COMMUNICATIONS TRAINER EC-796 INSTRUCTIONS MANUAL 1 DESCRIPTION The digital communications trainer is an ideal instrument to help teach basic types of digital modulations (ASK, FSK, PSK, DPSK, DQPSK and QAM), as well as their forerunners: sampling and quantization (PCM signals). Modulations of various channel types and states are studied in such a way that the advantages and limitations of each can be compared easily. Given the essentially educational nature of the instrument, discrete integrated circuits are used so that modulation and demodulation circuits can be understood directly. No commercial integrated circuits using digital modulation/demodulation subsystems are employed, making internal signals clearly observable. Thus circuit operation can be checked from various test points accessible to the user. Meanwhile it must not be forgotten that the focus of modern digital communications is on systems. Basic features are emphasised in training, such as the limitations of each type of modulation according to the channel, the comparative advantages between them and how modulator and demodulator circuitry is made up. Thus it is our intention to provide a basically experimental training which will subsequently enable the student to understand the bases of applications such as telemetering, modems, some satellite receivers, private communications networks, etc., and help the student to be able to come to terms with more complex modulations, the basis of systems such as GSM, ERMES, DECT and TFTS. More advanced elements such as the interpretation of eye diagrams and constellation plots are left to the teacher's discretion and educational aims.
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- 3-2 SPECIFICATIONS 2.1 Emitter module specifications SIGNAL INPUTS: Coax. 1: Function Generator Input Maximum level: ± 2 V Pass band (with antialiasing filter): 250 Hz - 3,400 Hz Input impedance: 5 kω at 1 khz Connector: BNC female Coax. 2: TTL Signal Input Level: 0-5 V Maximum pass band: DC - 300 khz (baseband) Input impedance: > 100 kω at 1 khz Connector: BNC female Mic. 1: Microphone Input Minimum sensitivity: 6 mvpp Maximum sensitivity (without compander): 80 mvpp Pass band (with antialiasing filter): 280 Hz - 3,400 Hz Input impedance: > 20 kω at 1 khz Connector: 3.5 mm female mono jack PCM SIGNAL. BASEBAND Sampling and Quantization Clock: 1.333 MHz (4 MHz/3 crystal) Bit T: 12 µsec. 11-bit scan: 1 start, 8 code, 1 stop and 1 parity T scan: 132 µsec. (144 at worst) Sampling frequency: 7.575 khz (6.945 at worst) Antialiasing filter: bandwidth at 3 db: 280 Hz - 3,400 Hz. Microphone compressor and expander: NE 571 (Philips) MODULATOR CHARACTERISTICS: ASK Modulator (OOK): MARK frequency: 390 khz (± 2%) SPACE frequency: DC Modulator bandwidth: DC - 60 khz
- 4 - FSK Modulator: MARK frequency: 390 khz (± 4%) SPACE frequency: 560 khz (± 3%) Modulator bandwidth: DC - 60 khz (DFD reception) DC - 200 khz (PLL reception) BPSK and DBPSK Modulator: Carrier frequency: 333.3 khz (± 1%) Modulator bandwidth: DC - 45 khz QPSK and DQPSK Modulator: Carrier frequency: 166.6 khz (± 1%) Modulator bandwidth: DC - 45 khz QAM Modulator (APK Amplitude Phase Keying): Carrier frequency: 166.6 khz (± 1%) Modulator bandwidth: DC - 45 khz Levels: 8 EMITTER CHARACTERISTICS: Twin Cable Emitter: Output level (measured at connector): - receiver not connected: 0 at ±4 V (according to modulation) - receiver connected: 0 at ±3 V (according to modulation) Connector: banana female adapter Coaxial Cable Emitter: Output level (measured at connector): - receiver not connected: 0 at ±4 V (according to modulation) - receiver connected: 0 at ±3 V (according to modulation) Connector: BNC female adapter. Fibre Optic Emitter: Emission by LED Emission wave-length: 850 nm (red) Infrared Emitter: Emission by LED Emission wave-length: 950 nm
