EE-4022 Experiment 4 Digital Modulation ASK and FSK

Transcription

1 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page 4-1 EE-4022 Experiment 4 Digital Modulation ASK and FSK Introduction: In this experiment the student will experiment with (a) modulation of a sinusoidal carrier by a digital message signal using amplitude shift keying (ASK), (b) demodulation of an ASK signal to recover the digital message signal, and (c) digital modulation and (optionally) demodulation using FSK. Published Resources: TIMS-301 User Manual (Issue No. 1.6, October 2004) describes each basic TIMS module. Communication Systems Modelling with TIMS, Vol. D1 and D2 Fundamental Digital Experiments by Tim Hooper (Issue No. 4.9, 2004) Instructor s Manual to accompany Communication Systems Modelling with TIMS includes notes on the TIMS experiments Equipment Needed: TIMS system with the following modules: o Audio Oscillator module o Master Signals module o Frequency Counter module o Sequence Generator module o Multiplier module o Tunable Low Pass Filter (LPF) module o Utilities module (for the Rectifier) o PC-Based Instrument Inputs (earlier TIMS called this Scope Selector) if desired o Voltage-Controlled Oscillator (VCO) module (for FSK modulator steps) o Variable DC module (for FSK modulator steps) Oscilloscope (e.g., Agilent MSO-X-3014A oscilloscope (100 MHz, 4 Gsa/s)) Prelaboratory Investigation: 1. Review information on digital modulation methods, such as amplitude shift keying (ASK) and frequency shift keying (FSK), from EE-4022 lecture material and available textbook(s) on electronic communication systems. 2. Read through the Laboratory Investigation and Appendix sections of this laboratory document that pertain to ASK. 3. If you are assigned to also do the FSK steps (ask your instructor), then read through the Laboratory Investigation and Appendix sections of this laboratory document that pertain to FSK. If any alternative or additional FSK experimentation is assigned by your instructor (such as other items within the TIMS FSK experiment within the document titled Communication Systems Modelling with TIMS, Vol. D1 and D2), then read those sections as assigned.

2 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page 4-2 Laboratory Investigation: Part A. Constructing the ASK Modulator Background: In this part, the equipment will be configured to generate a kbps digital message signal having a pseudorandom data pattern, and use that message signal to modulate a sinusoidal carrier at khz. The digital modulation method will be amplitude shift keying (ASK), more specifically, on-off keying (OOK). The frequency spectra of the pseudorandom data signal and corresponding ASK signal will be observed on the oscilloscope for a system that does not band-limit the digital message signal going to the modulator, and again for a system that first band-limits the digital message signal using a low-pass filter (LPF) and then modulates the filtered message signal. The SEQUENCE GENERATOR module is used to obtain a pseudorandom binary message signal at kbps. One method of generating an ASK signal is to use an electronic switch that is controlled by the data signal, as illustrated on Figure A1-2 in Appendix 1. Whenever the data signal is at logic-1 then the electronic switch allows the incoming carrier to go through to the ASK output (that is, an ON condition), and whenever the data signal is at logic-0 then the electronic switch blocks the incoming carrier from going through to the ASK output (that is, the output is then OFF). Hence this is called on-off keying (OOK), and the OOK signal is illustrated on Figure A1-1 in Appendix 1. If an electronic switch is used, its control signal is digital, and in this case the bandwidth of the resulting ASK signal cannot be controlled by smoothing out (i.e., low-pass filtering) the message signal that controls the switch. Therefore the switch shown on Figure A1-2 is sometimes replaced by an analog multiplier, and the binary message signal levels are chosen to be 0V and some positive voltage value such as 5V, resulting in an on-off ASK signal. If an analog multiplier is used, the message signal can be filtered prior to multiplication. The carrier signal that will be passed or blocked in this experiment will be at khz, shown on Figure 1 going from the AUDIO OSCILLATOR module to the MULTIPLIER module. On Figure 1, the khz TTL signal from the MASTER SIGNALS module is used to synchronize the AUDIO OSCILLATOR output. Steps: 1. Put both DIP switches on the SEQUENCE GENERATOR module circuit board in the UP position. This will result in a 32-bit-long pseudorandom bit sequence (PRBS) that is repeatedly transmitted. Build the ASK modulator of Figure 1, and put the multiplier AC/DC switch in the DC position. Note that the AUDIO OSCILLATOR needs to be tuned near khz prior to connecting its synchronization input to the khz TTL signal from the MASTER SIGNALS module. This synchronization causes the kHz carrier frequency to be synchronized with the 2.083k bps data rate. Why would we want these synchronized? O

