ST2207 TDM

Four Channel Analog TDM Trainer ST2207

Operating Manual Ver.1.1

An ISO 9001 : 2000 company

94-101, Electronic Complex Pardesipura, Indore- 452010, India Tel : 91-731- 2570301/02, 4211100 Fax: 91- 731- 2555643 e mail : [email protected] Website : www.scientech.bz Toll free : 1800-103-5050

ST2207

Scientech Technologies Pvt. Ltd.

2

ST2207 Four Channel Analog TDM Trainer ST2207 Table of Contents 1.

Features

4

2.

Technical Specifications

5

3.

Theory

6

•

Experiment 1 To Study Double sideband AM generation.

12

•

Experiment 2 Study of 4-channel analog Time Division multiplexing.

15

•

Experiment 3 To Study 4-channel analog Time Division De-multiplexing and Double sideband AM reception by envelope detector.

16

4.

Switched Faults

5.

Warranty

6.

List of Accessories


17

3

ST2207 Features •

Self-contained trainer with built in power supply

•

On-board sine wave generator(synchronized)

•

On board DSB/DSBSC modulators

•

Crystal controlled carrier frequency generator

•

Four Analog input time multiplexed channels

•

Four Envelope detectors

•

On board four 4th order low pass filters

•

Input-output & test points provided on board

•

8 Switched faults for fault simulation.


4

ST2207 Technical Specifications Crystal frequency

:

1 MHz

On-board Analog signals

:

250 HZ, 500HZ, 1 KHz and 2 KHz (Adjustable amplitude)

Analog Input channels

:

Four

Multiplexing

:

Time division multiplexing

Modulation

:

DSB/DSBSC modulation

On-board control Signal frequency

:

8 KHz, 16 KHz

Test points

:

29

Interconnections

:

2mm sockets

Power

:

230V +/- 10%, 50/60 HZ, 4VA (approx)

Dimensions (mm)

:

W 325 x H 90 x D 255

Weight

:

1.8kg (approx)

Accessories

:

Manual Set of patch cord, line cord


5

ST2207 Theory Time Division Multiplexing (TDM) :

Multiplexing is the process of combining signals from different information sources so that they can be transmitted over a common channel. Multiplexing is advantageous in cases where it is impracticable and uneconomical to provide separate links for the different information sources. The price that has to be paid to acquire this advantage is in the form o f increased system complexity and bandwidth. It is a technique of transmitting more than one information on the same channel. As can be noticed from the figure 1 below the samples consists of short pulses followed by another pulse after a long time interval. This no-activity time intervals can be used to include samples from the other channels as well. This means that several information can be transmitted over a single channel by sending samples from different information sources at different moments in time. This technique is known as Time Division Multiplexing.

Time Division multiplexing of two signals

Figure 1

Analog TDM : In this technique of multiplexing, analog signals appear at the input of multiplexer and samples of signals are taken at different instants of time and are transmitted on the same channel by interweaving them.

In analog communication systems like AM, FM the instantaneous value of the information signal is used to hang certain parameter of the carrier signal. The time division multiplexing system can be simulated by two rotating switches, one at transmitter and the other at receiver. (See figure) The two wipers rotate and establish electrical contact with one channel at a time.

Figure 2 Scientech Technologies Pvt. Ltd.

6

ST2207

Each signal is sampled over one sampling interval and transmitted one after the other along a common channel. Thus part of message 1 is transmitted first followed by part of message 2, message 3 and then again message 1 so on. It can be anticipated from above process that the receiver switch has to follow two constraints: 1.

It must rotate at the same rate as the transmitter switch.

2.

It must start at the same time as the transmitting switch and it must establish electrical contact with the same channel no. as that of the transmitter. If these two conditions are met, the receiver is said to be in synchronization with transmitter. If constraint one is not met, the samples of different sources would get mixed at the receiver.

Figure 3

Digital TDM :

In this technique, digital signals like TTL or CMOS is multiplexed. It is different from analog multiplexer since no sampling is done instead each input signal is selected by digital control logic. Output at any time depends on the control bit governing input data selection. Basics of amplitude modulation :

Amplitude modulation is defined as a s ystem of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage. In short information signal (modulating signal) is used to control the amplitude of the carrier wave. As the information signal increases in amplitude, the carrier wave is also made to increase in amplitude. Likewise, as the information signal decreases, then the carrier amplitude decreases.


7

ST2207

A double sideband transmission was the first method of modulation developed and, for broadcast stations, is still the most popular. By looking at figure below, two inputs are coming in amplitude modulator block one is the information signal i.e. modulating signal and the other is high frequency carrier signal. We can see that the modulated carrier wave does appear to 'contain' in some way the information as well as the carrier.