- 5-27 MHz Emitter: Output level on 50 Ω: Aerial: Monopole. Connector: Carrier frequency: Modulation on AM: 10 dbm 5 mm cable, 150 cm length BNC female 27 MHz (crystal) Modulation index of 10 to 40%, according to selected modulator. POWER REQUIREMENTS: Voltages: 110, 125, 220 and 240 V AC Mains frequency: 50 or 60 Hz Stabilised power supply (internal): + 5 V DC, 360 ma - 5 V DC, 240 ma Power consumption: 6 W Fuses: 220 and 240 V AC, 0.50 A 110 and 125 V AC, 0.75 A DIMENSIONS: WEIGHT: W. 400 x H. 100 x D. 280 mm 2.8 kg 2.2 Receiver Module Specifications RECEIVER CHARACTERISTICS: Twin-Line Cable Receiver: Type: Direct Connector: Banana adapter Coaxial Cable Receiver: Type: Direct Connector: BNC adapter Fibre Optic Receiver: Type: Photo-diode Reception band: 400-1,100 nm (for 90% efficiency) Infrared Receiver: Type: Photo-diode Reception band: 800-1,000 nm (for 50% efficiency) 27 MHz Receiver: Type: Envelope detector Reception band: 27 MHz Aerial: Monopole. 5 mm cable, 150 cm long Connector: BNC female adapter
DEMODULATOR CHARACTERISTICS: ASK Demodulator (OOK On-Off Keying): - 6 - Type: Pass band Filter, envelope detector and comparator. Pass band: - Referring to microphone and signal inputs: at least the entire antialiasing filter. - Referring to the TTL input: DC - 60 khz. Pass band filter: - Central frequency : 380 khz. - Bandwidth: 40 khz (Q=9.5). FSK Demodulator (dual filters, DFD): Type: Pass band filters, envelope detector and comparator between two loops. Pass band: - Referring to microphone and signal inputs: at least the entire antialiasing filter. - Referring to TTL input: DC - 60 khz. Pass band filter 1: - Central frequency : 380 khz. - Bandwidth: 40 khz (Q=9.5). Pass band filter 2: - Central frequency : 550 khz. - Bandwidth: 60 khz (Q=9.2). FSK Demodulator (PLL): Type: Direct detector for PLL. Pass band: - Referring to microphone and signal inputs: at least the entire antialiasing filter. - Referring to TTL input: DC - 200 khz. BPSK and DBPSK Demodulators: Pass band: - Referring to microphone and signal inputs: at least the entire antialiasing filter. - Referring to TTL input: DC - 45 khz. QPSK, DQPSK, and QAM (APK) Demodulators: Pass band: - Referring to microphone and signal inputs: at least the entire antialiasing filter. - Referring to TTL input: DC - 45 khz.
- 7 - OUTPUT CHARACTERISTICS: Earphones output: Output stage: Class AB Output power: 160 mw on 32 Ω Connector: Female mono jack BNC outputs: Connectors: BNC female adapters Output level at S1 (analogue signal): of the Coax. 1 input (f=3 khz) Output level at S2 (TTL signal): 0-4 V POWER REQUIREMENTS: Voltages: 110, 125, 220 and 240 V AC Mains frequency: 50 or 60 Hz Stabilised power supply (internal): + 5 V DC, 310 ma - 5 V DC, 130 ma Power consumption: 6 W Fuses: DIMENSIONS: WEIGHT: 220 and 240 V AC, 0.50 A 110 and 125 V AC, 0.75 A W. 400 x H. 100 x D. 280 mm 2.8 kg
- 8 - ACCESSORY ITEMS UNITS DESCRIPTION 1 Instructions manual 1 Theory manual 1 Work book 1 Dynamic microphone 1 Earphone 2 Radio aerial cables 1 Optical fibre 1 Banana cable/black banana (Ref. CC-12) 1 Banana cable/red banana (Ref. CC-13) 3 BNC coaxial cable/bnc (Ref. CC-03)
- 9-3 INSTALLATION 3.1 Power requirements This instrument is ready for connection to mains voltages of 110-125-220 or 230/240 V Ac 50-60 Hz. The mains voltage can be selected on the rear panel. Figure 1.- Changing the mains voltage. 1. Remove the fuseholder lid. 2. Place the correct fuse for the required mains voltage. 3. Replace the fuseholder lid, aligning the arrow [A] with the required mains voltage marked on the cover [B].
