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Monday, 4 February 2019

AM Signal Generator


The block diagram of an AM Signal Generator is shown in Figure. All the blocks have been labeled.

Block Diagram of AM Signal Generator
(a) Description of the Block Diagram and Working:

The standard signal generator consists of a highly stable RF oscillator. This oscillator is tunable in the frequency range of 100 kHz to 80 MHz. The tuning may be done in the conventional LC circuit. Some generators use a master oscillator for the highest frequency range and use frequency dividers to obtain the rest of the ranges.

A fixed frequency oscillator which works at 400 Hz or 1000Hz or both by switching is used to modulate the radio frequency signal in a modulator. The carrier level and the depth of modulation are indicated by the indicating instrument on the front panel.

The modulated output from the modulator is given to a coarse attenuator. Through the fine attenuator the modulated signal is connected to the output terminals. Provision will be made to control the amplitude of the audio signal to control the percentage of modulation. The common level is 30 % modulation. A.F. output is also available from a separate socket for testing work. The standard signal generator will be provided with standard dummy antennas for the medium waves and short waves to make the system complete.

The different RF ranges (scales) can be selected by a switch. A dial is provided which also has a vernier to correctly tune to the required frequency in a given range. Provision for external modulation is made, to externally modulate the radio frequency signal for special measurements. The calibration of the standard signal can be checked periodically using the internal crystal oscillator.

The power supply used is a regulated power supply that has very low ripple. The power supply to the master oscillator is regulated by temperature compensated reference circuit.

(b) Importance of Shielding in AM Standard Signal Generator:

When equipment of the type of signal generators is designed importance is to be given to effective shielding. As these generators produce signals of different frequencies and also of considerable amplitude, the interference of these signals can effect the performance of nearby equipment.

Therefore the main oscillator is to be provided with a good magnetic shield. These magnetic shields usually are made with copper or aluminium. The shield will be firmly fixed over the chassis. The wires connecting the different subsystems of such signal generators are to be shielded wires. The tuning condensers if used are also to be provided with shield. 

The bottom portion of the chassis will have to be provided with a metal shield. This shield is to be connected to earth point of the chassis. Metal cabinets are preferred for these signal generators as the cabinet also can serve as a shield. With the above provisions the radiation from the main oscillator, and the associated circuits can be minimized. However RF signal generators may radiate the signal because of the high frequency in and around the area of use in spite of the provision of shields.

(c) Front Panel Controls of an AM Standard Signal Generator: 

The front panel of the AM standard signal generator is described below. The list and functions of the various controls is given below:

1 On off switch                                9. AF output
2. Dial (frequency)                        10. AF attenuator
3. Vernier                                      11. RF output
4. Tuning knob.                             12. RF coarse attenuator
5. Range selector.                         13. RF fine attenuator
6. Carrier level.                              14. Crystal Oscillator On/ Off
7. AF level.                                    15. Ext Mod Input.
8. 'S' meter.                                   16. CW., Int., Ext., Mod Selector

1. On off control: To switch on and off the instrument.

2. Frequency range selector: A rotary switch which is used to select the desired frequency range.

3. Frequency dial: Dial with frequency marking and a vernier for measurement of frequency to which the generator is adjusted. This scale is to be read in consultation with the frequency range selector switch.

4. Carrier level control: A potentiometer that controls the carrier level (amplitude).

5. Modulation level control: A potentiometer that controls the depth of modulation by controlling the amplitude of the modulating signal.

6. RF attenuator: A coarse and fine attenuators are provided to control the RF output in steps using the coarse attenuator and continuously using the fine attenuator. Both are calibrated in volts (RF output voltage).

7. AF attenuator: A fine attenuator of the potentiometer type for the control of the AF output from the separate AF output socket.

8. AF selector: A switch that connects either 400 Hz or 1000 kHz signal or external audio signal for modulation of the radio frequency carrier.

9. Crystal oscillator On/Off: To work the crystal oscillator to calibrate the frequency dials periodically.

10. `S' meter: To indicate the carrier level and also the percentage of modulation

(d) Specifications of AM Standard Signal Generator:

Power supply - 230 V ± 10 % 50 Hz 50 W

Frequency range - 100 kHz to 72 MHz in six bands

Accuracy - Within ± 1 %

Dial - In addition to the main dial in steps respectively through a dial with 200 divisions may be provided which is a vernier dial.

AF Output - 400 Hz or 1000 Hz switch selected.

AF voltage - More than 1 V fixed voltage.

Internal Modulation - Selectable audio frequency by switching to 400 Hz or 1000 Hz

Depth of modulation - Zero to 80 % adjustable

External modulation: 50 Hz to 10 kHz by switching.

The specifications are self explanatory.

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