Figure 4 Depth of Modulation :

The amount by which the amplitude of the carrier wave increases and decreases depends on the amplitude of the information signal and is called the 'depth of modulation'. The depth of modulation can be quoted as a fraction or as a percentage. Percentage modulation = V max− V min x 100% V max + V min Here is an example:

Figure 5

In above Figure we can see that the modulated carrier wave varies from a maximum peak-to-peak value of 10 volts, down to a minimum value of 6 volts. Inserting these Figure in the above formula, we get: Percentage modulation


10 − 6 = 10 + 6 x 100%

8

ST2207

=

4 16

x 100 %

= 25% or 0.25 Double sideband (DSB) Modulator :

We are using balanced modulator, which is based on IC MC1496, to modulate our information signal. In figure 6 we can see that the amplitude of the carrier is increased and decreased in sympathy with the incoming information signal.

AM Modulation Process Figure 6

To emphasize the connection between the information and the final waveform, a line is sometimes drawn to follow the peaks of the carrier wave as shown in figure 7. This shape, enclosed by a dashed line in out diagram, is referred to as an 'envelope', or a 'modulation envelope'.

Figure 7

The modulated signal is now nearly ready for transmission. If the modulation process has given rise to any unwanted frequency components then a band pass filter can be employed to remove them. Double sideband (DSB) Demodulator :

We are using envelope detector i.e. diode detector to demodulate the modulated signal. Diode Detector :


9

ST2207

The function of the diode detector is to extract the audio signal from the modulated signal. It performs this task in a very similar way to a half wave rectifier converting an AC input to a DC output. Figure 8 shows a simple circuit diagram of the diode detector.

Figure 8

In Fig.8, the diode conducts every time the input signal applied to its anode is more positive than the voltage on the top plate of the capacitor. When the voltage falls below the capacitor voltage, the diode ceases to conduct and the voltage across the capacitor leaks away until the next time the input signal is able to switch it on again. See figure 9.

Figure 9

The result is an output which contains three components: 1.

The wanted audio information signal.

2.

Some ripple of the high frequency.

3.

A positive DC voltage level.

A low pass filter followed by the envelope detector on the board is used to remove the high frequency ripple and a capacitor blocks the DC voltage level. Figure 9 shows the result of the information signal passing through the diode detector and low pass filter.


10

ST2207

The input to the diode detector

Output of diode detector includes: a DC level, the audio signal and ripples.

Output after filtering Figure 10

Again the information signal is getting at the output of the filter block. If you varied frequency of the information signal corresponding changes occurs in the output frequency of the filter.

Testing Instruments Needed for Experiment 1.

Oscilloscope 20 MHz, Dual Trace, ALT Trigger.

2.

Oscilloscope probes X1 – X 10 etc.


11

ST2207 Experiment 1 Objective : To study Double sideband AM generation Procedure :

This experiment investigates the generation of double sideband amplitude modulated (AM) waveforms, 1.

Turn all potentiometers of analog signal generator unit to clockwise position.

2.

This all are information signal of balanced modulator block. Turn ON the power to the trainer. It is indicated by lighting of the power switch.

3.

Observe sine wave of 250 Hz at TP1 and 500 Hz at TP2 on oscilloscope.

4.

Observe sine wave of 1 KHz at TP3 and 2 KHz at TP4 on oscilloscope.

5.

Set analog signal amplitude equal to 4V (Approx.) at TP1, 2, 3 & 4.

6.

Observe carrier signal of frequency 1 MHz at TP5.

7.

Turn all Balance adj. potentiometers of balance modulator block to clockwise position.

8.

Make connection as shown in figure 11.

Figure 11


12

ST2207 9.

Next, examine the output of the individual balanced modulator block at TP6, 7, 8 & 9 on oscilloscope. Trigger the oscilloscope on the TP6 & 7 signal. Check that the waveforms as shown in figure 12

t. p. 6

Figure 12

The output from the balanced modulator block (at TP6, 7, 8 & 9) is a doublesideband amplitude modulated waveform, which has been formed by amplitudemodulating the 1 MHz carrier sine wave with the audio-frequency sine wave. The frequency spectrum of this AM waveform is as shown below in figure 13, where fm is t he frequency of the audio modulating signal.

Figure 13 10.

Now vary the amplitude of all modulating signals, by rotating the potentiometers of analog signal generator unit. Observe the effect of varying each potentiometer has on the amplitude modulated waveforms (TP6, 7, 8 & 9). The amplitude of the modulating signal reduced to zero by rotating the potentiometers of analog signal generator unit in anticlockwise direction and observes the signal at TP6, 7, 8 & 9 .It becomes an un-modulated sine wave of frequency 1 MHz, indicating that only the carrier component now remains. Return the amplitude pot to its maximum position (clockwise direction).