3 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page Use two scope inputs to simultaneously observe the PRBS data pattern and the ASK signal. Connect the SYNC signal out of the sequence generator to a third scope input and trigger the scope from the SYNC signal. The PRBS data signal is not as good for triggering because of the pseudorandom data pattern. The scope HF Reject filter on the trigger function might be needed for good triggering. Use the scope horizontal Fine mode to adjust the sweep rate to show 4 bits (at kbps) over one display division: (4 bits/division) / kbps = 1.92 ms/division Capture the data and ASK signal waveforms. 3. Now view on the scope the kbps data signal waveform while also viewing the frequency spectrum of the same data signal (suggest a span from 0 to 10 khz; adjust the horizontal sweep rate to give an appropriate sampling rate for this spectrum, calculated as follows: If spectral frequencies up to 10 khz are to be observed then the sampling rate needs to be at least 20 khz, so the time between sample points cannot exceed 1/20k or 0.05 ms. Assuming that the MSO-X-3014A oscilloscope uses approximately points across the horizontal display, which has 10 divisions horizontally, there are 6400 points per horizontal division. The 6400 points cannot span more than (6400)(0.05 ms), or 320 ms. Therefore the horizontal control setting cannot be greater than 320 ms per division because 320 ms/ corresponds to sampling at 20k samples per second. Therefore use a setting below 320 ms/. 4. On the TUNABLE LPF (Low-Pass Filter module), select the Normal frequency range using the front panel switch (this allows a tunable cutoff frequency between 900 Hz and 5 khz) and turn both TUNE and GAIN controls fully clockwise, resulting in maximum gain and a cutoff at approximately 5 khz. Insert this LPF between the SEQUENCE GENERATOR message data output and the MULTIPLIER input that normally accepts the message data signal. (One scope channel should now be connected to the filter input and a second scope channel connected to the filtered data signal at the LPF output.) 5. Adjust the filter GAIN on the LPF so that the filtered data signal on the scope goes between 0V and +5V (same levels as at filter input). Now adjust the LPF TUNE control until the filtered data signal s spectrum displayed on the scope appears to be cut off at about 2 khz. What is the bandwidth of this filtered data signal? 6. Now view both the waveform and frequency spectrum of the ASK signal (suggest a frequency span between 0 and 20 khz). Use the cursors with the displayed spectrum (i.e., the MATH function) to estimate the bandwidth of the ASK signal, from the displayed spectrum. This might be difficult assume that the bandwidth is defined to exclude the frequency regions where all of the spectrum components are at least 12 db below the peak spectrum component levels if you ignore the discrete frequency component spike at f C). Also estimate the carrier (that is, center) frequency from the displayed spectrum. 7. While observing the effect on the ASK spectrum, bypass the LPF that was inserted by disconnecting the connections to the LPF output and connecting them to the connections at the LPF input. When the LPF is bypassed, do the unfiltered lobes that were observed on the spectrum of the unfiltered data signal now appear on each side of the carrier frequency? Part B. Constructing the ASK Demodulator In this part, two TIMS modules will be used to construct an envelope detector for demodulating the ASK signal that was generated in Part A. The original data signal will be compared to the recovered data signal out of the ASK demodulator. 1. Because the LPF is bypassed (see step 7), disconnect anything that is currently connected to the LPF input, without upsetting connections between things that were all connected to the LPF input. The LPF is now available to be used as part of an envelope detector.

4 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page Without disturbing the operation of the ASK modulator having an unfiltered data signal (from step 7), connect the ASK output to the input of an envelope detector (that is, a diode followed by a LPF as shown on Figure 2). On the scope, use three oscilloscope inputs to simultaneously observe the original data signal from the SEQUENCE GENERATOR, the ASK signal, and the recovered data signal at the demodulator output (that is, out of the LPF on Figure 2). Adjust the GAIN and TUNE controls on the LPF to get the demodulator output to look similar to the original data input signal, and capture the scope image. Part C. FSK Modulation and Demodulation Perform steps below and/or other steps from the TIMS experiment on FSK (refer to the Communication Systems Modelling with TIMS, Vol. D1 and D2 Fundamental Digital Experiments), as assigned or suggested by your instructor. Steps: 1. On the VCO module, put the on-board switch in the VCO position. Put the VCO front-panel switch on LO. If we want the carrier to be at khz and the peak frequency deviation to be 2 khz, then the VCO outputs will be chosen as khz for logic-0 input and khz for logic-1 input. 2. Connect a ground to the VCO input and adjust its fo control to get an output at khz. Then connect the VCO input to +5V (available on the VARIABLE DC module) and adjust the VCO gain control to get an output at khz. 3. Connect the FSK generator of Figure 3. Observe both the data signal from the SEQUENCE GENERATOR and the FSK signal from the VCO on the oscilloscope. Suggested sweep rate is 1 ms/division. Also observe the spectrum of the FSK signal.

