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Sunday, 26 May 2019

AM/FM Signal Generator Block Diagram and Working

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AM/FM Signal Generator

The A.M./F.M. signal generator serves the purpose of generating the amplitude modulated signals as well as the frequency modulated signals. It is a two in one generator. The A.M. section will be useful in testing A.M. receivers. The F.M. section will be useful in testing the F.M. receiver circuits. A block diagram of such an A.M. / F.M. signal generator is shown in Figure.

Block Diagram of an A.M./F.M. Signal Generator  


The block diagram of an A.M./F.M. signal generator is show in Figure below.

AM/FM Signal Generator Block Diagram

Working of A.M./F.M. Signal Generator

The R.F. signal is generated from 100 kHz to around 200 MHz. This is done by an R.F. oscillator. This oscillator will be a well-designed oscillator. The frequency of this oscillator is tunable in different bands of frequency using a range selector switch. Provision will be made to effect frequency modulation of audio signal. This is done using a varactor diode. The varactor diode will be across the tuned circuit of the R.F. oscillator. Variation of the bias on the varactor diode causes frequency modulation. This variation will be produced by the application of the modulating signal to the varactor diode circuit, when the F.M. mode of modulation is selected through the modulation mode switch.

The modulator stage modulates the audio signal in the amplitude modulation mode. In frequency modulation mode its modulating input will be disabled. So in the F.M. mode it only acts as an R.F. amplifier amplifying the frequency modulated signal. However in the amplitude modulation mode, its input is the modulating signal supplied through the modulation mode selector switch.

Hence it acts as a modulator circuit. The amplitude modulated signal, or the amplified frequency modulated signal (depending on the setting of the modulation mode switch) will be applied to the R.F. amplifier. This enhances the magnitude of the signal. 'Through an attenuator (coarse and fine) the output reaches the output socket.

The audio frequency signal for modulation is generated from a stable audio frequency oscillator. The waveform of this signal is made as pure as possible. The controls for the audio signal amplitude, amplitude of R.F. signal are provided. The provision to control the modulation index is effected by providing a monitor for audio signal level, as well as R.F. signal level. A single indicating instrument can be used with a switch to monitor both the levels.

In the amplitude modulation mode, the audio signal is directly connected to the modulator from the audio frequency oscillator. Amplitude modulation takes place. The amplitude modulated signal will as usual be present at the output terminals.

If the modulation mode switch is put on to F.M. side the audio signal will reach the varactor diode circuits. Hence the (carrier) R.F. signal will be frequency modulated. This reaches the modulator. The modulators modulating input is disconnected in this mode.

Hence it acts as only an R.F. amplifier and gives in its output an amplified radio frequency signal. The signal is further amplified and given to the output terminals through an attenuator.

Friday, 24 May 2019

Alignment of Radio Receiver

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Radio Receiver Alignment


The alignment of an amplitude modulated radio receiver consists of two types of adjustments of the different tuned circuits present in the receiver. The first part is to align the I.F. transformers to the correct intermediate frequency of the receiver. This is called the I.F. alignment. The second part is to align the input tuned circuits to match with the scale/dial markings such that the wanted station (frequency) is producing the correct value of I.F. when tuned on the dial to the frequency marked on the dial. This is called the R.F. alignment. The R.F. alignment is also called TRACKING.

There are certain preliminary arrangements to be done and special precautions to be taken either for I.F. alignment or for the R.F. alignment. These are explained here under. 

 I.F. Alignment  


The setup for the I.F. alignment of a radio receiver is shown in Figure. We use a power output meter in addition to the Standard Signal generator to measure the power output of the radio receiver.


 Preliminary Steps for I.F. Alignment  

The following arrangements are to be made before alignment :

1. The A.V.C/A.G.C. is to be made inoperative.
2. The volume and tone controls are to be kept in maximum resistance position.
3. The loud speaker is to be replaced by an A.F. power output meter in the proper impedance range equal to that of the impedance of the loud speaker.
4. The radio receiver is to be tuned to the medium wave band.
5. The radio receiver is to be tuned to the lowest frequency in the medium wave band.
6. A capacitor of 0.1 μF is to be connected in series with the R.F. lead, when the signal is injected.
7. The cores of I.F. transformers are to be tuned using only nonmagnetic screw drivers or with special core tuning alignment tools. 

Step-by-Step I.F. Alignment Procedure  

The following steps are to be followed.