- 10 - WARNING: THE INSTRUMENT HAS BEEN SET IN THE FACTORY AT 230/240 V. BEFORE CONNECTING THE INSTRUMENT, SET TO THE PROPER VOLTAGE ON THE SELECTOR. BEFORE CHANGING VOLTAGE ENSURE THE INSTRUMENT IS NOT CONNECTED TO THE MAINS. 3.2 Installation Precautions For safety purposes, ensure both the receiver and the emitter are earthed. Abide by the channel configurations (do not use BNC/banana adapters, etc.). Abide by the recommended 25 cm distance between emitter and receiver. Be careful when adjusting the potentiometer: do not touch other points. Avoid placing any equipment that creates strong electromagnetic interference nearby: motors, transformers, switching sources, etc. Ancillary equipment: Use the earthed connection where this has been provided. N.B.: Using components other than those supplied with the Communications Trainer may cause damage to the instrument and should therefore be avoided.
- 11-4 DESCRIPTION OF CONTROLS Both the emitter and receiver modules have a switching logic system which controls the activation/deactivation of each component in the system. The state of activation is shown by light emitting diodes (LED) which light up when the corresponding component is active. The external elements of this control system comprise a series of push-buttons so that the user can configure the mode of operation appropriate for each case. 4.1 Emitter 4.1.1 Front panel The emitter module has five push-buttons to set configuration: [1] Power switch: this switches on the instrument, once connected to the mains. [2] INPUTS button: to activate one of the three possible inputs. The active input is shown by a yellow led. [3] FILTER/COMP button: to activate the antialiasing filter and/or compressor for the generator and microphone inputs. [4] MODULATIONS button: to select the type of modulation to be selected. [5] CH. SIMULATION button: this button enables you to choose whether to transmit the signal without distortion (direct), pass the signal through a low-pass filter (BW) or to introduce interference, noise or attenuations (Channel Degradations). [6] TRANSMISSION button: to select the type of channel on which to transmit information.
- 12-4.1.2 Left Side [10] Coax. 1: BNC input for function generator. (BNC 1). [11] Coax. 2: BNC input for TTL signals. (BNC 2). [12] Mic. 1: 3.5 mm jack, microphone input.
- 13-4.1.3 Right Side [20] Tx. 27 MHz: BNC emitter aerial. [21] I.R.: Infrared emitter. [22] F.O.: Fibre optic output connector. [23] Coax.: BNC output for coaxial line. [24] Twin.: BANANA output for twin line.
- 14-4.2 Receiver 4.2.1 Front Panel The receiver module has three push-buttons to control configuration: [1] Power switch: this switches on the instrument, once connected to the mains. [2] RECEPTION button: to select the input channel. [3] DEMODULATION button: to select the various demodulators. [4] FILTER/EXPANDER button: to select use of reconstructor and expander filters. [5] OUTPUTS button: to select between the audio, signal and TTL filters. For TTL output the reconstructor and expander filters are automatically disconnected.
- 15-4.2.2 Left Side [40] Twin.: BANANA input for line. [41] Coax.: BNC input for coaxial line. [42] F.O.: Fibre optic input connector. [43] I.R.: Infrared receiver. [44] Tx. 27 MHz: BNC receiver aerial.
- 16-4.2.3 Right Side [50] S2: BNC TTL output. (BNC 2). [51] Volume control for earphones. [52] 3.5 mm mono jack for earphones. [53] S1: BNC oscilloscope output. (BNC 1).