5 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page 4-5 Report Requirements: 1. Explain your observed waveforms and frequency spectra. 2. Discuss your methods of measuring and/or estimating bandwidth of the signals. 3. When band-limiting is applied to the data signal, estimate the lowest cutoff frequency that would seem feasible for data at kbps. APPENDIX 1. ASK Generating ASK: Amplitude shift keying (ASK), in the context of digital communications, is a modulation process which uses a sinusoidal carrier, and uses two or more discrete amplitude levels that are to represent the digital information being conveyed. The number of discrete amplitude levels is related to the number of characters there are in the set of characters being transmitted. For example, for binary data there are two possible characters (logic 0 and logic 1), and two discrete signal amplitudes can be used to convey the two characters. One type of ASK, called on-off keying (OOK) uses two discrete amplitudes, one of them being zero amplitude (or OFF signal) and the other having a nonzero amplitude (or ON signal). For OOK, the modulated waveform consists of bursts of a sinusoid. Figure A1-1 illustrates an ASK signal (actually an OOK signal), and just above the ASK signal is shown the binary message signal that is conveyed by the ASK signal. Neither signal shown on Figure A1-1 has been band-limited Figure A1-1. Binary digital message waveform (top) and corresponding ASK/OOK signal (bottom) There are sharp discontinuities shown at the transition points for both the message signal and ASK signal on Figure A1-1. These result in the signals having unnecessarily wide bandwidths. Band-limiting is generally introduced before transmission, in which case these discontinuities would be rounded off. The band-limiting may be applied to the digital message, or to the modulated signal itself. To get an integer number of carrier cycles in a bit interval, the data rate can be made a sub-multiple of the carrier frequency. This is the case for the waveform on Figure A1-1. One of the disadvantages of ASK, compared with FSK and PSK, is that the ASK signal does not have a constant envelope. This causes the power amplification to be more difficult, since linearity becomes an important factor. However, it does allow the use of simple demodulation techniques, such as the use of an envelope detector. A block diagram of a basic ASK generator is shown on Figure A1-2. This ASK generator includes band-limiting using a band-pass filter after modulation.

6 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page 4-6 The switch is opened and closed in accordance with the binary sequence (message). Figure A1-2. ASK generator (providing filtered OOK waveform) Bandwidth containment: As stated above, sharp discontinuities in the ASK waveform of Figure A1-1 can result in a bandwidth wider than necessary. A significant bandwidth reduction can be accepted without causing a substantial affect on the ability to demodulate the data with a low probability of bit errors. This can be brought about by band-limiting (that is, low-pass filtering called LPF) the message before modulation, or by band-limiting (that is, band-pass filtering called BPF) the ASK signal itself after modulation. Both these options are illustrated on Figure A1-3. In this experiment, the low-pass filtering (LPF) of the message will be implemented and tested. The frequency-domain process of band-limiting can be thought of as pulseshaping of the waveform in the time domain. binary digital message waveform sinusoidal carrier Figure A1-3. ASK generator showing both message-signal LPF and ASK-signal BPF for band-limiting Figure A1-4 shows the signals present in the ASK generator shown on Figure A1-3, where the message has been band-limited. The shape, after band-limiting, depends upon the amplitude and phase characteristics of the band-limiting filter.