1. The signal generator is to be tuned to the Intermediate frequency of the radio receiver.

2. It is to be kept in the internal modulation position. The depth of modulation is to be adjusted to 30% or above.

3. The attenuator is to be adjusted to minimum value.

4. The I.F. signal is to be applied to the collector of the second I.F. amplifier, through the 0.1 μF capacitor. The attenuator is to be adjusted to get a convenient reading in the power output meter. Now the core of the second I.F. transformer is to be adjusted to get a maximum reading in the power output meter.

5. The I.F. signal is to be shifted to the base of the second I.F. amplifier. There will be an improvement in the output as indicated by the power output meter. This is due to the gain of the amplifier. Again the core is to be adjusted to get the maximum output.

6. The same procedure is to be repeated at the collector and base of the first I.F. amplifier. The cores of the corresponding I.F. transformer are to be adjusted for maximum output in the power output meter.

7. The signal is to be shifted now to the base of the frequency converter transistor. In this case the attenuator is to be kept in the minimum position. Adjustment of core of the I.F. transformer in the collector of the frequency converter is to be done now. i It is to be adjusted to get maximum response.

8. Finally the R.F probe must be kept at a short distance from the ferrite antenna. The power output meter is to be disconnected and the loud speaker is to be connected. A reasonable 400 Hz note will be heard from the speaker showing the receivers sensitivity for I.F.

9. The alignment of the I.F. stages is to be repeated two to three times to finally arrive at correct setting.

10. When once arrived at final setting the cores are to be sealed to prevent tampering of the cores by the user. 

Step-by-Step R.F. Alignment or Tracking  

All the preliminary arrangements as listed for I.F. alignment are common for the R.F. alignment. The change is that we will select the different bands for tracking.

Further we will use a dummy antenna recommended for the particular frequency band under consideration. The following are the steps to be followed:

The Standard signal generator is to be tuned to the desired frequency as listed here under. The signal will not be directly connected to the input of the receiver, but the probe will be kept close to the antenna coils of the receiver. The frequencies to which the standard signal generator and the receiver are to be tuned and the adjustment to be made are given step by step here under in Table. 

Procedure for tracking the Radio Receiver


S.G. Frequency
Position of pointer
Adjustment to be made for getting maximum output
SWITCH THE RECEIVER TO THE MEDIUM WAVE BAND (550 kHz to 1600 kHz)
550 kHz
550 kHz
M.W. Oscillator coil
1600 kHz
1600 kHz
Trimmer of M.W. Osc. Coil
Repeat this adjustment twice then:
840 kHz
840 kHz
Slide the M.W. antenna coil on the ferrite rod.
Repeat the above two operations twice to get tuning as close to the dial marking as is possible.
SWITCH THE RECEIVER TO THE SHORT WAVE BAND (4.5 MHz to 16 MHz)
04.5 MHz
04.5 MHz
S.W. Oscillator Coil
16.5 MHz
16.5 MHz
Trimmer S.W. Osc. Coil
Repeat this adjustment twice then:
05.0 MHz
05.0 MHz
S.W. Antenna Coil
Repeat the above two operations twice to get tuning as close to the dial marking as is possible.


If the receiver has several short wave bands, the a different above process is to be repeated at all the different S.W. bands tuning to the lowest frequency, highest frequency, adjusting Oscillator Coil at the lowest frequency and adjusting the oscillator trimmer at the higher end. At the mid frequency in the band the antenna coil is to be tuned. This completes the process of tracking the receiver.

Thursday, 23 May 2019

Audio Frequency Oscillator Working

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AF OSCILLATOR

The block diagram of an Audio Frequency Oscillator is shown in figure. All the blocks have been labelled
Block Diagram of an AF Oscillator

Description of the Block Diagram and Audio Frequency Oscillator Working  

The first block is that of the sine wave oscillator. These oscillators are of the Wien bridge or phase shift type. They produce pure wave form. However when the output voltage required is large, beat frequency oscillator is preferred to generate the audio signal. The design and use of the beat frequency oscillator is complex. Cost wise also the beat frequency oscillator is uneconomical. Most of the audio signal generators use Wien bridge oscillator. The oscillator will be provided with a frequency range selection switch for coarse frequency change. A fine frequency control is also provided. The output of the sine wave oscillator goes to the amplifier.

The amplifier amplifies the signal generated by the oscillator. The amplifier is provided with negative feedback to maintain stability of operation. A voltage follower circuit is used to provide a very high input impedance, very low input current, very low output impedance and unity gain for the amplifier. The voltage follower will be the final stage in the amplifier section.