11.

Turn balanced modulator block potentiometers. It is this block that we will use to perform double-side band suppressed carrier amplitude modulation . Turn potentiometer until the signal at TP6, 7, 8 & 9 is as shown in figure 12.


13

ST2207

t. p. 6 Figure 14

The balance pot varies the amount of the 1 MHz carrier component, which is passed from the modulator's output. By adjusting the pot until the peaks of the waveform (A, B, C and so on) have the same amplitude, we are removing the carrier component altogether. We say that the carrier has been 'balanced out' (or 'suppressed') to leave only the two sidebands. This is called Double sideband suppressed carrier (DSBSC) modulation.


14

ST2207 Experiment 2 Objective : This experiment investigates the four channel analog time division multiplexed signal. Procedure : 1.


2.


3.

Make connection as shown in figure 15.

4.

Turn ON the power to the trainer. It is indicated by lighting of the power switch.

5.

Observe time division multiplexing output on oscilloscope by connecting TDM O/P terminal to oscilloscope.

6.

Trigger the signal to view clean multiplexed signal. This TDM output signal carries component of all four channels multiplexed in time domain.

Figure 15

7.

Observe control signal at TP10 and TP11 on oscilloscope.

8.

Now vary the potentiometers of analog signal generator unit and balanced modulator block. Observe the effect of varying each potentiometer has on the TDM waveform at TP12. This point clears the concept of time division multiplexing.


15

ST2207 Experiment 3 Objective : To study four channel analog time division demultiplexed signal and double sideband AM reception by envelope detector. Procedure : 1.

Make connection as shown in Figure 16.

2.

Connect 2 mm patch cord between TDM O/P and TDM I/P socket.

3.


4.


5.

Turn ON power to the trainer. It is indicated b y lighting of the power supply.

6.

Observe all four demultiplexed outputs on TP14, TP15, TP16 and TP17. On oscilloscope.

7.

Now vary the potentiometers of analog signal generator unit and balanced modulator block. Observe the effect of varying each potentiometer has on the

Figure 16

demultiplexed waveform at TP14, TP15, TP16 and TP17. This point clears the concept of time division demultiplexing


16

ST2207 8.

Next examine the output of individual envelope detector block on TP18, TP19, TP20 and TP21. The function of diode detector is to extracts the information signal from the modulated signal. But the output of the diode detector contains the information signal, a positive DC voltage level and some ripples of high frequency. A low pass filter followed by the envelope detector on the board is used to remove the high frequency ripple and a capacitor blocks the DC voltage level

9. 10.

Observe outputs of individual low pass filter block on TP22, TP23, TP24 and TP25 on oscilloscope. Now vary the potentiometers of analog signal generator unit and observe the effect of varying each potentiometer has on the output waveform at TP22, TP23, TP24 and TP25. Switched Faults

This chapter lists the switched faults on the four channel analog TDM trainer. There are four fault switches on the module. Open circuit faults : 1.

Fault disables the output from the 1 MHz Crystal oscillator (at TP5), by disconnection the carrier signal in all balanced modulator block. The result is a modulator circuit is not properly working in the absence of carrier signal.

2.

Fault disconnects the time division multiplexed output (TP12) by disconnection the TDM input (TP13).Causes the all low pass filter and envelope detector block’s output affected.

3.

Fault disables the output of 250 Hz input balanced modulator block by removing the carrier signal in modulator circuit

4.

Fault disconnects the output at tp20 to the low pas filter block by disconnecting the diode of envelope detector circuit to the input of filter.


17

ST2207

1.

2.

Warranty We guarantee the product against all manufacturing defects for 24 months from the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

The guarantee will become void, if a)

The product is not operated as per the instruction given in the operating manual.

b)

The agreed payment terms and other conditions of sale are not followed.

c)

The customer resells the instrument to another party.

d)

Any attempt is made to service and modify the instrument.

3.

The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

4.

The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

For any Technical Problem Please Contact us at [email protected]

List of Accessories 1.

Patch cord 16”black .............. .............. ................. ............... .............. ...... 2 Nos.

2.

Patch cord 16”blue ............... .............. ................. ............... .............. ...... 9 Nos.

3.

Mains cord ................. ............... .............. ................. ............... ................ 1 No.

4.

e-Manual .............. ................. .............. ................. ............... .............. ....... 1 No.

updated 01-05-09


18

ST2207 TDM

Recommend Documents