- 17-5 OPERATING PRINCIPLE 5.1 Block Diagram of the Emitter Module The following diagram shows the structure of the emitter module, and the various operating choices it offers. The module can be divided into various easily distinguishable blocks: inputs, A/D conversion (analogue digital), modulations, channel simulation and emitters. There are three different inputs: microphone, function generator and TTL levels. The signal from the microphone passes through a preamplifier to attain the correct level. Having selected TTL input, this goes directly to the modulators as it is already a digital signal. Neither the A/D converter nor the UART are involved here. Signals sent by the microphone or the generator go to the A/D conversion block. This block comprises the antialiasing filter and the compressor. The compressor (alongside the expander located in the receiver) is designed for voice signals and allows fewer bits to be used in the A/D conversion without loss of quality in the audio reconstruction of the microphone signal. A/D conversion works at a sampling frequency of 7.6 khz as the instrument is designed to handle telephone channel quality signals (300-3,400 Hz). If a signal of higher frequency is introduced, an error occurs in the sampling as there are too few samples to reconstruct the signal. This effect is called aliasing (overlapping) and occurs when you sample (A/D conversion) a signal of a higher frequency than half the sampling frequency (Nyquist criterion). When this occurs the antialiasing filter comes into play, this eliminates all signals of higher frequency than is permissible. The antialiasing filter acts only on microphone and generator inputs, and not on TTL inputs.
- 18 - The PCM signal generated by the UART from samples obtained by the A/D converter is sent to the modulator block. There are various modulations in this block, apart from being able to transmit in baseband. These are: ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), BPSK (Binary Phase Shift Keying), DPSK (Differential Phase Shift Keying), QPSK (Quaternary Phase Shift Keying), DQPSK (Differential Quaternary Shift Keying) and QAM (Quadrature Amplitude Modulation), the latter is understood as being a special case of APK (Amplitude and Phase Keying). In real circumstances, the modulated signal would be transmitted on a specific channel and during transmission the signal might be attenuated, modified by external interferences, by noise or by the bandwidth of the channel. These effects may be simulated by the instrument. You can opt to transmit the direct signal, pass it through a low-pass filter where the cut-off frequency can be altered or through a block which will enable you to add interferences of different frequencies, random noise or even attenuate the signal. From here the signal passes on to the emitters. The most common emissions for digital modulations are via radio, coaxial cable or optical fibre. In addition, the instrument offers other channels such as twin-line cable and infrared, useful for educational purposes.
- 19-5.2 Block Diagram of the Receiver Module The following diagram shows how the receiver module is made up. As with the emitter module, there is a clear division into blocks. There are the receivers, demodulators, D/A conversion and the outputs. The receiver module has blocks similar to the emitter but arranged so as to produce the inverse function, except, of course, for the channel simulator. The signal received by the receivers goes to the demodulators. Each modulator is linked in the receiver with its demodulator. In the case of FSK the module offers two different ways to demodulate the signal: one is by dual filters (a detector named 'Optimum non-coherent') and the other by using a PLL. Having recovered the PCM signal, the last step in reconstructing the emitted signal is to pass it through the D/A converter, unless you choose the TTL output. In this case the signal that comes from the demodulators does not go through the D/A converter but goes directly to the output. Inside the D/A converter there are the reconstructor and expander filters. By playing with the reconstructor filter you can see the effect on the quantization noise present in the reconstructed signal. The expander plays the opposite role to the compressor. The module also offers the possibility of reducing the number of bits in the D/A conversion. This is useful, for example, when simulating errors in the reception of bits, as the eliminated bit will be read as a zero. In addition, by experimenting with the quality of the reconstructed signal (detected both by the earphone and the oscilloscope) you can appreciate on a practical level the relation between the number of bits necessary to establish communication and its subjective quality. Remember that the fewer the number of bits transmitted, the quicker communications will be established. As for outputs, there is one for signals picked up in the emitter by the function generator (BNC connector), another for earphones (jack type connector) and another for TTL (BNC connector).