The amplifier's output goes to the attenuator through a switch. The switch selects the sine wave output direct from the output of the amplifier. Setting the switch in to position will connect the output of the wave shaping network to the attenuator.  The wave shaping network is supplied from the output of the amplifier. The sine wave is shaped to wave a square wave in the output of the wave shaping network. 
The attenuator has a coarse adjustment and fine adjustment. The attenuator is calibrated to indicate the output voltage from the audio oscillator. 
The frequency counter and its display shown in the block diagram are optional. The present day signal generators use frequency counters in them for the correct display of frequency. The frequency counters frequency range switch and the oscillator's range switch are gauged to get the correct display of frequency. When frequency counter is used to display the frequency of the oscillator, the dial need not be very accurately calibrated. 

Specifications of an Audio Oscillator 

The following are the specifications of an AF oscillator

1. Frequency range : It specifies the range in which the instrument has to generate the signal. A range of 10 Hz to 50 kHz is generally specified.

2. The output power or voltage : The output voltage is specified. For general purpose application 5 V, is sufficient. For better work the range can be up to 20V.

3. The output impedance : Two standard impedances are specified, one is 60M and the other is 100 Ω. Both the outputs are to be provided over the output sockets. The 600Ω impedance is for matching, with general transmission lines. The 100Ω impedance is for the low impedance test work.

4. Dial resolution and accuracy: Precise calibration of dial is essential. For accurate setting of the frequency a vernier is to be provided. Provision of a frequency counter is the best solution for tuning problems. The accuracy of the dial is important to have accurate setting of frequency.

5. Frequency stability: The frequency of the oscillator must be stable for long periods. Hence the long term stability is important.

6. Amplitude stability: The amplitude of the signal must be maintained constant throughout its operation.

7. Distortion: Distortion in the output of the signal generator is to be avoided. When the distortion is present, presence of harmonics will lead to spurious results in signal analysis and measurements. 

 Typical Specifications of an Audio Oscillator 

Power Supply : 230V ±10 % 50 Hz, A.C. Mains.
Frequency : 15 Hz to 150 kHz in five ranges.
Accuracy : ± 3% ( +0.15 Hz)
Stability : i. Less than ± 0.05% short term drift after initial 1 minute.
                ii. Change of frequency with temperature is typically less than ± 0.1% per degree.
Output Impedance :  i. 600 f with attenuator ± 2%
Output Voltage : Continuously variable up to 2.5 V.

It has been stated that the range of an audio oscillator is from 10 Hz to about 50 kHz in above paragraphs. The actual range of their audio oscillator depends on the manufacturer. It should cover the audio frequency range. That is the main requirement. 

 Audio Frequency Oscillator - A.M. Standard Signal Generator a Comparison

1. The audio oscillator produces sinusoidal signals or if required square wave voltages, with in audio frequency range and may have its range from 10 Hz to around 100 kHz. Though the frequency range at the upper end is in the low R.F. range it is not a modulated signal.
The standard signal generator in turn is basically an R.F oscillator. It has its frequency range from 100 kHz to around 80 MHz. It produces sinusoidal voltages in this frequency range under continuous wave (carrier wave) mode. Normally its output is a modulated signal. The R.F. signal will be amplitude modulated by an audio signal of 400 Hz or 1 kHz.

2. An audio oscillator has no provision for modulation. Its use is limited to audio frequency applications. The standard signal generator has provision for modulation. Modulation can be effected using internal audio frequency generated by an audio frequency oscillator. There is also provision for external modulation using an audio oscillator externally.

3. The output voltage of an audio oscillator can he from around 2 V to as large as 100 V, in special types. The Standard signal generator has its modulated R.F. output voltage restricted to a maximum value of around 2 V.

4. In a standard signal generator a socket is provided to take out the single frequency (fixed frequency) audio signal with or without attenuation. Such a facility is useful in trouble shooting audio circuits when a separate audio oscillator is not available.

5. Provisions like dummy antenna, S meter, modulation switch are not necessary in audio oscillators. They are essential in standard signal generator.

6. The shielding requirements of R.F.signal generators are critical. Effective shielding is necessary to prevent interference of radio frequency signals with the neighboring circuitry. Audio oscillators have less rigid shielding requirements.

