Study of amplitude modulator

LABORATORY WORK №2

STUDY OF AMPLITUDE MODULATOR

Purpose of the work: to study the principle of operation of an amplitude modulator and acquire the skills of experimental research of qualitative indicators of modulators.

Figure: 1.1. Amplitude modulator schematic diagram

Laboratory task

1. 
   Investigate the static modulation characteristic (SMX).
   
2. 
   Investigate the dynamic modulation response (DMX).
   
3. 
   Investigate the influence of the operating mode of the transistor on the quality of modulation.
   
4. 
   Investigate the influence of load parameters on the quality of modulation.
   

Methodical instructions

1. Investigate the static modulation characteristic.

CMX is the dependence of the amplitude of the first harmonic of the output current I1 on the modulating voltage. To study the CMX, the bias voltage is used as the modulating voltage.

To study the CMX, you must do the following:

1. 
   Download the program. Set up the oscilloscope, for this signal amplification set 1B / div in both channels and set the signal sweep to 0.1ms / div.
   
2. 
   Press the "Yes" button next to the inscription to remove the CMX transistor, then press the "LC" button. Set the signal frequency f = 14000 Hz and the signal amplitude Uω = 1 V.
   
3. 
   Connect test point # 2 in the first channel and test point # 4 in the second channel.
   
4. 
   Change the bias voltage Еcm from = -3 V to 0 in 0.2V increments. For each value of Ecm, record the milliammeter readings showing the value of the current amplitude. Enter the results of current measurements in the second line of Table 1.1.
   

Table 1.1.

Есм, В	- 3	- 2,8	- 2,6	…	- 0,6	- 0,4	- 0,2	0

Based on the data obtained, build a graph of the dependence I1 = f (Ecm).

2. Investigate dynamic response (DMX)

Dynamic modulation characteristic (DMX) characterizes the dependence of the amplitude modulation factor "M" on the amplitude of the modulating voltage UΩ

To learn DMX, do the following;

• 
  2.1. Set the bias voltage Ecm corresponding to the middleof the straight section of the SMX, removed, for 1B, install at the outlet of the MHF amplitude 1 V, press the "no" button next to the inscription remove the CMX transsistor.
  
• 
  2.2. Set the frequency of the signal to 14000 Hz and the amplitude of the signal Uω =1 B.
  
• 
  2.3. Set the signal frequency to the LFO to 500 Hz. Changing the amplitude Up of the LFO signal from zero to 2V, at which distortions of the AM signal at the output of the modulator become noticeable (the shape of the AM signal is monitored by an oscilloscope), measure the parameters Аmin and Аmax (Fig. 1.2) of the modulated signal, perform 6-8 measurements for different values ​​Data enter in table 3.2.
  

Table 3.2.

UΩ, В	0	0,2	0,4	0,6	0,8	…	1,8	2
Аmin, В
Аmax, В
М, %

• 
  2.4. According to table 1.2. calculate the modulation coefficients using the formula:
  

=		^−	∙ 100%
		+

Figure 1.2. Signal shape at the output of the modulator.

3. Investigate the influence of the operating mode of the transistor on the quality of the implementation of modulation. In this point of the laboratory work, the effect of the bias voltage on the modulation quality is investigated. Undistorted modulation is possible provided that the sum of the modulating voltage UΩ (t) and the bias voltage Ecm do not go beyond the straight section of the CMX.

• 
  3.1. Set Ecm corresponding to the middle of the straight section CMX, taken for B.
  
• 
  3.2. Set the oscillation amplitude Uω = 1 V at the output of the MHF.
  
• 
  3.3. Set the amplitude of oscillations at the LFO output equal to half the length of the linear section of the SMX. Observe and sketch the uninterested AM waveform.
  
• 
  3.4. Increase Ecm to values ​​when the difference between the signal shape at the LFO output and the shape of the AM signal envelope becomes noticeable, which indicates distortion during modulation. Sketch the signal at the output modulator at Ecm = -3.5V.
  

4. Investigate the effect of load parameters on the quality of the modulator

• 
  Set the bias voltage Еcm = -2.5V and the frequency on the MHF equal to 50 Hz, draw the timing diagrams of the AM signal with the oscillating circuit switched on (LC button) and load R (R button).
  
• 
  Set the bias voltage Еcm = -2.5V and the frequency at the MHF equal to 25,000 Hz, draw the timing diagrams of the AM signal with the oscillating circuit switched on (LC button) and load R (R button).
  

Content of the report

The report should contain:

1. Title and purpose of the work.

2. Schematic diagram of the modulator.

3. Table of measurement data and graphs of CMX.

4. Table of measurement data and graphs M = f (UΩ).

5. Oscillograms of output AM signals.

Test questions:

1. Why do you need modulation? What types of modulation do you know?

2. What kind of modulation is called amplitude modulation?

3. Why should an electrical circuit designed to produce amplitude-modulated oscillations be substantially non-linear?

4. What power polynomial approximates the current-voltage characteristic at which distorted modulation is provided?

5. What is the modulation depth ratio? Write his analytical expression.

6. Display the time and spectral diagrams of the amplitude modulated oscillation with a harmonic signal.

7. Draw a schematic diagram of a bias modulated transistor modulator.

8. Give the definition of static modulation characteristics.

9. Explain (graphically and analytically) how it is possible to obtain a static current-voltage characteristic of a nonlinear element

modulation characteristic?

10. How to choose the mode of operation of the modulator according to the static modulation characteristic?

11. How will changing the bias voltage of the modulated amplifier affect the AM waveform?

12. How will the detuning of the oscillating circuit relative to the carrier frequency affect the shape of the AM oscillation?

13. Is it possible to obtain a good quality of modulation with an aperiodic load of the transistor?

14. Why does the modulation coefficient change when the Q-factor of the circuit in the transistor load changes?