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- 21-6 DESCRIPTION OF THE CIRCUITS 6.1 Power Supply The diode bridges act as double-wave rectifiers supplying a signal which, filtered by condensers C100 and C102 and +5 V voltage regulators IC62 and IC63, produces symmetrical direct voltage of ±5 V with a maximum current of 500 ma. The diagram of the power supply used is shown below: 6.2 Inputs At the microphone input there is a preamplifier to amplify the obtained signal. The signal reaches a stage in the common emitter formed around the transistor (BC547). Finally the signal travels through a low-pass filter (R26, C20) and a noninverting amplifier (IC17). The generator and TTL inputs contain two Zener diodes to limit the input voltage. The input diagrams are shown below:
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- 23-6.3 Outputs The system is provided with a power amplifier to directly feed low-impedance earphones. This amplifier consists of a class AB stage made up of transistors T10-T11. The circuitry of the outputs is shown on the following page:
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- 25-6.4 Channel Simulator Characteristics such as bandwidth, attenuation and the influence of interferences for each emitter are listed in the section on emitters. As the channel is not ideal, this may degrade the signal, thus producing errors in reception. In order to simulate the effects of a non-ideal channel, a channel simulator has been incorporated in the instrument, and gives you a choice of three options: to send a direct signal, to pass it through a low-pass filter or to distort it (in other words, to add interferences, random noise or attenuate it). All interference can be generated independently, and added to the signal in the desired proportion through the potentiometers. 6.4.1 Low-Pass Filter The low-pass filter used is the following one: The filter here, as you can see, is a first order RC network (P1, C71), where a variable resistance has been added so that you can adjust the cut-off frequency of the filter and observe the effect caused on different modulations. 6.4.2 Interferences Four interferences can be generated at different frequencies. In generating these sinusoidals four Colpitts oscillators are used whose signals are added and amplified using a non-inverting amplifier (IC54D), as shown in the figure below.
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- 27 - Potentiometers P2, P3, P4 and P5 permit the level of each interference to be adjusted. 6.4.3 Noise There are various ways to generate random noise. The method that seems most obvious is to start from a noisy element, a resistance (thermal noise), transistor, Zener diode, etc., and amplify the noise produced. In fact, this is the method used to measure noise in very high frequency professional equipment. For lower frequency bands, the usual solution is to use pseudo-random sequences (PRBS, Pseudo- Random Binary Sequence). Though not totally random (as the same circuitry reproduces exactly the same sequence of pulses), their statistical and frequential behaviour is very much like white noise (of similar power throughout the frequency bands that concern us here) where the sequence generated is long enough. The communications trainer uses a noise module that produces pseudo-random binary sequences, and then sends them through a low-pass filter. The circuit used is the following:
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- 29 - The circuit consists of a group of D bistables (IC56) and an exclusive-or gate (IC57) designed to produce a sequence of pulses of predetermined length, randomly distributed over this length. Integrated circuit 4015 is a shift register containing eight D bistables, producing a sequence length of 255 bits (2 n -1, where "n" is the number of bistables used). The bistables have a common clock, and each bistable leads to the next (the clock marking the duration of the pulse, 1.33 MHz). The two outputs used to obtain the desired sequence are applied to an exclusive-or gate whose output is applied to the shift register input. The noise generated by this structure covers the entire band of frequencies used in the modulations. The system requires a starter. To create one a pulse train is generated for a duration determined by C83 and R81. 6.5 Emitters The following pages show diagrams of the emitters. FIBRE OPTIC EMITTER T7 is the stage that excites the fibre optic emitter diode. The current across the diode is controlled by the voltage of emitter T7, which is proportional to the voltage of the emitter signal. Given that the trainer is an educational tool, it uses a low-power emitter generated by a led diode. It should be stressed that in real applications it is dangerous to look at the emitter source, particularly when a laser source is employed. INFRARED EMITTER The infrared emitter works in the same way as the fibre optic emitter. In this case a specially designed diode is used for emission in the infrared band.
- 30 - RADIO-FREQUENCY EMITTER The emitter in the trainer is based on a 27 MHz, Colpitts crystal oscillator, which modulates the selected signal (baseband, ASK, FSK, BPSK, DPSK, QPSK, DQPSK, QAM), the modulation index being dependent on the input. In the trainer diagram, multiplication is achieved by modulating the T10 emitter current through T12, which acts as a controlled current source for the modulating signal. The Colpitts crystal oscillator comes into effect around T9. Transistor T11 raises the power level of the modulated signal before it is transmitted by aerial.