A table showing the comparison between an audio oscillator and Standard signal generator is given below

Parameter
Audio Oscillator
A.M Standard Signal Generator
Frequency Range
10 Hz to 50 kHz
100 kH to 80 MHz (Modulated)
Wave form
Sine and Square
Audio Modulated Sine
Provision for Modulation
No need
Yes by audio Fixed frequency or by external audio signal
Output Voltage
2 V to 100 V
2V
Output attenuator
Yes coarse and fine attenuators
Yes Coarse & fine attenuator for both audio & R.F. output
Output impedance :
600 Ω & 1000 Ω
600 Ω with dummy antenna
Dial facilities
Yes generally without vernier scale

Currently digital frequency counter is provided to show output   frequency
Yes with precision vernier

Currently digital frequency counter is provided to show the output frequency
Output indicator
Usually no
Panel meter with switch to indicate both the R.F. and A.F. outputs to set correct  % modulation required
External inputs
No external input
Provision for External A.F. input.
Additional features
No
Dummy antenna, Modulation switch, A.F. output socket
Provision for calibration
No
Yes using internal crystal oscillator
Shielding
Simple shielding
Critical Shielding is to he provided to prevent R.F. radiation
Power supply
230 V ± 10%
50 Hz 50 W
230 V ±10 %
50 Hz 50W

Tuesday, 21 May 2019

Rectifier Amplifier Type AC Millivoltmeter

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R.F. MILLI VOLTMETER OF THE RECTIFIER AMPLIFIER TYPE  

As the amplifier rectifier type of millivoltmeters performance is limited by the band width of the amplifier, an alternative is thought over that gave raise to the rectifier amplifier type A.C. millivoltmeter.

In this instrument initially the input signal will be rectified. The rectified D.C. will be amplified. In order to avoid the loss of signal due to the connecting cable, the rectifier element is incorporated in the probe itself. 

Selection of Diode and Amplifier 

The diode that are used with conventional type of alternating current meters cannot be used for rectification of the high frequency signals of the order of gigahertz region. The reason is their capacitance, and reverse recovery time.

For use at very high frequency rectification, Schottky barriere or point contact diodes are used When diodes are used at low forward biased voltages the rectified output will not equal the peak of the input. This results in the fact that for lower amplitudes of signal voltages, the rectified output will further be low. Therefore chopper stabilized amplifier or an amplifier stabilized for D.C. drifts is required in the A.C. millivoltmeter circuits. 



 Block Diagram of Rectifier Amplifier Type of AC Millivoltmeter

The block diagram of an A.C. millivoltmeter of the rectifier amplifier type is shown in Figure. The output of the detector probe reaches the chopper stabilized amplifier.

Block Diagram of Rectifier Amplifier Type of AC Millivoltmeter

The output of the amplifier is supplied to the digitization circuit. A display is connected to the digitization circuit. The magnitude of the input will be displayed on the display. 

Probes for Measurement  

The probe arrangement that can be used for voltage measurement with high impedance is shown in Figure.

Probe for Voltage Measurement
This probe will be used within circuits at which the impedances change and where the circuit cannot be isolated and terminated externally. 

Another probe arrangement mostly suitable for measurement of R.F. power of the order of nanowatt is shown in Figure.
.

Probe for Voltage Measurement

This probe is useful with high frequency circuits that can be disconnected and terminated in 50 ohm externally. The power measurement made with this probe will not yield true r.m.s. value. Mostly when modulated signals are measured the results are to be interpreted carefully. 

Measurement Method 

The type of probe is to be selected first depending on the fact whether the circuit can be disconnected and terminated, or not. After selecting the probe, the probe is to be connected to the signal source. The range selector switch of the milli-voltmeter is to be adjusted from the highest range to obtain an accurate read out from the display, which gives the magnitude of the input signal.

Applications of R.F. millivoltmeter

This type of R.F. millivoltmeter is useful at very high frequencies.
It can be used for R.F. power measurement at frequencies of the order of gigahertz. 

SPECIFICATIONS OF A.C. MILLIVOLTMETER 

Voltage Range : 1.5 mV to 500 V full scale in 12 ranges in 1.5 and 5 sequence.

dB range : — 60 dB to + 60 dB in 12 decade ranges with 1 mW as '0' dB reference dissipated in a resistance of 600 ohm.

Frequency Range : 1 Hz to 5 MHz

Accuracy : ± 1% of reading, ± 1% of full scale reading

Input impedance : 5 MΩ

Predeflection : Less than 100 μV

Power requirement : 230 V, 50 Hz





Sunday, 19 May 2019

AC Millivoltmeter Block Diagram

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RF Voltage Measurement


Measurement of radio frequency voltage is difficult especially when the magnitude is very small and the frequency is very high. Naturally when small magnitudes are to be measured, we certainly need amplification of the signal, that too at the signal frequency.

