct entry of a 28-bit binary pattern. In this case, the 0 or 1 value of each data bit is repeated for 1000 points in the record, which requires a record length of [...]

22 The simplest example of digital modulation is to turn the carrier on or off, depending on the state of the modulation data. On-off keying (OOK) can be directly implemented by multiplying a carrier by the 1 or 0 value of the data pattern. This example uses a 10.7 MHz carrier created in a 28,000 point record to match the record length of the data [...]

23 The modulating data alters the carrier frequency in frequency-shift keying (FSK). A digital modulation index of 0.5 is used in this example; that is, the frequency shift will be 1 ⁄ 2 the 40 kbaud data rate or 20 kHz. If the carrier remains centered at 10.7 MHz, this results in the two data frequencies of 10.710 MHz and 10.690 MHz. Figure 28 s[...]

24 As previously mentioned, the AWG’s two binary marker output signals can be modulated with a data pattern. Figure 30 shows how this can be used as a tool for testing or troubleshooting digital receivers. One marker output is programmed to generate a trigger pulse at the beginning of each 700 µs record (top trace). The second marker is programm[...]

25 Multi-level data modulation splits the amplitude, frequency, or phase of the carrier into more than two discrete states. 8-PSK previously demonstrated direct control of the phase Φ in the equation A cos( ω c t + Φ ) ; A was constant. The eight symbols were equally spaced points around the polar axes. Alternatively, the I-Q mapping can be used[...]

26 Figure 32. Quadrature amplitude modulated (QAM) signal generated by combining an amplitude modulated cosine carrier (upper) and an amplitude modulated sine carrier. There are 16 symbols, so this is 16-QAM. I In I/Q Modulated RF Out Discrete Q Signal Discrete I Signal Ch. 2 Out Ch. 1 Out AWG RF Generator Controller (PC) Oscilloscope (DSO) Q In DU[...]

27 One effect of the edge transitions in digital modulation patterns is a wider than desired occupied spectrum of the transmitted signal. The solution is to filter the baseband digital signal before it modulates the carrier. The two most common filter types for this application are Gaussian and Nyquist filters. Application of the Gaussian filter is[...]

28 The convolution result is 30,000 points long. Note that the impulse response is 2000 points long, which is longer than the 1000 points per data bit. This means that each data bit affects more than the 1000 points that it immediately occupies. Hence, a possible anomaly must be accounted for in the convolution process. The AWG assumes that the dat[...]

29 Figure 36 compares the original and filtered data patterns. The upper two traces are the unfil- tered data pattern and its spec- trum. The lower two traces are the filtered data pattern and its spectrum. Note how the spec- trum of the filtered version rolls off more quickly. The spectrum of a modulated carrier shows the same results. Figure 37 s[...]

30 The final example of digital modulation spreads the energy in a BPSK signal by amplitude modulating the carrier with a spreading pattern. In the same way that the baseband data pattern spreads the energy of an unmodulated carrier, a spread- ing pattern further spreads the energy of a modulated carrier. Pseudo-random sequences are generally used [...]

31 For More Information on T ektronix Instrumentation Tektronix offers a broad line of signal sources and electronic measurement products for engineering, service, and evaluation requirements in virtually every industry. For detailed information about the Tektronix tools used in developing this booklet, consult the appropriate brochures and data sh[...]

32 Tektronix AWG Arbitrary Waveform Generators give the most extensive capabilities for editing waveforms, with 8 or 1 2 bits of vertical resolution and waveform frequencies to 500 MHz. AWGs contain a high speed, high resolution digital to analog convertor with sophisti- cated triggering and mode settings, plus up to 4 megabytes of internal memory [...]

33 The TDS 744A represents the next generation of digitizing scope performance. This versa- tile general-purpose instrument introduces Tek’s new InstaVu ™ acquisition feature and sets a benchmark in waveform capture rate for DSOs. The TDS 744A can display more than 400,000 acquisitions per second—a rate 2,500 times faster than the most advanc[...]

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36 4/97 WCI 76W–10555–1 Copyright © 1997, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registered trademarks of T[...]