7 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page 4-7 Figure A1-4. Original binary digital message (lower), band-limited message (middle), and ASK signal resulting from band-limited message (upper) ASK demodulation: It is apparent from Figures A1-1 and A1-4 that the ASK signal has a well-defined envelope. Thus it is amenable to demodulation by an envelope detector. A synchronous demodulator would be an alternative to the simple envelope detector demodulator. The synchronous demodulator would require the use of a phase-locked local carrier signal and therefore carrier acquisition circuitry, but would have superior biterror-rate performance. With band-limiting of the transmitted ASK signal, the envelope detector will not recover the original binary waveform but instead will have an output that is a band-limited version of the message waveform, such as the middle waveform on Figure A1-4. In this case, further processing by some sort of decision-making circuitry for example would be necessary. This demodulation process would then be a two-stage process: 1. recovery of the band-limited bit stream 2. regeneration of the binary bit stream Figure A1-5 illustrates typical waveforms out of each stage of this two-stage process. time Figure A1-5. Typical output waveforms for envelope detection of band-limited ASK and regenerated digital waveform

8 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page 4-8 ASK signal bandwidth: It is easy to estimate the bandwidth of an ASK signal. Refer to the block diagram of Figure A1-3. This is a DSB-SC transmitter. It is an example of linear modulation. If we know the message bandwidth, then, as in the case of DSB-SC, the ASK bandwidth will be twice this, centered at the carrier frequency. The factor of two is because the message bandwidth is replicated above and below the carrier frequency (upper sideband and lower sideband, respectively). Even though you may not have an analytical expression for the bandwidth of a pseudo-random binary message sequence, you can estimate that it will be approximately the same as that of a square-wave data sequence of alternating logic ones and zeros (that is, ). Recall that for the case of a binary message sequence of alternating ones and zeros, if the average signal voltage is positive (as in the case of TTL voltage levels of 0V and 5V), then there will be a DC component and additional components at the fundamental square-wave frequency f 0 (which is one-half the bit rate R b) and at odd harmonics of that fundamental frequency. Because the only component necessary to recover the data is the fundamental frequency component, the essential bandwidth is then equal to the fundamental frequency component, which is equal to 0.5 R b. As stated above, the bandwidth of the ASK signal is twice that of the message signal, and therefore the minimum bandwidth of the ASK signal is B ASK = R b. This ASK signal bandwidth is centered at the carrier frequency. Note that if the message data sequence is pseudo-random but has a DC level, then its spectrum will be continuous (with a sin Kf / Kf shape) but will also have a discrete component at frequency zero due to its DC level. This causes the ASK signal spectrum to have a discrete component at the carrier frequency but otherwise have a continuous spectrum (with a sin K(f - f C) / K(f - f C) shape), centered at the carrier frequency, f C. This ASK spectrum is depicted on Figure A1-6. Amplitude f C - R b f C f C + R b Frequency Figure A1-6. Typical ASK magnitude frequency spectrum when modulated by pseudo-random binary data message at bit rate R b APPENDIX 2. FSK Generating FSK: As its name suggests, a frequency shift keyed (FSK) transmitter has its frequency shifted in accordance with the message. Although there could be more than two frequencies involved in an FSK signal, in this experiment the message will be a binary bit stream, and only two transmitted frequencies will be involved.

9 EE-4022 MILWAUKEE SCHOOL OF ENGINEERING 2015 Page 4-9 The word keyed suggests that the message is of the on-off (mark-space) variety, such as one (historically) generated by a morse key, or more likely in the present context, a binary sequence. The FSK output from such a generator is illustrated on Figure A2-1 below Figure A2-1. Binary digital message waveform (top) and corresponding FSK signal (bottom) The FSK transmitter could consist of two oscillators (at frequencies f 1 and f 2), with only one being connected to the output at any one time, as shown in block diagram form on Figure 2 below. binary message at bit rate R b Figure A2-2. Functional block diagram for FSK transmitter Unless there are special relationships between the two oscillator frequencies and the bit clock rate, there will be abrupt phase discontinuities of the output FSK waveform at the times that mark the bit interval boundaries. The FSK waveform shown on Figure A2-1 does not have these phase discontinuities, and it is therefore referred to as continuous phase FSK (CPFSK). FSK signal bandwidth: In practice, the two transmitted frequencies, f 1 and f 2, have a separation that is selected to result in a particular FSK signal bandwidth, and these frequencies are often chosen to be integer multiples of the bit rate. This leads to the possibility of continuous phase, which offers advantages, especially with respect to bandwidth control. Alternatively, the frequency of a single oscillator (VCO) can be switched between two values, thus guaranteeing continuous phase FSK (called CPFSK). The continuous phase advantage of the VCO is not accompanied by an ability to ensure that f 1 and f 2 are integer multiples of the bit rate. This would be difficult to implement with a VCO. Refer to course lecture material for FSK bandwidth calculations.