Radio frequency signal amplification is complicated and requires special devices, and has several restrictions on the component size, orientation, and shielding is very important. In spite of all the precautions taken the bandwidth restriction again poses a problem. Stability of the gain of the amplifier is yet another problem. For accurate measurement care is to be taken in the design of the instrument considering all the above constrains.

R.F. millivolt meter can be either of the amplifier rectifier type or the rectifier amplifier type. thee amplifier rectifier type again can be the average reading type, peak reading type or true R.M.S. reading type.



In amplifier rectifier type of instrument initially the radio frequency signal will be amplified in a wide band amplifier. The output of the amplifier will be used in a detector to obtain an indication in the indicating instrument, which reads the value of the input voltage by proper calibration of the instrument.

In the rectifier amplifier type of radio frequency voltage measurement, the input signal will initially be rectified. The rectified D.C. will be amplified in a D.C. amplifier. The output of the D.C. amplifier will be connected to an indicating instrument which will be calibrated in terms of the input signal.


In this blog the following topics are covered :

Classification of amplifier rectifier type of A.C. Millivoltmeters
Principle of working of the amplifier rectifier type of A.C. Millivoltmeter
Applications of amplifier rectifier type of A.C. Millivoltmeter
Principle of working of the Rectifier amplifier type of A.C. Millivoltmeter
Types of Probes used with the rectifier amplifier type of instrument
Applications .

 Amplifier Rectifier Type A.C. Millivoltmeter 


In this type of instrument the input signal will first be amplified in a wide band amplifier. The output of the wide band amplifier will be used in a rectifier to get a D.C. output which will be indicated in an indicating instrument. 

(a) Classification  


Depending on the type of rectifier arrangement used in the circuit we have three types of A.C. millivoltmeters as listed below:
(i) The average reading type
(ii) The peak reading type
(iii) The true R.M.S. reading type 

(i) The Average Reading Type 


In this type the dial calibration will be made multiplying the average output of the rectifier by the form factor 1.11, for the sinusoidal waves. For square wave inputs the reading will be more than the true R.M.S. value by 11%. For triangular wave inputs the reading will be less than the true R.M.S. value by 4%. 

(ii) The Peak Reading Type 


In this type the detectors response will be quick for increasing amplitudes of the input signals. i The response will be slow for the decreasing amplitudes. As it is the peak detector that is employed, it "holds" the peak indefinitely. Practically a suitable decay will be obtained by properly choosing the time constant selecting the value of capacitor and load resistor in the peak detector. 

(iii) The True R.M.S. Reading Type


The true R.M. S. type indicates the R.M.S. value of the input signal for any type of waveform. It is for this purpose the amplifier and the detector must be confined to their linear region of operation. The true R.M.S. indication will be specified for a given maximum value of "crust factor", which is less than 5. The crest factor will be more than 5 in special cases like the pulse train that have a low duty ratio. Therefore the true R.M.S. meter is generally specified for the crest factor value of 5, for an input signal having an R.M.S. value that equals the full scale deflection. 

(b) Description of the Block Diagram


The block diagram of the a.c. millivoltmeter is shown in Figure. From the block diagram it can be seen that the input signal to be measured will reach the wide band amplifier through a calibrated attenuator.


The output of the wide band amplifier is supplied to the linearized detector. The output of the detector is given to the indicating instrument. Two more detectors which are optional have been shown in the block diagram to indicate the three types of arrangements that can be hand in this type of instrument. The two other blocks of detectors are shown to have been connected in dotted lines. The choice of the detector is optional. Further the output of the wide band amplifier can be terminated over a socket as an optional output terminal, as is shown in the block diagram.
Block Diagram of AC Millivoltmeter


(c) Measurement Method 


The zero setting of the indicating instrument is to be adjusted correctly to zero after the normal warming time of the instrument. The signal input is applied to the input terminals. The range switch is initially is to be set to the largest range. By adjusting the range switch, a reading close to the full scale reading is to be obtained which accurately gives the magnitude of the input signal. 

(d) Applications 


Noise analysis
Turbulance studies
R.M.S. power measurement in power systems using thyristor. 

(e) Limitations


The rectifier amplifier type of A.C. millivoltmeters performance is limited by the bandwidth of the amplifier. They can be used up to around 5 MHz, beyond which their accuracy will be poor.