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- 34 - Impedances should be adjusted in order to attain improved energy transference between the subsystems configuring the emitter and receiver. In this case, the first component that must be adjusted is the aerial. This means that to make the maximum use of the power it receives, it must 'see' a resistance equal to its own. The resistance of an aerial such as the one used in the training instrument (monopole) depends, amongst other things, on its distance (measured in wavelengths) from nearby conductors. These may be metal structures such as laboratory tables, the human body itself, the casings of apparatus, etc., which will produce noticeable differences in how the instrument operates as this will depend on its working environment. Where these are powerful, the emitter and receiver aerials should be placed closer together. The following figures demonstrate the variations in resistance of an aerial according to its distance from the earth plane (or metal conductors), measured in wavelengths (λ). For 27 MHz the wavelength is approximately 11 metres. Variation in aerial resistance with respect to its distance (H, measured in wavelengths). Mutual impedance between two dipole aerials of λ/2 length.
- 35-6.6 Receivers The circuit diagrams used in the receivers are shown below. FIBRE OPTIC RECEIVER The photo-detector used in the reception of fibre optics is equivalent to a current generator. T1 operates as a current-voltage converter. The output signal of T1 is amplified at IC14. INFRARED RECEIVER The infrared receiver is equivalent to a current generator. T2, T3 and T4 form a high-gain current-voltage converter. The output of this stage is filtered at IC15A, IC15B and IC15C, and amplified at IC15D. RADIO-FREQUENCY RECEIVER The radio-frequency receiver is based on an amplifier tuned to the working frequency (T5). The input of the amplifier is obtained from the aerial. The output reaches an integrated amplifier (IC16) whose output reaches a peak detector (which acts as an amplitude demodulator) formed by D7, C34 and R49. The demodulated signal is amplified by IC18.
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- 39-7 TEACHER'S NOTES 1 You are advised to work at all times with compensated attenuator probes (x10) 2 The limitations of infrared caused by noisy transmissions and bandwidth reductions are appreciable when working with phase modulations. 3 The RF emitter has a large bandwidth in order to support all the modulations of the instrument. So each specific modulation allows more noise to be passed than is necessary (bandwidth greater than the modulation). This effect is particularly noticeable in baseband transmissions. 4 On connecting the instrument, the loop between the A/D converter and the UART emitter may stop the order of a new A/D conversion for 12 microseconds once the UART has emptied the transmission buffer. This effect is due to transients and noise entering the UART while the instrument is being connected, and does not effect operation. It just entails a longer sampling period of 12 microseconds. 5 Remember that the PARITY ERROR led of the UART receiver will light up to TTL signals, even though the UART is not in operation. It does not logically ensue that there are communications errors. Furthermore, for phase modulated TTL signals the automatic phase discriminator is not operational, as it is based in the UART. The phase reference will have to be found manually, as shown in each exercise where this is necessary.
TABLE OF CONTENTS 1 DESCRIPTION... 1 2 SPECIFICATIONS... 3 2.1 Emitter module specifications... 3 2.2 Receiver Module Specifications... 5 3 INSTALLATION... 9 3.1 Power requirements... 9 3.2 Installation Precautions... 10 4 DESCRIPTION OF CONTROLS... 11 4.1 Emitter... 11 4.1.1 Front panel... 11 4.1.2 Left Side... 12 4.1.3 Right Side... 13 4.2 Receiver... 14 4.2.1 Front Panel... 14 4.2.2 Left Side... 15 4.2.3 Right Side... 16 5 OPERATING PRINCIPLE... 17 5.1 Block Diagram of the Emitter Module... 17 5.2 Block Diagram of the Receiver Module... 19 6 DESCRIPTION OF THE CIRCUITS... 21 6.1 Power Supply... 21 6.2 Inputs... 21 6.3 Outputs... 23 6.4 Channel Simulator... 25 6.4.1 Low-Pass Filter... 25 6.4.2 Interferences... 25 6.4.3 Noise... 27 6.5 Emitters... 29 6.6 Receivers... 35 7 TEACHER'S NOTES... 39
GENERAL INDEX EC-796 INSTRUCTIONS MANUAL...1 THEORY MANUAL...2 PRACTICES MANUAL...3 DIAGRAMS ANNEX...4